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Skin-on-Frame: Differences between SOF and fuselage frame kayaks

During the winter hibernation from boating activities, I have considered which kayak I would like (not need) to build next, and how to do it. It should be an SOF, for the ease of building and the low cost, considering the ply and other wood I have lying around. In the forest around my house I can get hazel, oak, juniper, birch and other woods for traditional construction, and the lumber yard has long boards of cheap and good knot-free Norway pine for the longitudinals. But should it be fuselage, which I am familiar with, or the more traditional steambent ribs? In this post on Wooden Boat Forum, Dave Gentry brought up what might be an important difference between fuselage construction and the traditional rib construction.
Re: SOF - Ply Frames vs Steamed Ribs 10-31-2013, 11:04 AM
 Originally Posted by LittleGoat 
Dave, when you say that the two types (fuselage vs ribs) feels different in waves, how are they different? I'll probably be in waves since I'll be paddling off from the Oregon Coast. Thanks.
They are different merely in that a traditionally built and lashed boat flexes more through the waves. It's a cool sensation - but one that is not particularly noticeable in flat water or small chop. I'm sure that James' mostly authentic baidarka replica flexes even more than any I have paddled. Whether or not that is beneficial in waves is certainly open to debate (and I seem to recall one can find lots of "expert" opinions on the subject being bandied about - much like in the debate over the purpose or usefulness of a bifid bow).

The flex inherent in a traditionally built boat will, in my opinion, make the frame somewhat more crush resistant that the rigid frames of more modern construction methods. That's not often important, but could be if you intend to go in harms way. Surf boaters dropping off a wave onto a big rock, for instance, or getting swept into a tree by the current, or rolling your truck over (which has been done, with little damage to the traditional qajaqs on top of it).

The traditional kayak is like a woven basket, the fuselage frame is like a wooden box. You can kick the basket without damage but not the box. One could make some assumptions based on this difference. The rigid hull could be fastened by any method: lashings, screws or nails, or epoxy glue. But the traditionally-built hull, to maintain flexibility, should be fastened with a flexible method: lashings, flexible PU glue, screws in certain places, but not epoxy. The rigid fuselage hull is susceptible to fracture on impact, so has to be built strong enough to resist breakage of the plywood frames and the longitudinals. What forces causing breakage are likely to occur? Traditional ribs are flexible so the longitudinals may be significantly thinner than on the fuselage frame as both the ribs and longitudinals together absorb a blow by bending. The flexible hull needs only to be strong enough to resist disadvantageous overflexing in the waves it is likely to be used in, or when heavily loaded. If the chine stringers are rectangular in section, they could be placed flat (long side facing in and out) or on edge (short side facing in and out) to promote flexibility in the former, or stiffness in the latter position.

The Inuits would always have tried to build a boat with as little wood as possible, and experience would teach them how small the dimensions and number of ribs and stringers were needed to prevent overflexing in use, or even breakage under extreme loads, like landing on a rock or ice ledge, or carrying too great a load in waves. The much greater number of stringers of the Western arctic kayaks (7-11, Adney and Chapelle, p192), used in the open ocean, than the Greenland kayaks (as few as 3), used in more sheltered waters), would support this evolution. We could rely on their experience for the dimensions of a traditionally built kayak with ribs, but the dimensions of the parts of a fuselage SOF seem to be undefined. What if you do not “intend to go in harm’s way”? How thin can the plywood frames and longitudinals be, under what conditions of use, if you want to reduce the weight as much as possible? Is there any information that suggests that I shouldn’t use 5mm birch ply for the frames or 6mm x 12 mm pine for the stringers of a hard-chine Greenland kayak?

The same goes for the skin. If beautifully made and varnished strip-built kayaks can be used and still be protected from everyday scratches and dings, why shouldn’t a fabric skin be taken care of the same way, and be much lighter than usually used (8-12 oz Dacron or nylon)? Like Platt Monfort did.

Peter Lord