Famous Monitor's XI-inch Dahlgren Shell Guns

When John Ericsson designed the Monitor, he knew that a 15-inch Rodman existed, but that was an Army gun. He had hoped that a gun like that could go in his design. Unfortunately, the largest gun adopted by the Navy at that moment was the XI-inch Dahlgren Shell Guns, which is still really big, but not what he really wanted to install in his new creation. Dahlgren wasn't convinced that guns larger than 11 inches were safe, and in the confines of an armored turret, well, he had even more reservations about such big guns. So at his direction, Ericsson submitted his experimental plans for the Monitor tailored to fit two XI-inch Dahlgren Shell Guns in the turret.

ARTILLERY PROFILE
  • Model: XI-inch Dahlgren Shell Guns
  • Type: Muzzleloading Smoothbores
  • In Service With: U.S. Navy, Aboard the U.S.S. Dacotah, transferred to the U.S.S. Monitor
  • Under the Command of:
    • Lieutenant John Lorimer Worden, in command of U.S.S. Monitor, Feb. 25, 1862 - Early Sept. 1862
      • Lieutenant Samuel Greene, Executive Officer, supervised loading and firing of one Dahlgren
      • Acting Master, Louis N. Stodder, supervised loading and firing of one Dahlgren
    • Commander John P. Bankhead, in command of U.S.S. Monitor, Early Sept. 1862 - Dec. 30, 1862
  • Purpose: All Purpose Naval Armament on Turret Ironclad
  • Gun Placement:
    • Gun 27: U.S.S. Monitor Turret, Port Side
    • Gun 28: U.S.S. Monitor Turret, Starboard Side
  • Used in Battle: March 9, 1862, Battle of Hampton Roads, Virginia, against ironclad C.S.S. Virginia
  • Invented By: John A. Dahlgren, USN
  • Lost at Sea: On-board the sinking U.S.S. Monitor, Atlantic Ocean, southeast off Cape Hatteras, on December 31, 1862
MANUFACTURING
  • US Casting Foundry: West Point Foundry, Cold Springs, New York
  • Year of Manufacture: 1859
  • Tube Composition: Cast Iron
  • Registry Numbers: 27 & 28
  • Trunnion Markings: Not Available
  • Foundry Numbers: Not Available
  • Inspectors Mark: Not Available
  • Additional Engraving: added during a maintenance period in October of 1862...
    • Gun 27: "WORDEN. MONITOR & MERRIMAC."
    • Gun 28: "ERICSSON. MONITOR & MERRIMAC."
  • Purchase Price in 1859: $1,391.00 ea. (US)
WEIGHTS & MEASURES
  • Bore Diameter: 11 inches
  • Bore Length: 131.2 inches
  • Tube Length: 161 inches
  • Tube Weights:
    • Gun 27: 15,720 lbs.
    • Gun 28: 15,617 lbs.
  • Carriage Type: Turret Carriages
  • No. of Crew to Serve: 7 men per gun
PERFORMANCE
  • Rate of Fire: One round, every 7 to 8 minutes each
  • Rifling Type: None, Smoothbores
  • Standard Powder Charge: Up to 15 lbs. Cannon Grade Black Powder
    • Later, charges safely increased to 30 lbs., too late for Hampton Roads
  • Muzzle Velocity: 1,120 ft/sec.
  • Effective Range (at 5°): 1,712 yards (0.97 miles)
  • Projectile Flight Time (at 5°): 5.81 seconds
  • Maximum Range (at 15°): 3,650 yards (2.07 miles)
  • Projectiles: Round Balls, 166 lb. Solid Shot or 133.5 lb. Shells
HISTORY OF THE MONITOR'S DAHLGRENS

John Ericsson had been assured that two XI-inch Dahlgren shell guns would be provided for the new Monitor project. When it was discovered that the intended guns had not shipped, and were not available, a search for available guns was made. The U.S.S. Dacotah which just happened to be docked nearby, had two slide-mounted pivot guns installed, these just happened to be lightly used XI-inch Dahlgren Shell Guns, Registry numbers 27 & 28. It was just what they needed.​
The Dahlgren guns were removed from Dacotah, and mounted aboard the Monitor, inside the new armored rotating turret.​
Back in 1860, before the Monitor was designed, during a test firing, a Dahlgren shell gun exploded. To prevent any catastrophic gun bursting within the confined turret on the Monitor, each of the XI-inch Dahlgren guns was restricted to using 15-lb gunpowder charges by the always cautious Commander John Dahlgren.​
When the Monitor entered it's first Battle at Hampton Roads, it fired it's Dahlgrens in anger against the C.S.S. Virginia, formerly the Merrimack. Forty-one shots were fired by the Monitor in that engagement, but with the restricted gunpowder charge of 15 lbs., even though the 165 lb. solid shot easily dented and scuffed the armor plate on the Virginia, it didn't do any serious damage to the iron-clad vessel.​
Tests conducted after the battle confirmed that using 30 lbs. of black powder in the 11-inch Dahlgren would have easily punctured the Virginia’s hull.​
After the Battle of Hampton Roads, the Monitor attempted to engage the Virginia when it came out on May 8th, firing a few shots at distance, but the Virginia didn't take the bait. The Confederates abandonded the City of Richmond a few days later, burning the Virginia in their wake.​
Free from patrolling the Virginia, the Monitor moved on to participate in the Battle of Drewry's Bluff, firing at a few targets with the Dahlgrens and scoring hits, but finding it difficult to elevate their guns effectively at short range.​
When the U.S.S. Monitor was ordered to move down to North Carolina in late December, it took a voyage that it wouldn't sail home from. In the evening of December 30th, a storm hit off the coast of Cape Hatteras, and waves caused the ship to take on water and begin sinking. Later that night the doomed ship took 16 men with it to the sea floor, and the two XI-inch Dahlgren Shell Guns.​
ARTIFACT RECOVERY
  • Wreck of USS Monitor Discovered: August, 27, 1973
  • Location of Wreck: 35°0′6″N 75°24′23″W, designated as Monitor National Marine Sanctuary
    • Atlantic Ocean, about 16 mile SSE of Cape Hatteras Lighthouse, North Carolina, about 230' below the surface.
  • Turret / Dahlgrens Recovery Date: August 5, 2002
  • Dahlgrens Current Disposition: Undergoing Conservation at the Mariners' Museum in Newport News, Virginia
After the turret was raised in 2002, conservators began the long process of excavating the fragile cannons from the turret and stabilizing them. The cannons were removed from the turret in 2004 and placed in conservation tanks. The guns underwent an extended soaking process to remove chlorides from the iron. This process took approximately five years. Additional work to remove concretions outside and inside the guns has been completed. Both guns are currently undergoing electrolytic reduction and desalination in the Batten Conservation Laboratory Complex.​

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Photos, L to R: USS Monitor Turret Recovery 2002, Monitor's Dahlgrens going into Conservation Tanks,
and Excavating the Bore of one of the Monitor's Dahlgrens. Photos from NOAA.gov
OFFICIAL REPORT
Navy Official Reports, North Atlantic Blockading Squadron
Report of Lieutenant Jeffers, U. S. Navy
Regarding ammunition expended by the U. S. S. Monitor

U. S. CASED BATTERY MONITOR,
Hampton Roads, March 16, 1862.

SIR: In answer to your enquiry I have to report that the Monitor expended forty-one solid cast-iron shot in her engagement with the Merrimack, equally divided between guns 27 and 28.

