Skin-on Frame Kayak Building Jig

The ribs are in, copper-nailed to stringers and gunwales. I lost three to breakage. I didn't realize the generator was disconnected and I didn't get a full steaming of one load. The frame will be done tomorrow.

 

Hull framed. Later I added the deck frames.
The copper nails were driven through and when the boat was removed from the molds the nails were bent over and while bucked from the inside the clipped (to 1/4") ends were driven back in with a punch, so each nail formed a J. This is because I am working alone and normal clenching isn't possible.
I used unskilled help to buck the nails when punching.

 

Very nice, Alan

 

Agree In spades. Less than a month from a cold start, too!

 

Agreed, Very Nice!

 

Thank you. Yeah, about three and a half weeks from the drawing of the midsection. I guess I now know how long it takes to do a new design (unless I loft, which I may do next time).
I'm now hesitant to build another until sea trials on the first. But I need to skin this one while the water warms up anyhow. Few good instructions on the web about skinning, so I will probably havr to buy a book.

 

Here's another photo showing the ribs. I still have to make three ribs but I have to laminate two 1/8" strips to get the tight radius. How necessary these ribs at the ends are is in question, but they are light and can't hurt.
AK, I need your advice on coaming design--- what shape is needed for a spray skirt to stay on, for example.

 

Alan
What does it weigh at this stage?

Rick W

 

33 lbs is a rough guess now with all the framing done--- 31 as you see it in the outdoor photo, with the foredeck frames but not the afterdeck frames.
So the skin must weigh 5 lbs and the coaming maybe 2 lbs. A foot brace and knee braces, another two lbs. That makes 40 lbs and I will probably lose a pound of moisture from the spruce gunwales and stems, as the wood was outdoors at the yard and wet. That would be 39. Or am I off with the skin guestimate?

 

Alan: here's a net page I made a note of some time ago that has good info on the skinning process, and you can find definitions of the various stitch types specified in Wikipedia:-
http://capefalconkayak.com/howtoskinakayak.html

If you Google kayak coaming you will find several pages and sites with good information.

Here's one really good page with links to pages on coamings, skinning etc: http://www.guillemot-kayaks.com/Building/SkinOnFrame/index.html

I build canoes not kayaks (so far at least) so I am not an authority on coaming design and construction, but the following is about what I would do rather than what I've done. You have probably already researched these topics however, and may well know more than I at this stage. Good luck either way:!:

There are several styles of coaming construction that I know of: 1) steamed solid wood 2) horizontal laminated stack of thick ply (or solid wood cutouts arranged cross-grained) and 3) several vertical laminations of thin bent ply. Since you're already into steaming the steamed wood approach may be most attractive to you.

As far as design is concerned, based on the plastic ones I have there should be a rolled edge that the spray skirt fits over, the inside edge should be smooth to aid entry/exit, and give a lot of thought to getting in and out, also consider where your weight will rest during entry/exit so you can reinforce those locations.

BTW -and I don't want to sound patronizing - but if you haven't actually paddled a kayak before I strongly suggest you try out one before finalising the coaming design

Another suggestion: unless you are making the spray skirt it would be wise to buy one first so you can make sure the coaming fits it!

I have looked at the construction of several Inuit kayaks at the Canadian Canoe Museum and the coaming appears to be attached to the skin more than than the frame. However, that is likely to be a reflection of the limitations of their materials rather than recognition of some obscure design requirement. Here is a view of the kayak area of the museum you can scan around using the mouse:- http://www.canoemuseum.ca/docs/PANS/3ARCTIC.swf

 

Thanks for those links. I'll check them all.
Yes, I've seen that SOF kayaks have holes in the coaming and the skin is drawn up into the inside and sewn to the coaming. It appears that little else but the stretched fabric holds the coaming down and this is exactly what I don't want.
I prefer to have a coaming that's solidly mounted to the boat. To do this I will have to have a perfect fit between an inner and outer steamed set of ovals. One shows, and is ash, which would be 1/4" by maybe 2 1/2" high. It would have a lip on the outside at the top. I guess that lip has to be just big enough to accept a spray skirt. 1 1/2" of that coaming would stand above the deck, and one inch would be below the deck. Another 1/4" by 1" hoop would fit around the first coaming.
Now get this: The outer hidden hoop is not glued together (the coaming, however, shows is scarfed and glued, or has a decorative joining piece).
The other (unseen) hoop isn't glued but is adjustable by means of an all stainless hose clamp that's cut and simply screwed to the two joining parts.
Now this would be a problem if the shape isn't round or at least egg-shaped, since flat sides wouldn't allow tension against the outside, but I'm doing an egg shape. Attaching the unseen hoop to the boat makes most sense. That allows the fabric to be stapled to the inside of the hoop first. The fit can be checked against the coaming before the stapling to ensure that little tightening will need to be done (so no wrinkle appears at that point).
the adjustable hoop then is easily screwed to the frame at several points. Maybe forward, aft, and to each side to the knee braces. The adjuster can be anywhere there's space.
Now the coaming is held in with tension alone, and some sealant, which also keeps it from coming loose.
I had originally thought of a 3/8" coaming, but with the added support of a hoop, I can go with 1/4". A 1/2" x 1/2" spray skirt keeper, if that turns out to be the right thing, will be glued to the top of the ash coaming.

