How dense are boats?

[Anaraxes]

Originally Posted by Anthony

For cargo ships, that is. For warships, it isn't.

True. Warships also tend to attract their own odd rules for measurement -- 2/3 loaded, or politically-inspired measures like those in the Washington Treaty before WW II.

[johndallman]

The warship rules are based on displacement, which means actual weight, because that was a better measure of fighting power than volume. Still not a perfect measure, but better. The politics in that largely stemmed from different nations having very different requirements for range, and thus fuel capacity.

[Rupert]

Originally Posted by Anthony

For cargo ships, that is. For warships, it isn't (and this means a 10,000 ton warship is in fact a different size from a 10,000 ton cargo ship...).

Warship tonnages are just about always displacement tons, but even then you have to check to find out what kind of load is being referred to. To make matters more confusing, aside from the 'standard displacement' defined in the Washington Naval Treaty, different navies used different definitions for 'normal', 'full load', 'deep', etc. To make matters even worse, they also used to alter those definitions for some classes of ship, or 'cheat' by removing stores, etc. to make weight, and so on. Full load displacement is generally the best choice for comparisons, because 1) it varies least between navies, assuming they aren't just lying, 2) it best reflects the actual size and mass of the ship, and 3) it also best reflects the likely state of a ship in wartime service.

As for density, aside from oddities like the Monitor the most dense surface ships were the late 19th century battleships, with their minimal superstructures and low freeboards. They had densities higher than 0.5 (i.e. over half their volume was below their waterline).

Next would be 20th century dreadnoughts, which by WWII tended to about 0.5. Aircraft carriers were (and are) much less dense. Compare the carrier,USS Hancock, with the battleship, USS New Jersey in these two photos:

http://www.bb62museum.org/images/g291047.jpg
https://s-media-cache-ak0.pinimg.com...8446a7906b.jpg

Hancock was an Essex-class carrier, of about 35,000 tons, while New Jersey was an Iowa-class battleship of about 55,000 tons. Their length and beam were similar, but the battleship obviously had a deeper draught. But the striking thing is that the carrier clearly would tower over the battleship were they side by side.

As an aside, the battleship developed 40% more power to drive almost 60% more mass the same speed - larger ships tend to require less power per ton for the same speed, especially if they grow deeper rather than simply bigger all over.

Originally Posted by johndallman

The warship rules are based on displacement, which means actual weight, because that was a better measure of fighting power than volume. Still not a perfect measure, but better. The politics in that largely stemmed from different nations having very different requirements for range, and thus fuel capacity.

Displacement is also a better measure of likely cost than volumetric tonnage (which as noted has little to do with actual volume, and hasn't for some considerable time).

[davester65]

Try reading this: http://en.wikipedia.org/wiki/Builder...ld_Measurement

I happened to be curious about how they rated old sailing ships recently. That is the formula the Royal Navy used to use to estimate the carrying capacity of wooden sailing vessels. It normally seems to work out that tonnage is about 2/3 displacement.