On inspection of the bore with a mirror no trace of injury can be observed. I have no means of examining the vent by taking an impression.

Unless absolutely necessary I shall fire no more cast-iron solid shot, as I am satisfied that shells are not more liable to fracture. The bronze coated shot I shall reserve for especial occasion. The wrought-iron shot I shall send on shore to remove the temptation to fire them. I am satisfied that the Merrimack can not seriously injure the Monitor, but an explosion of a gun might destroy the turret.

I have the honor to be, very respectfully, your obedient servant,

WM. N. JEFFERS,
Lieutenant, Commanding.​
Flag-Officer L. M. GOLDSBOROUGH,
Commanding North Atlantic Blockading Squadron.
NAVY OR, Series I--Volume 7, From March 8 To September 4, 1862. pp. 1-81

FOR FURTHER READING
  • The Story of the Monitor: The First Naval Conflict Between Ironclad Vessels - Archive.ORG (Free)
    by William S. Wells, Issued by the Cornelius S. Bushnell National Memorial Association, New Haven, CT; 1899.
  • Shells, and Shell-guns by John Dahlgren, King & Baird, Philadelphia, 1856. - Google (Free)
  • The Big Guns: Civil War Siege, Seacoast and Naval Cannon
    Olmstead, Edwin, Wayne E. Stark, and Spencer C. Tucker, Alexandria Bay, NY: Museum Restoration Service, 1997.
ASSOCIATED LINKS
 
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Back in 1860, before the Monitor was designed, during a test firing, a Dahlgren shell gun exploded. To prevent any catastrophic gun bursting within the confined turret on the Monitor, each of the XI-inch Dahlgren guns was restricted to using 15-lb gunpowder charges by the always cautious Commander John Dahlgren.
I'm not sure you can draw a causal link here. The 15 lb charge was the full normal charge of the Dahlgren gun in contemporary tables - they don't even mention a 20 lb charge - and it would seem odd to me for Dahlgren to make a positive announcement to the effect that the permitted charge is exactly the same as before.
 
I'm not sure you can draw a causal link here. The 15 lb charge was the full normal charge of the Dahlgren gun in contemporary tables - they don't even mention a 20 lb charge - and it would seem odd to me for Dahlgren to make a positive announcement to the effect that the permitted charge is exactly the same as before.
Portions of the following have been highlighted with BOLD TEXT by me for emphasis....

