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Re: Skin-on-Frame: Differences between SOF and fus

Thank you all for your helpful and prompt replies. Even the questions help me clarify my aims. I have built two fuselage SOF boats that were too heavy, so am a bit obsessed about getting the weight down on my next one.

Steve: The idea is that fuselage frames are comparatively rigid, regardless of the means of fastening. A flexible fastening allows some movement. The hull by the traditional method deforms rather than breaks, so in theory could be built lighter than fuselage method. Which weighs less, 20 or so ribs or 6 ply frames? And by how much? Maybe not enough to make a difference. But the limits of fuselage have not been explored, at least I could not find any reports of breakage, except of frames breaking during construction at a stringer notch (Jeff Horton, I think). I am familiar with Jeff Horton’s methods, have his book, read his helpful website. If they haven't broken, they are too heavy. If they have, they are too light (N.G Herreshoff?). No great loss if an experimental lightweight kayak breaks, unless I am out alone when it does.

Mark. I don’t have a definite goal for the weight, just an aim to get it as light as possible without it breaking under the rather gentle use I put my boats to, on the lakes and the archipelagoes where I live in Sweden. Day trips, I have a S&G kayak for overnight trips. Short steep waves. If the boat is light, I can hop out and lift it when coming to shore instead of running it onto a rock that I can’t avoid. Can’t get and don’t want to use Corey’s Goop. I will try Bill Hamm’s method as I can get 3.5 oz aircraft Dacron and various weights of fibreglass, and good boatbuilding epoxy. I enjoy the zen of squeegeeing epoxy into glass.

With both construction methods, don’t the gunwales and keelson take most of the stresses while paddling in waves? Should the stringers be placed flat, or on edge, or should they be square? Can you use thinner stringers with multi chine construction than with single hard chine? Less stress on each one?

Thomas. I am asking myself, what are the lower limits of weight of fuselage construction? I could try the 5mm Finnish birch ply that I have for the frames, this ply is about 25% stiffer and heavier than gaboon, and could glass it for more stiffness and strength. Make the hole in the middle smaller where it doesn’t matter. Double up the stems. A hybrid construction is something to consider, traditional ribs and masik around the cockpit. Or build two similar kayaks, one with each method. I am retired. It’s a hobby. I also like the idea of cutting and splitting local green wood to make ribs, cockpit rim, masik, etc. But that takes time away from paddling and sailing. Priorities...I have the rest of the winter to decide.

Peter

If you are primarily concerned with bending up and down due to waves, then the gunwales and keel will take most of the load. Either style of kayak has a lot of flexibility whether you fix the joints firmly (epoxy) or you lash them because there are long runs of flexible stringers which provide the bending resistance. Certainly the lashed joint will have more flexibility. When either one will break is a mystery to me.

Plywood frames obviously work, but the wood direction and amount are certainly not optimum for light weight.
If you don't want to do the extra work to create forms and laminate a ring frame you might think about making a minimal plywood frame and laminate an interior flange. Like an I-beam. I had modified a kayak to give more knee room by raising the shape of the frame at the front of the cockpit. That experience told me that the laminate frame was stiffer and stronger than the plywood frame.

I think your 5mm frame is going to buckle sideways between the stiffener attach points, and that was with epoxied joint which would add more stiffness than lashed. That is what happened to me on a 6mm frame (not okume).

If you want lighter stiffeners, you might try laminating epoxy/ glass to the inside and outside surface of the stiffener (not talking about gunwales). You could also make the stiffeners like an I-beam.

In my own boats, I added a lot of weight in the cockpit seating platform - for me I'd attack that first.
The large ply forms at bow and stern are also probably much more than necessary.

Last, I've always been interested in Bill Hamm's method of light dacron and glass/ epoxy, but never felt I understood what the weight benefit actually was.

Good luck, I hope you will let us know how it works and what you did.