# displacement value Help

Discussion in 'Stability' started by kag65, Mar 26, 2011.

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### kag65Junior Member

What is the displacement for a boat 12.7 feet long by 2 feet wide and 8 inch keel to gunals. How to calculate? Surface long sleek hull? Can some one point me in the direction to get displacement value.

Thanks for any help

www.millit5.com

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### PARYacht Designer/Builder

You're kidding right?

How much does an airplane weigh, with 12.7' wings and a 2' round fuselage with 8" windows?

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### JRDSenior Member

Put the boat in your swimming pool, sit in it and the measure how much water comes out.
PAR.... is there a prize if we can guess the aeroplane weight correctly?

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### gonzoSenior Member

Weigh the boat, that is the displacement.

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Kag65; The boys are messing with you as we often do. No offense intended. You have not told us nearly enough about the boat in question. The displacement potential of the boat depends heavily on the shape of the underwater parts. If the boat was a box shape with blunt ends and plumb sides it would contain about 16.75 cubic feet. That would amount to about 1045 pounds in fresh water. But that is for a box. You said long slender hull................ You need to find some information about prismatic coefficient in order to calculate actual displacement of a curvy shape like a boat. A wild assed guess for a long skinny boat, say like a kayak, the Cp (prismatic coefficient) might be around 0.52. Multiply that by the volume of the box and you will get somewhere around 500 pounds. Remember, I said "wild assed guess". You can also find displacement with the use of section measurements and Simpsons Rule. Check Wikipedia for either of those concepts.

Get a book such as Skenes Elements of Yacht Design to learn about this stuff. The book is cheap, available, and not hard to read. Try Amazon. Another good book is The Nature of Boats; by Dave Gerr. That one is pretty basic and easy to follow. Learning about boats is both useful and fun.

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### Tim BSenior Member

How to calculate? Numerical integration of the immersed hullform. Mathematically, this is [triple integral of P(x,y,z) dx dy dz] where P(x,y,z) is a function which defines the hull-surface.

Several methods are available, using bonjean areas based on section is the classic method.

Or my prefferred solution... discretise the hull surface into triangles, extrude each triangle to the waterline and the immersed volume is the summation of the volume of all the triangular prisms. Of course, you need to work out how to calculate the projected area of a general triangle.

Tim B.

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Tim B; The guy asked a simple question. OK so he did not know that we needed a lot better description of the boat. If he knew a lot about the subject he would not have needed to ask. Let's not punish him for failing to know what you know about complex mensuration.

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### Tim BSenior Member

It's a simple answer. He's interested in the volume inside the hull, which can be found by using [triple integral of P(x,y,z) dx dy dz] where P(x,y,z) is a function which defines the hull-surface. No punishment intended, but I don't think I can conscientiously give another answer to the rather broad question. From that he needs to do a little research. Either on the algebraic definition of hull surfaces (eg. WIGLEY Hull or part thereof), or numerically, using either sectional areas, or an extruded surface mesh.

I am happy to point people in the right direction. I am not happy to tell them the exact answer to a vague question (unless I'm feeling very generous or they're paying me).

Tim B.

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