Skin-on Frame Kayak Building Jig

An update. Although I've replaced all the wood, modifying the ealier pieces, I still have to remove it all again for making measurements and templates. It would be far more gratifying if this were a one-off!
anyway, the design is coming together. It measures out to be a high capacity touring sea kayak. Dimensions are 17' 4" x 23 1/2", swede form hull. It looks like a West Greenland kayak but it's got a lot more cubic volume. I like the greenland kayak look, but this will have a longer waterline for its length and a larger (egg shaped) 30" coaming. Easier to get into and out of, and it will still take a spray skirt.

 

That's looking really good, Alan! How much rocker have you got there? With the deep forefoot it will track well but might be a bit hard to turn without some rocker.

 

Should be about three inches of rocker, very close to the depth, which will probably be about 3 1/2"-4". She's well rockered.

 

I'm getting down to the steamed ribs, which are now 1 1/8" at 8" centers. the trick is to carry a fair curve athwartship, ending with the upper end of each rib seated neatly in a mortise in the underside of the gunwales. Getting this fair curve at each point along the whole length is tedious.
So I'm using a piece of ash 1/16" thick to test each rib location. I'm pretty much ready to mortise the gunwales. I have no mortising bits so I'll use a drill and connect with a router fluting bit chucked into the drill press, running fast. With a fence and a vise grip clamp I should get pretty accurate mortises. I'll leave the rounded ends where I've drilled and just round off the rib ends to match. The only other thing left to do is to make two more gunwales (but these are for setting the ribs so the tops have something to rest against. Then I'll remove the temporary gunwales and tap the actual gunwales down over the rib ends.
This method will be accurate because the temp gunwales will allow placement of the ribs into open mortises (pockets) rather than closed ones as are in the actual gunwales. That solves the problem of doing the ribs first and then the gunwales. I'm certain it will work, since the pockets on the temporary gunwales will be positioned exactly where the actual gunwale mortises will be.

 

Steamed ribs sounds great, although we usually barbeque ours Seriously though -

:?: How is the steaming process itself? What thickness are the ribs, looks about 1/8. Is this still looking like a good production build method?:?:

 

Ribs are 1/4", maybe slightly less. Of course, rather than say "production", I'd rather say fast and repeatable while being an excellent boat.
I don't want to sacrifice any quality but I need to make money and so while many others are cutting CNC frames out of plywood, making non-flexible boats, I want something different, something with ribs that absorb shock well, allowing movement and eliminating point loading, and I can use that as a selling point.
The fastest way to build a SOF kayak right now is of course precutting sections using a computor-controlled router. Good for kits for beginners or someone who wants a fast build. If I can get close to that speed with steamed ribs, I'll be real happy.

 

How long did you steam the ribs? If they are 1/4" or less, using the 1 hour per inch rule of thumb, you wouldn't have to wait long to start fitting the first rib and after that you could work as fast as you like.

As an experiment I tried making a laminated rib this afternoon, using 3 layers 1/16" think. I just glued the layers together flat, clamped the ends only and bent it to shape: the laminations were kept together without clamps. It held its shape and there was very little spring-back. A bit messy but not as fiddly as I expected and with a bit of development it might be a viable technique. Cutting the strips would be a bear but I believe the material is sold precut for basketwork. I might try it out on a boat one day.

 

Just a hint that might help somewhere.

Steaming ribs for a canoe is a tiresome business, but the wood will break if not steamed.

I found that if you plane good quailty wood down to say 3/16", and apply f/g and epoxy to the outside of the curve, that you can get some good bends in without steaming, and without the wood breaking.

Having the canvas resting against a waterprooof side of the frame isnt a bad thing either.

Might be worth trying a test piece to check the possibilities

 

Thanks for the tip. I will try that if for no better reason than it's easy to try and it is going to be useful on some project. Besides, my girlfriend's teeth are killing her from the chewing.
Currently, the ribs are in the bathtub sunk by a brick. I will give them three days underwater. I could have used green ash rather than this kiln dried stuff, but my sawyer didn't come through on time. I like one aspect of dried wood---- it is nicer to joint and rip.
I don't anticipate problems with the ash since no wood seems to steam as well. Ash is just very bendy wood, even not steamed.
Just glass-tape a wide piece and rip it after? I've got some 4" tape.

 

Neat idea, ensures wood is under compression instead of tension and is much less likely to break! Actually, I read somewhere that most of the distortion that occurs when wood is steamed and bent is on the inside of the bend.

I have some data on strength and elasticity of wood; for example southern longleaf pine with a modulus of elasticity of about 2,000,000 psi has a yield point about 8,400 psi suggesting that it will distort by up to 0.42% before breaking, at least on the inside. Assuming that it will stretch by, say half that on the outside gives a total distortion of 0.63% or 1 / 158. The data for all the woods that I have give very similar results, so a bending radius of about 200 x thickness should be OK without steaming, although steaming will enable the wood to bend much more and hold the new shape better.

I am looking at a new boat design which has a coaming bend radius of only 14 inches in one place, suggesting the wood will have to be steamed to allow a sensible thickness to be used; 3/8" lumber would be OK but that's is only 1 / 40 of the radius. To avoid steaming I may try ply with the outer grain running vertical: I might be able to get away with 1/4" which should be adequate in ply.

 

I haven't steamed the ribs yet. i am ready now to do so, though. I had to fiddle around with the forms today. The next boat won't be but a few hours to build but the first one is a lot of work.
All you need is the right blade. An 80 tooth 10" will rip very smooth. My trying batten is 1/16" or so and it is nice and smooth. My blade was cheap too, about $30.00.

 

If you have a girlfriend who will chew ribs for you - you dont really need a boat

Yes, sawing a wider section will work fine.

I find that tape tends to pucker on the edges, so I tend to cut my own strips, but it will work fine.

http://au.blog.360.yahoo.com/blog-9Lzn38s2aaNjbox8Tf4JBaMLBFaBlmI-?cq=1&l=11&u=15&mx=46&lmt=5&list=

illustrates the bend I put in my own canoe using the technique after breaking 3 other pieces of timber.

I discovered the technqiue after having cheap exterior ply start to break when I was bending it while building a skateboard ramp. 1 layer of epoxy and 6oz cloth, and it was a breeze to curve afterwards

 

What model canoe are you building?

 

New twist. I will be dropping ribs into the upright hull cavity, as described, and tonight I figured out how to hold the rib tops exactly in place. I will raise the gunwales 3/4" using spacers under them. Then 38 individual "clips" (already made) will position the rib tops. The clips will be spring-clamped onto the gunwales (pictures to follow). One by one, I'll remove the clips and tap the gunwales down. This will be fast and accurate. It was work making the clips out of hard maple, but now I've solved another problem and if it goes like I think it will, I'll have the ribs and gunwales done in no time.

 

I steamed half of the ribs tonight. I had to build a steam box and set up the steam generator, which worked out well. It's a small steam box anyhow, about 6" x 6" x 40".
The ash had been soaking for a day in the tub. The foremost rib has a very small radius as you can see in the picture. Maybe a 3" radius. Of course, ash steams beautifully.
Note the clips m,entioned before. They are seen at the top of every rib and just below the gunwales. They get removed as the gunwale drops 3/4". Right now, the gunwale is raised exactly 3/4" to accomodate the clips.
Tomorrow I'll finish up the ribs.