Boat displacement in weight vs. cubic feet.?

Question

I recently found out that water is about 64 lbs per cubic foot. so I got very confused when looking at a boats gross weight vs. the displacement weight. Should they be the same? the more weight you add, the larger the displacement area becomes right? Am I correct in thinking this or is there some formula I'm missing here.

Answer 1

You're dead right, maybe the difference in quoted weights is for a boat less fuel and oil and all accessories, ropes, anchors, oars, life jackets etc., versus its ready- to- use weight.

Answer 2

The two are the same.  the Greek mathematician Archemedes is supposed to have discovered the principal on getting out of a bath (supposedly famously exclaiming "Eureka"): the weight of water "displaced" is equal to the weight of the object immersed.  

But the object cannot "displace" more than its own weight in water.  That's when it floats -- when it weighs less than the water it displaces.

So if the object (a boat) floats, then the amount of water "displaced" is exactly equal to the weight of the boat.

And "displacement area" is an incorrect concept.  The area of the hull wetted by floating it will vary by the design of the boat.  A flat-bottom barge will have a much larger wetted area than a narrow sailboat of the same displacement that has a deep draft.

In terms of cubic feet, that's the determining factor as to whether an object of a given weight will float.  If it is less dense than water (weighs less per cubic foot), then it will float (assuming it doesn't leak!).  If it doesn't, it will sink.