THE ORDNANCE BUREAU.
Report of Capt. Dahlgren
The New York Times, December 4, 1862​
The report from the Ordnance Department, by Capt. J.A. DAHLGREN, abounds in interesting and important matter. He first urges that suitable provision of ordinance material be made for the probable future necessities of the navy, and after suggesting various other steps which should at once be taken to increase the efficiency of his department, turns to the discussion of "Iron-clads and Ordnance." He sketches the progress made by our own, as well as by the French and English Governments in the construction and the improvement of ordnance, discusses the difficulties in constructing a ship impregnable to the heaviest shot, and in working a gun of the largest calibre. In the experiments which have been made in England, he acknowledges that the advantages remain with the guns, and there, he thinks, they will continue for the present. He adds:​
It would be unwise, however, to rush to the conclusion that armor is needless because the most powerful ordnance should, under skillful guidance, be able to pierce it. For even against such cannon a ship may delay the conclusive difficulty long enough to make its own guns of avail; and, when opposed to any but these heaviest pieces, will still be in effect impregnable.​
The case of the Monitor and Merrimac affords an illustration. No one supposes that either or these vessels could have escaped serious injury if subjected to a course of target firing from the most recent and powerful descriptions of ordnance; yet they sustained for four hours the utmost effort of each other's batteries. The Monitor was hardly more than scarred by the fire of the very guns which, on the preceding day, had, in a fourth of the time, acted most destructively on the hulls and crews of two fine wooden frigates. A very high official authority (Duke of SOMERSET, First Lord of the Admiralty, House of Lords, April 3, 1862,) has, to be sure, imputed the default of injury to life or limb in this combat to a lack of powder in the artillery which the two vessels carried -- which is no doubt true -- but it is equally true that no guns of light weight and kind now used in the British navy would have effected as much under like circumstances. These cannon are not "idle against iron-plates," as may be shown by their operations upon samples of Warrior targets on which they have been tried here. Nor are they rifled shell ordnance. Nor is their initial velocity as low as 900 feet, as stated by the Duke.​
The 11-inch guns of the Monitor were designed chiefly for shells, which were computed to leave with a velocity of at least 1,250 to 1,300 feet. In the action, shot were used with an initial velocity of 1,120 feet. But since that, the same class of gun has been ascertained to be capable of throwing solid shot of 169 pounds, with a charge of 30 pounds, giving an initial velocity of 1,400 feet. The 9-inch guns of the Merrimac were intended paincipally for a shell of 72 pounds and a charge of 10 pounds, and therefore giving an initial velocity of about 1,400 feet. But they can be safely used with a solid shot, and 13 1/2 pounds of powder, which produces an initial velocity of 1,290 feet. One of these guns was fired 750 times with its shell, and then five hundred times with shot and the high charge, showing no symptom of giving way after such severe usage. The Duke of Somerset more nearly approached the present state of the question, when he doubted the capacity of plates finally to resist the action of ordnance, but was in fault in overestimating the service to be expected of the Armstrong gun.​
With regard to the results spoken of as obtained in England which assign a superiority to the solid plates, it may or may not be that Capt. ERICSSON selected, or was compelled to use one-inch plates in making up his total thickness -- for at that time there was very little choice in the matter; but it is certain that the practice made here proves that several plates made into one are preferable on many accounts to one solid plate, and would be so altogether if it were not for the increased number of bolts that become requisite, and are the weakness of all such plating. Indeed, even with this disadvantage, it remains to be seen whether, by any process, a very thick solid plate can be made equal in its texture to the thinner plates. For in every instance where I have seen a solid plate pierced by a shot, the imperfection at the welds has been made manifest by their separation, although externally none such could be perceived. In this connection it may also be observed, that for want of information it is not possible for us to form a trustworthy opinion, from the incidents of the content, of the relative powers, offensive and defensive, of the two vessels. We know the armament of the Monitor, and also that she was not in any way crippled by the fire of the Merrimac; the most telling blow was on the pilot-house, but this did not affect the immediate efficiency of the vessel.​
On the other hand, we do not know to what extent the Merrimac was injured by the fire of the Monitor, nor, with any certainly, what guns she carried. Among those recovered with the Norfolk Navy-yard was a 9-inch, which, by the mutilation of sight masses, trunnions, &c., must have been one of those which our men attempted to destroy on evacuating the yard in April, 1861. Its muzzle being broken off, induces the belief that it was also one of the guns of the Merrimac, which, in the official report of her proceedings on the 8th of March, is stated to have had the chase shot off. The end of an elongated shell was also found in the plates of the Monitor, showing that such projectiles were fired by the Merrimac. These facts seem to confirm a current rumor that the vessel had 9-inch guns on the sides, and rifled guns at each end -- probably 32-pounders -- as we find pieces of this, description used very commonly by the Confederates, or else one of the Blakely pattern, a few of which are also known to have been furnished to them. If this be true, then the metal was entirely, too light to affect the Monitor, and will account, in a measure, for the little damage she sustained.​
As so much interest was, at the time, attracted to the material of the shot used by the Monitor, and it was averred by high authority that if she had not been restricted to those of cast iron, but had been allowed to fire the wrought-iron shot put on board for the purpose, the Merrimac would have been sunk, I am induced to say a few words in order to correct any erroneous impression that may still exist on the subject. The cast-iron shot fired by the Monitor average about 169 pounds, and, being used with 15 pounds of powder, had an initial velocity of about 1,120 feet. After the action I caused ten of the Monitor's wrought-iron shot to be weighed; they were found to be 186 to 187 pounds; therefore, (with a charge of 15 pounds,) their initial velocity would have been about 1,050 feet. So that, by using wrought-iron shot, the projectile velocity would have been reduced; and though in no great degree, yet to that extent, whatever it was, would have increased the very cause of insufficient power -- if, as generally conceded, the offset of shock be proportional to the V2xW. Secondly, if the criticism were intended rather for the inferior tenacity of the cast iron, which was therefore assumed to lessen the effect of the cast-iron shot when compared with those of wrought iron, then it is only to be said that the theory is directly at variance with the facts as exhibited in the practice executed under my immediate direction, and witnessed by yourself and many others. The cast-iron shot does break, and the wrought-iron is only crushed; but while the latter lodges in the 4 1/2-inch plate, the former (both being of 11 inches) passes completely through the plate, and nearly through the wooden backing of 20 inches, making a larger hole and badly cracking the plate.​
The proper mode of increasing the power of the Monitor's guns was to have increased the charge, which the gun was capable of enduring safely, to the extent of 30 pounds; but this was not known at the time, and all will admit that the occasion was not one when any risk was to be incurred unnecessarily. The Monitor only fired 42 shot, and of these, many no doubt failed to strike, from the unavoidable difficulty of aiming with perfect precision.​
Setting aside the disadvantage due to a first essay in a combat of this kind, the circumstances were entirely favorable to exhibiting the full powers of iron-clad chips like the Monitor and Merrimac -- the distances being short and the object but little above or below the level. When much elevation is required, particularly on account of the height of the object, the fire of any ship-of-war loses proportionally in effect, by reason of the difficulty in aiming; and the vessel is also more exposed to damage; in the close ports of an iron-clad the former must be seriously aggravated, of which we had an example in the attack upon the work on the James River, when the Galena and Monitor participated, and could never make their guns tell fully as they would have done had the batteries been lower.​
The operations that have been conducted here, with reference to the power of different cannon and projectiles, as well as the resistance of iron plating, have been so far satisfactory that the results derived have been consistent. Still they are liable to such qualification as may be properly due to practice upon targets only, and in some sense favor the defence, because many sources of weakness which are unavoidable in the extensive structure of a ship are undisclosed in the strong, new and well-knit targets, but will appear when vessels are subjected to fire, and to the wear and tear of time and service, especially at sea. So long as the ponderous armor is merely attached to the ship, and is not made to contribute to the strength of the fabric, but severely taxes that strength, so long will there be involved a serious element of deterioration which will after a while impair the general capacity for endurance, and in the end unfit the ship for battle. In this respect, as in many others, the turret-class are to be excepted from much of the preceding remark; and are probably of greater and more certain endurance under severe fire than the ordinary plated vessel. So far, they are likely to find the most fitting sphere for their peculiar powers in the less troubled waters of harbors and rivers; though the ability that has devised them may also be able to give a wider scope to their usefulness."​
Capt. DAHLGREN then discusses the proper mode of arming iron-clads, comparing the respective advantages of rifled and smooth-bore ordnance. He says:​
"If, in battering an iron-clad, penetration only shall be the paramount consideration and other effects merely incidental, the rifle cannon must be selected. But if the concussion and shattering of the plate and its backing be preferred, with such penetration as might be consequent thereon, then the heavy, swift, round projectile will supply the blow heat adapted to such work.​
So long as the present mode of plating continues, there can be little doubt that it will be most effectively attacked by cracking and breaking the iron, starting the bolts, stripping off the armor, and breaking away large portions of the wooden structure within. And to this mode of action I feel more inclined, after witnessing its effects upon a number of targets plated with solid iron, or with thin plates bolted into one -- the direction of the fire being perpendicular or oblique.​
The iron-clad Galena suffered most in this way. The fire was so well directed (distance 600 yards) that most of the shot were clustered about the middle of the side exposed, as if the funnel had served for a mark. Several remained where they had lodged in the iron plating, which, by measurement, proved to be 64-pounders; others had only dinted the armor and fallen off, while a few had passed entirely through. Seen from the outside, the damage did not appear considerable, but within it was evidently irreparable, for the entire structure of the vessel was so shattered that there was little power of resistance left.​
The number of guns being very much reduced, of necessity, in iron-clads, particularly in the turrets, which will only accommodate a pair of them, it would naturally be supposed that the weight of broadside would also be limited. But the very large calibres that are likely to be adopted will so far compensate for the loss in number that the diminution or power will not be important, and in some instances there will be a gain in weight of projectile. Thus, the Merrimac originally threw 1,424 pounds from the broadside, and the recent changes have increased this to 2,440 pounds; whereas the Roanoke, which belonged to that class, and is now converted into a turret ship, will throw 2,700 pounds at an object abeam, when fully armed, and 900 pounds ahead or astern.​
There must be, however, a material reduction in the celerity of fire with guns and projectiles so large as the 11-inch, whatever may be the mechanical appliances which may be brought to assist. An 11-inch gun, with a well-disciplined crew, can be fired once a minute; but there must be much improvement in any mode now suggested, before a 15-inch gun can be fired once in thrice that time. As a certain capacity for repetition is essential to the general power of a batters, there is thus involved a disadvantage which can only be compensated to any extent by the great concentration of effect in the individual projectiles. For it may be conceived that the effects of shells of 330 pounds and shot of 450 pounds, will be damaging beyond any experience in former batteries.​
What may be the power of such ordnance against iron-cased ships -- comparative or absolute -- remains to be ascertained. This, as well as the piece itself, is yet but an experiment."​
Capt. DAHLGREN thinks it behooves us to look after our coast defences promptly. Regarding this port, he says:​
"My own views are, that a trustworthy and thorough defence can only be made by a harmonious and just combination of forts, iron-clads and rams, with such other minor auxiliaries as may be contributed to the general purpose; singly, neither would be effectual.​
1. The principal defences by land are to be at Sandy Hook, the Narrows, and upon all the little islands near the City. Whatever may be selected as the material of the interior structure, the exterior must be of iron. Lines of earthworks will also be useful in assisting and completing the position of the main work. But by no means should bare masonry be exposed to the action of rifle cannon.​
2. The sites of these works should be at a reasonable height above the water, because the fire of ships always loses much of its effect against elevated objects; and iron-clads have still greater difficulties to contend with from the very limited degree of elevation which their ports permit, and the obstacles that interpose in aiming deliberately. For these reasons the forts at the Narrows are illy calculated to hinder the passage of an iron-clad, or even to withstand its direct attack; less so, indeed, than the seemingly insignificant earthworks on the elevated plateau to the left.​
3. A sufficient number of iron-clads should be ready to assist the forts, and to fill in the gaps left between them.​
4. With these should act the most powerful rams that can be built. Their construction to be very powerful, their lines very fine, draft moderate, filled with steam power, and impervious to shot. This description of steamer ought never to act singly; for by a skillful and timely change in direction the vessel attacked might ellude the blow and leave no good opportunity to renew it for some time; a second or a third ram, cooperating with the first, would make it impossible to avoid a direct blow from one of them. It is obvious, also, that rams would be of little force in chase, unless their speed were very much greater than that of the other vessel.​
Obstructions of various kinds, may also be placed at different parts of the channel.​
It will be seen that I now have in view only the main passage to the city -- the Raritan Bay and the 'Kills' being easily defensible.​
The same remarks which apply to New-York are also applicable to many other seaports, such an Newport, Portland, &c. And on the Pacific side, most especially to San Francisco, which, though remote from the scene of the present struggle, is yet worth all attention and care in fortifying it against attack, for this superb harbor should not be exposed to the least mischance.​
The greatly-increased importance of Key West, and similar islands, demands improvements in their fortifications, with a sufficient number of iron-clads and rams."​
In conclusion, the positive necessity of at once instituting a school for instruction in naval gunnery, is insisted upon.​
 
