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Kayak and Canoe Design Bulletin Board
Tortured ply experiment
Posted By: Brian T. Cunningham
Date: Monday, 27 December 1999, at 9:34 p.m.
Hey folks,
What follows is a little exercise I tried out over the holidays.
Yes, I could have sat down with either a computer, a calculator, or just a lot of thought. But, there is something about experiments in the flesh that make results seem a lot more tangible.
I want to turn the CLC SailRig float into something that look more like the floats found on racing trimarans. What I want is a bow with a lot of flotation, to prevent stoving, and a flat transom to prevent squatting. So, I did a little experimenting over the holidays with tortured ply techniques.
I read in Sam Devlin's book about using stiff cardboard to model stitch & glue hulls. I cut my models to scale and taped them together. BTW when you do this _always_ cut the edges. If you simply fold it over you won't be modeling what plywood would do. Since you obviously cannot fold plywood! The rest of the 'scientific method' consisted of taking my model to a sink full of water and pressing down on the middle of the hull and seeing which end sunk first. Yeah real scientific here folks.
All models were cut from 11.0in by 0.70in pieces of cardboard. The same scale as the ply I will cut the floats from.
The latest racing tri's have a knife edged bow to cut through the waves leading back to a wide transom. I cut a gentle curve for a keel with most of the area forward. I let the transom start about 1/4 the way up the piece of cardboard. So I would have a generous transom. Now when you first think of this, where would your buoyancy be? At the back, as my first model proved. I put pressure in the middle and the bow sunk like a rock! It would promote pitch-polling. Just the opposite what I wanted, but more or less what I expected.
I knew that the bow needed to be deeper to account for the lack of width, but by how much? I could have sat down with a calculator, but at this stage I was having all together too much fun.
I recut the model with the bow almost vertical. I found you need a certain about of rake forward, spreading the sides apart bring angle the bow back. The keel was this time straight line, from where the line of the bow hit the bottom of the cardboard to halfway up the side of the rear. In other words the bow was twice as deep as the stern, and I made two straight cuts.
I returned to the sink and the buoyancy was just about where I wanted it.
Giving it a flat cut for the keel was a mistake though. When you spread the sides out it makes the keel bend up. I found that this was easy to fix. You just make the keel a little convex, and when you spread the transom it straightens right out. You need to do the same with the shear line. Of course having the bow reach up a little on a float is what you want, so I left it. If you don't want this, just curve the shear down at the bow.
In the end I got the hull to look, and act, just the way I wanted.
For all those thinking about using tortured ply, or 'regular' ply techniques, give it a try.
If not just to have a little model of what you trying to eventually build.
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