Skin-on Frame Kayak Building Jig

[alan white]

The idea of building a skin-on-frame kayak has been on my mind for some time. I've been studying the many ways they can go together, the flexible Eskimo way, where U-shaped ribs of ash support the long stringers, and either nylon, canvas, PVC, or Dacron are stretched over and sewn.

A 35 lb boat, 15 ft x 24 1/2" with a gently arced bottom and two chines (rather than the Greenland kayak's single chine).
So far (and I'll keep photos coming), I have the mold for the widest beam, which is mounted atop a 3" x 3" square steel beam on legs. The idea is to adjust length easily by clamping molds to the beam at intervals corresponding to a particular paddler's weight. Then I can build from 12 ft to 18 ft kayaks and the mold relocation will take no time at all (each mold is 3/4" plywood with an angle iron screwed to it. The angle piece is what's clamped to the beam.

Now here's the part that I hope will speed the process: The boat is built rightside up. The mold is female, not male. There are notches in the mold to exactly place the stringers, which go in first along with the stems.
this by itself is a reversal of methods I've seen.
Once the keel (a 3/4" x 1" piece of ash) and it's attached stems are sprung down into the bottom of the mold, the four chine stringers are sprung into their respective pockets and attached to their lands on the stems.
Here's the benefit of using this process: The steamed ash ribs are pushed down into place, caged in already by the stringers.
This means the stringers have already defined the hull's shape, as if one might go on to build a plywood boat.
It would be difficult to go wrong setting up the ash ribs, since their shape is determined by the stringers, and simple spring clamps can be used to hold them until dry.

Note that the sheer (gunwales) pieces (3/4" x 2 1/2") are not yet attached.
The rib tops are first trimmed (if necessary, though I intend to pre-cut them to exact measurements, using steel banding for obtaining length).
With the rib tops all 3/4" above marks on the mold, the mortises in the underside of the gunwales should tap nicely over them and be pinned.
That's pretty much it. I'll use a total of five molds and two permanent triangular intenal bulkheads (with holes in them) at the ends, giving 24" spacing.

My hope is to streamline the building process and see if I can build frames very quickly. I would sell finished kayaks in any length, and also see if some folks would buy bare frames to oil and canvas/paint themselves, which it appears is the time-consuming part of the process, but perhaps needing the least skill.
I'd be interested in what others think about this idea.

[duluthboats]

I like the way you are thinking Alan, I also have been toying with using a female type building jig. A little more work to start but repeat frames would fast and easy.

[ned L]

I've built a number of canvas covered kayaks over the years. I really like your idea, quite creative. It sounds like it should end up being quite a time saver.

[alan white]

It's heartening to hear since I've already begun building the strongback. Usually, a seemingly good idea has already been tried and discarded due to some glitch.
I have been searching the web for some indication that a female mold has been or is being used, but nothing so far.
Like I said, I'll document the process.
The station molds aren't split to release the boat. Instead, the notches for the stringers and gunwales are shaped to let the boat slip out vertically when the time comes.
Because the amount of material in a skin boat is so cheap, I'm looking at what labor could be saved as well in order to produce a tremendous value for a buyer. Because of this, I'm searching for a quicker way to lash or attach frame parts, if anyone has any ideas.

[alan white]

Here's the cradle mold, 3/4" plywood, ready to be clamped to a 3" x 3" steel beam about 15 ft long. You can see how the stringers aren't trapped but will release easily. To make sure they don't tend to rise, I can mount swing-away wood fingers on the mold. The top of the mold is shaped to allow easy clamping of the gunwales.

[alan white]

Some progress. The strongback is heavy steel. The molds are screwed to the angle irons, which are at 24" spacing. We'll see how fast this goes.

[ned L]

Looks like your idea should really take time out of the process. Nice!

[alan white]

Yeah, though getting started is slow. The real time savings will come once the bugs are worked out. I want to be able to shorten or lengthen according to paddler size later, using the same molds (only the stems will be different). This boat will be a bit over 17 ft, but 20" mold spacing makes a 14 footer, etc.. So there will be a lot more angle irons welded to the beam eventually.
Changing molds should take about ten minutes.
Which designs have you built, Ned?

[ned L]

I'm not sure the ones I have were to any real design. As a a kid I built a 14 ft one from scratch with my dad back in 1974. It is still doing well, even has the original canvas. The two others I have I picked up as piles of broken sticks with a few shreds of rotted canvas hanging on. The one on the right is the 1974 build. You can see what I sort of started with in the second picture for the one on the left (12ft).




They are fun little diversions to put back together,

[alan white]

Nice work. How are they to paddle?

[ned L]

They are very different from each other. The 14' on the right has a full length keel (parting strip 5/8"x3/4"), that and the hull shape make it track like a spear. You can point it at your destination and barely have to correct your track. Because of the narrow beam it is also fairly quick. It is also a bit like sitting on a tightrope & requires understanding the balancing routine. This is the one to use for distances in open water (growing up I used to take it out in the Atlanic off the Jersey shore). The other one is 'short & fat' with a perfectly flat bottom. It is like paddling a round bowl, not good for distance, but very manuverable. This one is so stable you can stand up in it.

[ned L]

This is my third one. Knowing a bit about where it came from I think it probably dates from the 1930's - 50's.

[alan white]

I only see lashings used on virtually every SOF kayak on the web, but I have the feeling other methods of frame fastening have been used in the past. I am contemplating using clenched copper nails to fasten stringers to ribs and elsewhere. How are your kayaks fastened?

[alan white]

Progress. No drawn plans, boat is being modelled in true 3D, by eye. This is very tedious, fairing the hull without lofting. I like seeing it take shape this way though. Three out of five molds are done.
Tomorrow I'll tackle the two aft molds. Once all of the parts are fitted, the whole boat will be disassembled and patterns will be made. Everything will be sanded on the bench.
Then it should only take about a half hour to replace everything that was removed.

[Rick Willoughby]

Alan
I look forward to your tally of hours spent and what you believe would be the target with a honed procedure. My interest is for comparison purposes with flat panel construction.

Rick W