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Portions of the following have been highlighted with BOLD TEXT by me for emphasis....

THE ORDNANCE BUREAU.
Report of Capt. Dahlgren

The New York Times, December 4, 1862​
The report from the Ordnance Department, by Capt. J.A. DAHLGREN, abounds in interesting and important matter. He first urges that suitable provision of ordinance material be made for the probable future necessities of the navy, and after suggesting various other steps which should at once be taken to increase the efficiency of his department, turns to the discussion of "Iron-clads and Ordnance." He sketches the progress made by our own, as well as by the French and English Governments in the construction and the improvement of ordnance, discusses the difficulties in constructing a ship impregnable to the heaviest shot, and in working a gun of the largest calibre. In the experiments which have been made in England, he acknowledges that the advantages remain with the guns, and there, he thinks, they will continue for the present. He adds:​
It would be unwise, however, to rush to the conclusion that armor is needless because the most powerful ordnance should, under skillful guidance, be able to pierce it. For even against such cannon a ship may delay the conclusive difficulty long enough to make its own guns of avail; and, when opposed to any but these heaviest pieces, will still be in effect impregnable.​
The case of the Monitor and Merrimac affords an illustration. No one supposes that either or these vessels could have escaped serious injury if subjected to a course of target firing from the most recent and powerful descriptions of ordnance; yet they sustained for four hours the utmost effort of each other's batteries. The Monitor was hardly more than scarred by the fire of the very guns which, on the preceding day, had, in a fourth of the time, acted most destructively on the hulls and crews of two fine wooden frigates. A very high official authority (Duke of SOMERSET, First Lord of the Admiralty, House of Lords, April 3, 1862,) has, to be sure, imputed the default of injury to life or limb in this combat to a lack of powder in the artillery which the two vessels carried -- which is no doubt true -- but it is equally true that no guns of light weight and kind now used in the British navy would have effected as much under like circumstances. These cannon are not "idle against iron-plates," as may be shown by their operations upon samples of Warrior targets on which they have been tried here. Nor are they rifled shell ordnance. Nor is their initial velocity as low as 900 feet, as stated by the Duke.​
The 11-inch guns of the Monitor were designed chiefly for shells, which were computed to leave with a velocity of at least 1,250 to 1,300 feet. In the action, shot were used with an initial velocity of 1,120 feet. But since that, the same class of gun has been ascertained to be capable of throwing solid shot of 169 pounds, with a charge of 30 pounds, giving an initial velocity of 1,400 feet. The 9-inch guns of the Merrimac were intended paincipally for a shell of 72 pounds and a charge of 10 pounds, and therefore giving an initial velocity of about 1,400 feet. But they can be safely used with a solid shot, and 13 1/2 pounds of powder, which produces an initial velocity of 1,290 feet. One of these guns was fired 750 times with its shell, and then five hundred times with shot and the high charge, showing no symptom of giving way after such severe usage. The Duke of Somerset more nearly approached the present state of the question, when he doubted the capacity of plates finally to resist the action of ordnance, but was in fault in overestimating the service to be expected of the Armstrong gun.​
With regard to the results spoken of as obtained in England which assign a superiority to the solid plates, it may or may not be that Capt. ERICSSON selected, or was compelled to use one-inch plates in making up his total thickness -- for at that time there was very little choice in the matter; but it is certain that the practice made here proves that several plates made into one are preferable on many accounts to one solid plate, and would be so altogether if it were not for the increased number of bolts that become requisite, and are the weakness of all such plating. Indeed, even with this disadvantage, it remains to be seen whether, by any process, a very thick solid plate can be made equal in its texture to the thinner plates. For in every instance where I have seen a solid plate pierced by a shot, the imperfection at the welds has been made manifest by their separation, although externally none such could be perceived. In this connection it may also be observed, that for want of information it is not possible for us to form a trustworthy opinion, from the incidents of the content, of the relative powers, offensive and defensive, of the two vessels. We know the armament of the Monitor, and also that she was not in any way crippled by the fire of the Merrimac; the most telling blow was on the pilot-house, but this did not affect the immediate efficiency of the vessel.​
On the other hand, we do not know to what extent the Merrimac was injured by the fire of the Monitor, nor, with any certainly, what guns she carried. Among those recovered with the Norfolk Navy-yard was a 9-inch, which, by the mutilation of sight masses, trunnions, &c., must have been one of those which our men attempted to destroy on evacuating the yard in April, 1861. Its muzzle being broken off, induces the belief that it was also one of the guns of the Merrimac, which, in the official report of her proceedings on the 8th of March, is stated to have had the chase shot off. The end of an elongated shell was also found in the plates of the Monitor, showing that such projectiles were fired by the Merrimac. These facts seem to confirm a current rumor that the vessel had 9-inch guns on the sides, and rifled guns at each end -- probably 32-pounders -- as we find pieces of this, description used very commonly by the Confederates, or else one of the Blakely pattern, a few of which are also known to have been furnished to them. If this be true, then the metal was entirely, too light to affect the Monitor, and will account, in a measure, for the little damage she sustained.​
As so much interest was, at the time, attracted to the material of the shot used by the Monitor, and it was averred by high authority that if she had not been restricted to those of cast iron, but had been allowed to fire the wrought-iron shot put on board for the purpose, the Merrimac would have been sunk, I am induced to say a few words in order to correct any erroneous impression that may still exist on the subject. The cast-iron shot fired by the Monitor average about 169 pounds, and, being used with 15 pounds of powder, had an initial velocity of about 1,120 feet. After the action I caused ten of the Monitor's wrought-iron shot to be weighed; they were found to be 186 to 187 pounds; therefore, (with a charge of 15 pounds,) their initial velocity would have been about 1,050 feet. So that, by using wrought-iron shot, the projectile velocity would have been reduced; and though in no great degree, yet to that extent, whatever it was, would have increased the very cause of insufficient power -- if, as generally conceded, the offset of shock be proportional to the V2xW. Secondly, if the criticism were intended rather for the inferior tenacity of the cast iron, which was therefore assumed to lessen the effect of the cast-iron shot when compared with those of wrought iron, then it is only to be said that the theory is directly at variance with the facts as exhibited in the practice executed under my immediate direction, and witnessed by yourself and many others. The cast-iron shot does break, and the wrought-iron is only crushed; but while the latter lodges in the 4 1/2-inch plate, the former (both being of 11 inches) passes completely through the plate, and nearly through the wooden backing of 20 inches, making a larger hole and badly cracking the plate.​
The proper mode of increasing the power of the Monitor's guns was to have increased the charge, which the gun was capable of enduring safely, to the extent of 30 pounds; but this was not known at the time, and all will admit that the occasion was not one when any risk was to be incurred unnecessarily. The Monitor only fired 42 shot, and of these, many no doubt failed to strike, from the unavoidable difficulty of aiming with perfect precision.​
