I'll make a few points:

1. Height of gun sills and throw weight

The Merrimack had the gun sills a little more than 3 ft 5 inches above the waterline (this is the height of the gun deck) and the Duncan 3 ft with 101 guns (she never carried 101 guns in active service, but instead 89 having the spar deck guns replaced with rifles on pivots*). Less than 5 inches makes little difference, in any sea state where the Duncan can't open her lower deck the Merrimack can't either.

The Merrimack had 24x 9" on the gun deck (12 per side), and 14x 8" (7 per side) and 2x 10" (chasers) on the spar deck. In a calm sea she has a broadside of 12x 9", 7x 8" and 2x 10", and only 7x 8" and 2x 10" in reasonable seas.

The Duncan has 36x 8" on the bottom gun deck, 36x 32 pdrs on the next deck and 1x 12 pdr, 12x 40 pdr, 2x 20 pdr, 1x 110 pdr Armstrongs on the spar deck. In a calm sea she throws 18x 8", 32x 32 pdrs and a 68 pdr, and in a sea state that closes the lower gun deck 32x 32 pdrs and the Armstrongs can still engage.

* Her spar deck in 1862 had the following Armstrongs: 1x 12 pdr, 2x 20 pdr, 1x 110 pdr, 12x 40 pdr, all capable of firing both sides.

The throw weight is thus:

All decks

Merrimack: 1,424 lbs

Duncan: 1,970 lbs

Sea state

Merrimack: 551 lbs

Duncan: 1,154 lbs

2. "Armour"

The Duncan was 36" oak walls, the Merrimack 24" of oak and pine (which is much weaker).

The Duncan is "immune" even to the 10", which

only penetrated 32" of oak at 500 yds at the normal. The weaker armour of the Merrimack wouldn't stop the long 32's at 1,000 yds and the heavy 8" will also penetrate at 1,000 yds. The rifles of course are effective at several thousand yards.

Thus in a calm sea at 1,000 yds, the effective throw weight is:

Merrimack- throw weight of effective rounds against Duncan: 0 lbs

Duncan - visa versa: 1,970 lbs

3. Speed

Merrimack under steam typically made 6 kts, and her greatest every speed under steam was 9.5 kts (unloaded with a following wind and with tide. Duncan made 13.3 kts on standard trials at Stokes Bay with 101 guns (average of 6 runs, both directions to cancel out wind and tide effects). In a sea state the Merrimack looses speed quicker than Duncan.