"Hull Down"

Dec 13, 2011

This isn't specifically a Civil War naval topic, but I'll ask Mark's forbearance in allowing it, as it may be of interest to some.

If you read much naval history or fiction, especially that takes place in the age of sail, you'll come across the term that another vessel is "hull down." This simply means that the vessel being described is far enough away that the curvature of the earth makes it impossible to see the entire ship above the horizon. If a ship is "hull down," only its masts and maybe a funnel is visible above the horizon; the rest is hidden below (i.e., beyond) the horizon.

The distance to the horizon depends on the height of the observer, but it's surprisingly short. If you're standing right at the water's edge, with your eye at 5 feet (1.5m) above sea level, the horizon is only about 3 statue miles away, or 2.6 nautical miles (4.8km). At a height of 20 feet (6.1m), about the elevation from which the photos below were taken, the horizon is only about 5.2nm (9.6km) away. At that distance, you could (theoretically) see right to the edge of the other vessel where the hull meets the water. At longer distances, less and less of the hull of the other vessel is visible, until the hull cannot be seen at all. The other vessel is then said to be "hull down."


This is all simple enough theoretically, but it may help to see it in reality. Saturday the weather in Galveston was just perfect, with unusually clear air that allowed a good view from the beach of the shipping anchored offshore in the Gulf of Mexico, waiting for a berth at one or another of the ports that ring Galveston Bay -- Galveston, Texas City, Houston and Baytown. So this afternoon I went up to the the Seawall at 37th Street, and shot some photos of ships offshore. I then looked them up on VesselFinder, overlaid that plot into Google Earth, and -- well, anyway, was able to identify most of them and plot out accurately how far away each is.


So here, then, are the ships that could be seen, shot from an elevation of about 20 feet (6.1m) above sea level, and their calculated distances in nautical miles. I haven't changed the images except to adjust the brightness and contrast levels to reduce the effects of a small amount of residual haze in the atmosphere. Note how the farther ships are indeed partially or fully "hull down" from the point of view of the observer on shore:




Hope this makes sense.
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Jan 4, 2017
I was a lookout for special details early in my navy career and taught lookouts on the USS Iowa. I was questioning the ranges of the ships at longer ranges from the shoreline then I reread the article and noticed he was 20ft above the shoreline. Hull down is still a term used. I remember one signalman that was very good at determining distances using his Mark 1 eyeball.

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