Setting aside the disadvantage due to a first essay in a combat of this kind, the circumstances were entirely favorable to exhibiting the full powers of iron-clad chips like the Monitor and Merrimac -- the distances being short and the object but little above or below the level. When much elevation is required, particularly on account of the height of the object, the fire of any ship-of-war loses proportionally in effect, by reason of the difficulty in aiming; and the vessel is also more exposed to damage; in the close ports of an iron-clad the former must be seriously aggravated, of which we had an example in the attack upon the work on the James River, when the Galena and Monitor participated, and could never make their guns tell fully as they would have done had the batteries been lower.​
The operations that have been conducted here, with reference to the power of different cannon and projectiles, as well as the resistance of iron plating, have been so far satisfactory that the results derived have been consistent. Still they are liable to such qualification as may be properly due to practice upon targets only, and in some sense favor the defence, because many sources of weakness which are unavoidable in the extensive structure of a ship are undisclosed in the strong, new and well-knit targets, but will appear when vessels are subjected to fire, and to the wear and tear of time and service, especially at sea. So long as the ponderous armor is merely attached to the ship, and is not made to contribute to the strength of the fabric, but severely taxes that strength, so long will there be involved a serious element of deterioration which will after a while impair the general capacity for endurance, and in the end unfit the ship for battle. In this respect, as in many others, the turret-class are to be excepted from much of the preceding remark; and are probably of greater and more certain endurance under severe fire than the ordinary plated vessel. So far, they are likely to find the most fitting sphere for their peculiar powers in the less troubled waters of harbors and rivers; though the ability that has devised them may also be able to give a wider scope to their usefulness."​
Capt. DAHLGREN then discusses the proper mode of arming iron-clads, comparing the respective advantages of rifled and smooth-bore ordnance. He says:​
"If, in battering an iron-clad, penetration only shall be the paramount consideration and other effects merely incidental, the rifle cannon must be selected. But if the concussion and shattering of the plate and its backing be preferred, with such penetration as might be consequent thereon, then the heavy, swift, round projectile will supply the blow heat adapted to such work.​
So long as the present mode of plating continues, there can be little doubt that it will be most effectively attacked by cracking and breaking the iron, starting the bolts, stripping off the armor, and breaking away large portions of the wooden structure within. And to this mode of action I feel more inclined, after witnessing its effects upon a number of targets plated with solid iron, or with thin plates bolted into one -- the direction of the fire being perpendicular or oblique.​
The iron-clad Galena suffered most in this way. The fire was so well directed (distance 600 yards) that most of the shot were clustered about the middle of the side exposed, as if the funnel had served for a mark. Several remained where they had lodged in the iron plating, which, by measurement, proved to be 64-pounders; others had only dinted the armor and fallen off, while a few had passed entirely through. Seen from the outside, the damage did not appear considerable, but within it was evidently irreparable, for the entire structure of the vessel was so shattered that there was little power of resistance left.​
The number of guns being very much reduced, of necessity, in iron-clads, particularly in the turrets, which will only accommodate a pair of them, it would naturally be supposed that the weight of broadside would also be limited. But the very large calibres that are likely to be adopted will so far compensate for the loss in number that the diminution or power will not be important, and in some instances there will be a gain in weight of projectile. Thus, the Merrimac originally threw 1,424 pounds from the broadside, and the recent changes have increased this to 2,440 pounds; whereas the Roanoke, which belonged to that class, and is now converted into a turret ship, will throw 2,700 pounds at an object abeam, when fully armed, and 900 pounds ahead or astern.​
There must be, however, a material reduction in the celerity of fire with guns and projectiles so large as the 11-inch, whatever may be the mechanical appliances which may be brought to assist. An 11-inch gun, with a well-disciplined crew, can be fired once a minute; but there must be much improvement in any mode now suggested, before a 15-inch gun can be fired once in thrice that time. As a certain capacity for repetition is essential to the general power of a batters, there is thus involved a disadvantage which can only be compensated to any extent by the great concentration of effect in the individual projectiles. For it may be conceived that the effects of shells of 330 pounds and shot of 450 pounds, will be damaging beyond any experience in former batteries.​
What may be the power of such ordnance against iron-cased ships -- comparative or absolute -- remains to be ascertained. This, as well as the piece itself, is yet but an experiment."​
Capt. DAHLGREN thinks it behooves us to look after our coast defences promptly. Regarding this port, he says:​
"My own views are, that a trustworthy and thorough defence can only be made by a harmonious and just combination of forts, iron-clads and rams, with such other minor auxiliaries as may be contributed to the general purpose; singly, neither would be effectual.​
1. The principal defences by land are to be at Sandy Hook, the Narrows, and upon all the little islands near the City. Whatever may be selected as the material of the interior structure, the exterior must be of iron. Lines of earthworks will also be useful in assisting and completing the position of the main work. But by no means should bare masonry be exposed to the action of rifle cannon.​
2. The sites of these works should be at a reasonable height above the water, because the fire of ships always loses much of its effect against elevated objects; and iron-clads have still greater difficulties to contend with from the very limited degree of elevation which their ports permit, and the obstacles that interpose in aiming deliberately. For these reasons the forts at the Narrows are illy calculated to hinder the passage of an iron-clad, or even to withstand its direct attack; less so, indeed, than the seemingly insignificant earthworks on the elevated plateau to the left.​
3. A sufficient number of iron-clads should be ready to assist the forts, and to fill in the gaps left between them.​
4. With these should act the most powerful rams that can be built. Their construction to be very powerful, their lines very fine, draft moderate, filled with steam power, and impervious to shot. This description of steamer ought never to act singly; for by a skillful and timely change in direction the vessel attacked might ellude the blow and leave no good opportunity to renew it for some time; a second or a third ram, cooperating with the first, would make it impossible to avoid a direct blow from one of them. It is obvious, also, that rams would be of little force in chase, unless their speed were very much greater than that of the other vessel.​
Obstructions of various kinds, may also be placed at different parts of the channel.​
It will be seen that I now have in view only the main passage to the city -- the Raritan Bay and the 'Kills' being easily defensible.​
The same remarks which apply to New-York are also applicable to many other seaports, such an Newport, Portland, &c. And on the Pacific side, most especially to San Francisco, which, though remote from the scene of the present struggle, is yet worth all attention and care in fortifying it against attack, for this superb harbor should not be exposed to the least mischance.​
The greatly-increased importance of Key West, and similar islands, demands improvements in their fortifications, with a sufficient number of iron-clads and rams."​
In conclusion, the positive necessity of at once instituting a school for instruction in naval gunnery, is insisted upon.​
Dahlgren was always very concerned with the possibility of a gun bursting at the breech. He had the personal motivation that a 32-lber he was testing on his birthday in 1849 almost killed him (it did kill the gunner who fired the shot). He also had the very practical reason that bursting guns were actually a major cause of combat casualties, and that men were unlikely to want to serve guns they thought might explode and kill them.

