15' Lapstrake Kayak.

A little more effort. My spanish windlass system. I put a taper on the last four of the inner stringers, fore and aft. 1/2" down to 1/4" at the ends. By placing the cross-pawls towards the ends and pulling the ends down, I'm forcing more curve into the ends of the kayak. Everything is in a state flux and floating, but I've acheived my required shape parameters and I feel comfortable with internal loading in the panel.

 

I'm zeroing in on the design now. I've made some changes to accommodate the new build process. Key points are limited curvature in the sheer on the flat pattern and a fairly consistent taper fore to aft in also in the flat pattern. Stability looks good and Cp is .59-.60.

 

How are you planning to do that cockpit ring, LP? I toyed with ideas for bending it in the vertical plane when i was doing the one for the Raptor, but never came up with a solution.

 

I will most likely do a simple horizontal lamination. I've got some doug fir stock that I will mill down to between 1/4" and 3/16" and build up layers directly on the deck. I'll stagger the joints and that will skew the grain from layer to layer. I've done it with a couple of kayaks aready, but with plywood rather than solid wood.

 

You might find Doug fir a bit hard to bend, other firs are, so I suggest you check the first piece before you cut all the laminations. I don't know if it steams well . . .

 

It will steam, but not all that well. From memory (it's been a while) I could get a bend radius of 50x thickness in small bits of Doug Fir without it misbehaving, but trying to go much tighter than that wasn't easy. Somebody else may be able to do better.

 

Better than I expected; applying the 1:50 figure to the rear corners of the cockpit coaming - about 6" radius - suggests 1/8" thick material. I would try a sample piece before committing a lot of material and time though.

The shape will be easier to form if the shape of the rear edge is changed to a single circular segment *. That would give a radius of about 9" and also eliminate the straight-to-curved transitions which can be tricky. It might make 3/16" material work too; but my money is on 1/8" thickness.


* the straight rear edge serves no purpose: a coaming should not be used as a back rest if that was the reason for it . . .

 

Thanks for all of the comments, guys. Build day for the cockpit ring is a ways off yet. I'm a learn/figure out as you go kind of guy on a lot of things and this will be no different. I used 3/16" plywood lams on a previous build and that was verging on the limit. I did have as failure because I clamped it improperly. Proper, progressive clamping allowed for continuing with the 3/16" material. The flat portion on the back of the ring is not based on any design considerations other than the fact that it sits over a ring frame that I typically place at the aft end of the cockpit. That's just how it is drawn right now will most likely change by the time I start cutting material. I steamed some 1/2" x 3/4" for frames for my current sailboat. I had some breakage, but had pretty decent results. I'm guessing anywhere from an 18" to 24" radius for those. If it truly is 18", that would be 36x. It helped to bend them in pairs where one backed up the other. My current plans don't call for steaming the cockpit ring components. Time will tell.

Hopefully, I can get moving on this project shortly. Work. bathroom remodels and software upgrades have me in a design/build holding pattern.

 

- I read someplace that wood is less likely to break when bending if there is a piece of metal on the inside of the bend; not sure how thick, probably thick enough to prevent stress concentrtion and thin enough to bend without forming; maybe a piece of 1/16 thick steel strip. I think the trick is to clamp the wood to the metal tightly at the ends and less tightly in between so the wood is under tension rather than compression. If you run into breakages this might allow you to ue some wood strips that would otherwide be unusable.

 

You want a metal strap on the outside of a radius when bending. The inside usually has the jig, mold or buck in place to hold things in the proper curve. The metal should be clamped with the bending stock as it goes down on the mold, typically starting at the greatest point of curvature and working out. Of course this assumes a male mold bend. If pushing into a female mold (why) I see no advantage of the metal strip on the inside, but it still could be useful preventing "blow outs" on the outside, where errant grain or run out might try to burst through, during the bend.

 

- Paul, I think the inside strap was used for freehand bending without a mold, but I have been unable to trace the original article to confirm that.