Cape Dory 25

[Tom Keevil]

Objects in water are buoyed up by a force equal to the weight of the water they displace. This is known as Archimedes' Principle, and basically is a result of the fact that there is always more water pressure on the bottom of the object (since it's deeper) than on the top.

One cubic foot of water weights 64 lb. If you submerged an object that weighs 128 lb (the downward force of gravity) and occupies one cubic foot, it would be buoyed up by 64 lb (128-64=64), and the net result would be a downward force of 64 lb - it would sink, but it would be easier to pull it up than if it were out of the water. If the 128 lb object occupied 2 cubic feet, it would displace 128 lb of water, and wouldn't go up or down (128-128=0). If the 128 lb object occupied 3 cubic feet of space, it would displace 192 lb of water. Now the net force would be 64 lb up (128-192=-64), and it would rise, break the surface, and keep going up until one cubic foot was out of the water. The 2 cubic feet still submerged would displace 128 lb - the weight of the object, and the net force would be zero. Now you have a boat! Your next job is to come up with a shape that works for you, but it will still float if it is the same volume

As for the difference between "stated displacement" and travel lift weight, either the stated displacement is wrong, or it assumes a completely empty boat, or your boat is filled with stuff, or the travel lift scale is wrong. Note that at 64 lb per cubic foot, 100 gal of water weights 900 lb, so full water tanks, full fuel tanks and full holding tank are a pretty good start to a heavy boat. Then there are the extra batteries, the tools, the cans of beer....

[mgphl52]

... the cans of beer....

Well I reckon that explains it all! We did have an 18-pack on board