Dahlgren had been prohibited by his superiors from testing armor penetration throughout the 1850s and into the Civil War. The Battle of Hampton Roads (March 8-9, 1862) was the crisis that broke the roadblock on that. Dahlgren immediately started testing the XI-inch with bigger charges in 1862, even before the armor-penetration came through at the end of March. By April, he was testing for armor penetration right after he got the approval (he had to move to a new testing range on the grounds of the Insane Asylum because of the number of tests he was conducting).

The report above was written after many months of intense testing. I think this report actually was sent to Congress by his bosses.
 
Portions of the following have been highlighted with BOLD TEXT by me for emphasis....
Having examined this report, and in particular the bolded section, I reiterate the post to which you were replying. The restriction to 15 lb charges does not appear to have been a restriction below any seriously contemplated battering charge in excess of 15 lbs; in order to demonstrate the contrary, it would be necessary to show in so many words that before the battle with Monitor (e.g. in 1860) a charge in excess of 15 lbs was contemplated and rejected on safety grounds.


Otherwise, it is simply the case that the Dahlgren 11 inch gun had a maximum authorized charge of 15 lbs, and always did, until Dahlgren's post-Hampton-Roads experiments confirmed that it was safe to fire at higher charges.

Of course nothing would prevent someone exceeding the charge in theory - nothing would prevent someone filling the gun to the muzzle with powder - but they would be going off doctrine. In the specific case of an active military engagement such as with the Virginia (where the Monitor is the only ship then present able to fight her) then risking a breech explosion that would knock out the turret crew is a catastrophically bad idea, which is also why the ungauged wrought iron balls were not used.
 
Having examined this report, and in particular the bolded section, I reiterate the post to which you were replying. The restriction to 15 lb charges does not appear to have been a restriction below any seriously contemplated battering charge in excess of 15 lbs; in order to demonstrate the contrary, it would be necessary to show in so many words that before the battle with Monitor (e.g. in 1860) a charge in excess of 15 lbs was contemplated and rejected on safety grounds.


Otherwise, it is simply the case that the Dahlgren 11 inch gun had a maximum authorized charge of 15 lbs, and always did, until Dahlgren's post-Hampton-Roads experiments confirmed that it was safe to fire at higher charges.

Of course nothing would prevent someone exceeding the charge in theory - nothing would prevent someone filling the gun to the muzzle with powder - but they would be going off doctrine. In the specific case of an active military engagement such as with the Virginia (where the Monitor is the only ship then present able to fight her) then risking a breech explosion that would knock out the turret crew is a catastrophically bad idea, which is also why the ungauged wrought iron balls were not used.

Dahlgren knew from the beginning that the gun could handle larger charges. The low powder charge made it easier to get the gun approved back in the 1850s (this is actually a rather routine issue in getting approvals on many different items in many different fields and lots of different times).

The 15-lb charge was not selected based on single-shot burst risk. The wear-and-tear of rounds fired affects the service life of a gun. The heavier the charges, the more long-term risk of gun failure, lowering the service life. The projectile fired also affects service life. The XI inch gun had a very long life and was eventually rated at 1000 discharges -- almost none of the guns manufactured ever burst.

OTOH, the XV-inch had a rating for 800 discharges -- only 1 of the 100 XV-inch in the Civil War made it past 800 and that one blew at 868. Others were withdrawn from service long before 800 rounds. "Of the one hundred 15-inch guns produced, fifty-five were never fired in service, two burst, and ten were deemed unfit for service after firing fewer than eight hundred rounds. Three 15-inch guns were condemned after firing fewer than one hundred rounds. "

When the crisis of the CSS Virginia gave him the opportunity, Dahlgren grabbed it to get testing ramped up. He started on heavier charge testing as soon as he had enough technical data about the Virginia to work on, then moved to testing armor penetration as soon as he got approval.
 
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Dahlgren knew from the beginning that the gun could handle larger charges.
Which rather contradicts:

Back in 1860, before the Monitor was designed, during a test firing, a Dahlgren shell gun exploded. To prevent any catastrophic gun bursting within the confined turret on the Monitor, each of the XI-inch Dahlgren guns was restricted to using 15-lb gunpowder charges by the always cautious Commander John Dahlgren.

The claim made in this specific paragraph is that Dahlgren ("always cautious") restricted the 11" gun to 15 pounds, and specifically links it to either or both of (preventing a gun bursting in the Monitor turret) and (the shell gun exploding during a test firing in 1860).

It's my position that the 15 pound charge was the full normal service charge for the gun and had been all along. There is a persistent myth going around (and has been for a century and a half or more) that the 15 lb charges used at Hampton Roads were "half charges" or otherwise reduced charges from normal service charges, but this is a myth.
 
Which rather contradicts:



The claim made in this specific paragraph is that Dahlgren ("always cautious") restricted the 11" gun to 15 pounds, and specifically links it to either or both of (preventing a gun bursting in the Monitor turret) and (the shell gun exploding during a test firing in 1860).

It's my position that the 15 pound charge was the full normal service charge for the gun and had been all along. There is a persistent myth going around (and has been for a century and a half or more) that the 15 lb charges used at Hampton Roads were "half charges" or otherwise reduced charges from normal service charges, but this is a myth.

Dahlgren always knew the gun could handle charges greater than 15 pounds. That is simply a number used to get the gun approved. It needs to be a charge that will allow the gun to carry out its' mission -- and it did. Using larger charges will cause greater stress on the gun. More stress reduces the service life.

In addition, of course, gun-crews are not particularly happy about working on guns that have a larger chance of blowing up and killing them. Having almost died in a gun explosion, Dahlgren understood their desire intensely.

Beyond that, Dahlgren's mission for the IX-inch and XI-inch was to produce a shell gun to use against wooden warships. This made accuracy important, not muzzle velocity. High muzzle velocity guns were less accurate at longer ranges.

Dahlgren had recommended armored warships as early as 1852. He had been requesting permission to test for armor-penetration for many years. He was probably as informed as anyone in the world about the details of gunnery, following the details of armored batteries and ships intensely. It was his superiors who repeatedly prevented him from testing guns to penetrate armor.
 
Beyond that, Dahlgren's mission for the IX-inch and XI-inch was to produce a shell gun to use against wooden warships. This made accuracy important, not muzzle velocity. High muzzle velocity guns were less accurate at longer ranges.
I'm sorry, but I want to focus in on this statement because I'm not sure I follow how it can be true. Given the same nature of gun (smoothbore vs. smoothbore), how is a high muzzle velocity gun less accurate at long range?

All else being equal we would expect there to be less time for effects like wind to influence the trajectory of a higher muzzle velocity projectile. The magnus effect is caused by the spin of the ball, but applies an acceleration (random for a smoothbore) and so would take effect over time, which a high muzzle velocity projectile would have less of.

I can see how a shell gun would be M.V. limited because of the wish to avoid shattering the projectile, but that's not what this paragraph mentions.
 
I'm sorry, but I want to focus in on this statement because I'm not sure I follow how it can be true. Given the same nature of gun (smoothbore vs. smoothbore), how is a high muzzle velocity gun less accurate at long range?

To start out, there are two classes: subsonic (under 1100 FPS) and supersonic (above 1100 FPS).

Supersonic projectiles suffer from an inherent instability when the speed drops back below the sound barrier. This would apply to both small arms and cannons.

IIRR, an XI-inch Dahlgren firing solid shot with a 15-pound charge would have a muzzle-velocity of about 900 FPS or so, well under 1100 FPS. With a larger charge, the velocity goes up. I think that at about 25-30 pounds it would be above the 1100 FPS velocity.

The closer to the sound barrier the velocity is, the sooner the drop below the speed of sound, the earlier the instability occurs. The earlier it occurs, the more it is likely to affect accuracy.

All else being equal we would expect there to be less time for effects like wind to influence the trajectory of a higher muzzle velocity projectile. The magnus effect is caused by the spin of the ball, but applies an acceleration (random for a smoothbore) and so would take effect over time, which a high muzzle velocity projectile would have less of.

It does not actually work with time. It is about speed and distance and mass-to-surface-area, not time. Think number of rotations and drag, not elapsed seconds.

Also, the effects are different for a round ball than an elongated bullet shape.

I can see how a shell gun would be M.V. limited because of the wish to avoid shattering the projectile, but that's not what this paragraph mentions.

No, that is not the shell-gun problem. What you optimally want with a shell gun is for the shell to actually hit the target and lodge in it before exploding. Too much velocity might actually mean you penetrated all the way through the ship and exploded somewhere on the other side, probably causing no damage with the explosion.
 
Supersonic projectiles suffer from an inherent instability when the speed drops back below the sound barrier. This would apply to both small arms and cannons.
Does that happen with a round projectile? You can obviously destabilize an elongated projectile, but I'm not sure how a round projectile can actually destabilize in direction given that, you know, it's round!




It does not actually work with time. It is about speed and distance and mass-to-surface-area, not time. Think number of rotations and drag, not elapsed seconds.

Also, the effects are different for a round ball than an elongated bullet shape.
I know it's different for a round ball, and of course it's random because of the bounce for a smoothbore, but since we're discussing different speeds for the same projectile the mass-to-surface-area doesn't matter. The equation I found for the magnus effect states that the force is:
1653352117129.png


Which is to say, the force is proportional to the cross product of the velocity vectors for the fastest side (relative to the fluid) and the slowest side. (Everything else is a constant here for the same projectile in the same air.)

This means that for a given rotational rate the velocity of the whole projectile does not affect the force (since the issue is the relative vectors between the fastest and slowest side, and consequently they don't change if you accelerate the whole system) and obviously if the travel time is smaller then the force has less time to produce acceleration (i.e. vector change) and for that vector to have an effect. In fact since the suvat equations (under constant force) state that s = ut + 1/2 at^2 (i.e. displacement is the square of time) and a projectile with double the velocity travels in half the time, the displacement from the magnus effect (at constant rotation rate) should go as the inverse square of the velocity; in other words, increasing velocity drastically reduces the amount of deflection from the magnus effect.

The only way in which greater velocity would cause the magnus effect to become worse, therefore, is if the muzzle velocity affected the rotational speed such that the force was affected to a power greater than two.
 
Does that happen with a round projectile? You can obviously destabilize an elongated projectile, but I'm not sure how a round projectile can actually destabilize in direction given that, you know, it's round!

Yes. You are simply thinking wrong about the problem: all projectiles will be destabilized as they drop through the sound barrier.

I know it's different for a round ball, and of course it's random because of the bounce for a smoothbore, but since we're discussing different speeds for the same projectile the mass-to-surface-area doesn't matter. The equation I found for the magnus effect states that the force is:
View attachment 440957
Incorrect. The mass-to-surface-area is very meaningful because it will substantially affect the deceleration of the projectile. A musket ball will act differently than a cannon ball.

Which is to say, the force is proportional to the cross product of the velocity vectors for the fastest side (relative to the fluid) and the slowest side. (Everything else is a constant here for the same projectile in the same air.)
You seem to be trying to assert you are right instead of trying to discover what the real actions of physics on the projectile are.

This means that for a given rotational rate the velocity of the whole projectile does not affect the force (since the issue is the relative vectors between the fastest and slowest side, and consequently they don't change if you accelerate the whole system) and obviously if the travel time is smaller then the force has less time to produce acceleration (i.e. vector change) and for that vector to have an effect. In fact since the suvat equations (under constant force) state that s = ut + 1/2 at^2 (i.e. displacement is the square of time) and a projectile with double the velocity travels in half the time, the displacement from the magnus effect (at constant rotation rate) should go as the inverse square of the velocity; in other words, increasing velocity drastically reduces the amount of deflection from the magnus effect.

The only way in which greater velocity would cause the magnus effect to become worse, therefore, is if the muzzle velocity affected the rotational speed such that the force was affected to a power greater than two.

You are trying to come at this from the wrong direction. Assume you might be wrong, step back, research and think through the problem.
 
The 11" Dahlgren was only fired in proof thus:

10x shell with 15 lb charges
1x shot with a 25 lb charge

It was only authorised to fire 15 lb charges in 1862. After experimentation, a distant charge of 20 lbs was authorised in 1863.
 
Incorrect. The mass-to-surface-area is very meaningful because it will substantially affect the deceleration of the projectile. A musket ball will act differently than a cannon ball.
But we're considering a Dahglren cannonball at 1000 fps versus the same one at 1200 fps. This isn't about how a cannonball behaves differently from a musket ball, it's about the idea that the same cannonball is less accurate at higher muzzle velocity which was your argument.

Yes. You are simply thinking wrong about the problem: all projectiles will be destabilized as they drop through the sound barrier.
But a round ball that is rotating gets destabilized; what is it doing now? At most it is rotating in a different direction, surely?

You are trying to come at this from the wrong direction. Assume you might be wrong, step back, research and think through the problem.
I looked up the equation for the force which the Magnus effect creates, and confirmed that it was a force generated by the rotation (and that it did not depend on the absolute speed of the projectile through the fluid); that is, that it was as I originally believed it (a constant force given a rotating projectile). How is this not researching?

What is the equation that is governing the effect you are describing?
 
For a round ball, the centre of gravity and the centre of aerodynamic pressure are identical. Ergo, it is physically impossible to destabilise a round ball in the transsonic region. It is this destabilisation that shooters of modern ogival bullets refer to.

However, drag is related in a rectilinear fashion to velocity in the region ca. mach 0.5 to mach 1.4 - i.e. the entire region we are concerned with. This is not true of ogival or other elongated bullets, which experience a huge spike in drag.

In fact, the faster the (round) ball, the less displaced it will be by the Magnus effect. Simply, with round balls, precision increases with increased velocity, diameter and density of the ball.
 
But we're considering a Dahglren cannonball at 1000 fps versus the same one at 1200 fps. This isn't about how a cannonball behaves differently from a musket ball, it's about the idea that the same cannonball is less accurate at higher muzzle velocity which was your argument.
Yes, I understand that a cannonball is not a musketball. However, the size and characteristics of the shot play a part in what happens during the ballistic flight. You keep trying to find a way to ignore real effects that you do not want to acknowledge.

In relation to the "cannonball at 1000 fps versus the same one at 1200 fps":
  • All ballistic projectile with a muzzle velocity above the speed of sound will be subject to a disruptive effect when it loses velocity and falls through the barrier.
  • Only ballistic projectiles with a muzzle velocity above the speed of sound will be subject to that effect.
  • No ballistic projectile that starts out below the speed of sound will be subject to that disruptive effect
Your 1200 FPS example will experience the impact as the velocity drops.
Your 1000 FPS example will not experience the impact as the velocity drops.

Dahlgren, of course, would have had little knowledge of the "sound barrier", a concept that only arose out of WWII (fast planes and rockets). Dahlgren merely felt that the US navy had over-emphasized muzzle velocity at the expense of accuracy. He would not have had any way to gather physical evidence about in-flight characteristics to prove his ideas.
 
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Yes, I understand that a cannonball is not a musketball. However, the size and characteristics of the shot play a part in what happens during the ballistic flight. You keep trying to find a way to ignore real effects that you do not want to acknowledge.
That is not what's going on. I am trying to derive what the effects in question are from known physical laws (e.g. the actual equation for the magnus effect on a spherical projectile) and am not finding something which is controlled by velocity as you describe.

In relation to the "cannonball at 1000 fps versus the same one at 1200 fps":
  • All ballistic projectile with a muzzle velocity above the speed of sound will be subject to a disruptive effect when it loses velocity and falls through the barrier.
  • Only ballistic projectiles with a muzzle velocity above the speed of sound will be subject to that effect.
  • No ballistic projectile that starts out below the sound barrier will be subject to that disruptive effect
Your 1200 FPS example will experience the impact as the velocity drops.
Your 1000 FPS example will not experience the impact as the velocity drops.
But what disruptive effect is this? An elongated projectile going transonic suffers from a yawing effect in an unpredictable direction and can end up pointing sideways, but this is minimized with a large, blunt projectile which is therefore not very elongated. A projectile that is not elongated at all wouldn't suffer this effect, and of course if it ended up sideways there's zero difference.
 
But we're considering a Dahglren cannonball at 1000 fps versus the same one at 1200 fps. This isn't about how a cannonball behaves differently from a musket ball, it's about the idea that the same cannonball is less accurate at higher muzzle velocity which was your argument.


But a round ball that is rotating gets destabilized; what is it doing now? At most it is rotating in a different direction, surely?


I looked up the equation for the force which the Magnus effect creates, and confirmed that it was a force generated by the rotation (and that it did not depend on the absolute speed of the projectile through the fluid); that is, that it was as I originally believed it (a constant force given a rotating projectile). How is this not researching?

What is the equation that is governing the effect you are describing?

There are four types of ballistics involved here: internal, transitional, external and terminal. Internal, Transitional and exterior ballistics affect accuracy

Internal ballistics occur inside the barrel. This is what leads to and creates the muzzle velocity.

Transitional ballistics (or intermediate ballistics) occur as the projectile leaves the barrel
When the projectile leaves the barrel, it breaks the seal that kept the pressurized gases in the barrel behind the projectile. The gasses then rapidly escape in any direction they can, creating what can be described as turbulence. The variables involved are not well defined and the results are inexact. There is a shockwave. There can also be a sonic boom effect at this time if the projectile is supersonic as well as a combustion flash. Transitional ballistics end when the pressure inside the barrel normalizes to local air pressure. The gasses may be moving faster than the projectile and thus pass the projectile before slowing down. This may be true even with a subsonic muzzle velocity. This is a very brief time increment. This is the area in which devices like flash and sound suppressors act.​

External ballistics start after transitional ballistics and end with the projectile strike.
The two big variables here are muzzle velocity and the ballistic coefficient. You can use those to determine a maximum range if you know the angle of elevation and thus the time of flight. With those and the gravity drop, you can calculate the trajectory.​
A high muzzle velocity is thus desirable because it will reduce the time-to-target and flattens the trajectory. There are implied disadvantages as well (usually materials and practical in nature) that must be traded off in terms of advantages and disadvantages in military practice. To take advantage of a high muzzle-velocity, what you need is a high ballistic coefficient.​
The ballistic coefficient has two components: the sectional density (SD) and the form factor (FF).​
SD is simply a ratio: divide the projectile weight in pounds by the square of the caliber in inches (if projectile weight is in grains, divide the result by 7,000). If everything else is the same, the bigger the caliber, the longer the range and the shorter the flight time to that range. However, everything else may not be the same.​
FF is a measurement of aerodynamic efficiency. It is more difficult to calculate. One of the reasons is that the same FF can perform differently above and below the speed of sound. The shapes that perform best at subsonic speeds are not the shapes that perform best at supersonic speeds.​
Round balls have poor ballistic coefficient values.​

Terminal ballistics occur after the projectile strike and so are not involved with accuracy.
 
Round balls have poor ballistic coefficient values.​

Yes they do, but that's a non-sequitur.

Round balls do not suffer instability and tumbling in the transonic region, because they are round, and hence are present the same shape to the aerodynamic flow in all orientations.

The transonic region is much wider than you credit (ca. Mach 0.8 to Mach 1.2 for modern projectiles), and the instability is caused by part of the airflow around the projectile being supersonic, and part being subsonic. This generates an unbalanced force which causes the projectile to yaw, which becomes a positive feedback loop because the yawing increases the unbalanced force for a modern ogival projectile. The source of this unbalance force is, of course, that the bullet is not perfectly aligned to the aerodynamic flow.

With round balls, there are no unbalanced forces and can't be, because a sphere is uniform. The wave structure around a ball is essentially uniform, in the super- and transonic regions. It only starts to become uneven well into the subsonic region, when hairpin vortices start forming and induce the Magnus effect.

With round balls, the projectile is completely stable until velocity drops to around Mach 0.8 to 0.9 (b in the figure below), when hairpin vortices start forming which cause the yawing known as the Magnus effect. At Mach 0.95 (c) the wave structure is completely uniform, and there are no unbalanced forces on the ball.

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There are four types of ballistics involved here: internal, transitional, external and terminal. Internal, Transitional and exterior ballistics affect accuracy
Forgive me, but you don't seem to have provided the specifics of the effect which produces the reduced accuracy you describe at higher velocity.


Specifically, a ballistic coefficient (which you do mention) influences drag, but a poor ballistic coefficient simply means that a projectile sheds velocity more quickly - that is, the drag is higher - and so all that means is that a round 1,200 fps projectile decelerates into being a 1,000 fps projectile more quickly than an elongated one would.

It's still the case that a projectile with a 1,200 fps starting velocity has no major difference in transverse forces (forces that would knock it off target) and reaches the target sooner so those transverse forces have less time to act on it; thus, it is more accurate.
 
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