I've found that virtually everyone out there building steamed rib kayaks is copying the Eskimo boat and probably often the builders don't bother to ask how some of the methods of yore could be done away with in favor of a much better method using modern materials.
I see lashings being espoused as "very strong and resiliant" but frankly the lashings were a result of a thousand year old screw and nail shortage. While lashings are strong and resiliant the stringers have to be grooved unless the thread is so thin that a million turns have to be done to do one joint. That groove makes a weak spot.
I tested the clenched nail method to destruction and found that the wood breaks first. Dyed in the wool Greenland or Baidarka builders claim no metals means no problems with galvanic corrosion. I used brass and stainless screws and I doubt the kayak will ever have to deal with conditions that could eat metals.
I'm also using construction adhesive (new "THREE TIMES STRONGER" adhesive that costs five bucks). I like it for bedding and screw backup. it will prevent the wood from moving at important points like the gunwale/stem joints and the stringer/stem joints. Plus it will keep water out of the joints.

 

The coaming as you have described it makes a lot of sense.

There is usually some emphasis on the flexibility of the frame when reading about SOF boats of all kinds. Partly that's because the skin adds a great deal of stiffness. I think that flexible joints between thin members like ribs and stringers allows the joint to give slightly so the load is shared with adjacent members.

However, the use of glue for more robust frame members like gunnels is probably good as it would make for a boat that holds shape better.

 

Alan,

I just came to this thread (been off the forum for a while) and I have a few observations and ideas to share. I am an engineer, and have built 9 kayaks and 14 boats over the last 30+ years, and I have considered various ways to make a "production" SOF kayak. I have built using traditional "native" methods, modern methods and "production" methods (pre-production really with an eye towards production). I have used lashings, wood pegs, morticed joints, bronze and stainless screws, fiberglass, alumunim, plywood, and many types of grown lumber for any and all parts.

If I was going to make several exactly alike (no two of mine have been the same) as in a limited production I have considered the strong back and mold method you are using, which would likely save the most time on the next one (just make sure you have the hull shape you want first!). I have done it several times that way, but I find it is much faster just to do it the "native" way: build up the gunwales and deck beams using a few plywood jigs to "eye ball" the form, and than flip it over and build up the ribs/stringers from there. The assembly just sits loose on top of saw horses (only to bring it up to work height but can also be done on the floor). Though I do use a couple or three temp blocks to hold the keel in place to control rocker and depth (and indirectly the curve of the bilge). from there I test fit a strip of plastic to find the length of each rib (I number their location), cut to length and boil them, and hand fit to each location. This is a very fast way to size and install ribs. I use 3/4" x 0.2 inch ribs, if I want a flatter bottom I make them .25 thick and using a joiner plane off each end down to .2" think for 8 inches (so each end is .2" think, and center at .25").

I have found that lashing the stringers and keel with polyester "artificial sinew" cord the fastest and strongest joints to make, I put an 1/8" hole through the stringer at each rib and found it works best (no lump from the lashings in the skin). I have also considered the copper rivet at each rib location, a good idea, but I have never tried it. Lashing is lighter, faster to make, corrosion proof, and stronger I think.

I found using a router on a router table the fastest way to make clean mortises in the gunwales, I attach the ribs with wood dowels through the gunwales and glue them in place (some say the glue is unnecessary but I want them to stay in place). I have considered the copper rivets like you have used, it is an excellent idea, but they cost more and there is no advantage at this location to the addition strength.

I have used both Alaskan yellow cedar and white oak for ribs (and aluminum too, but I like the wood better), I have found boiling them works much better than steaming. I have much less breakage (almost none in fact) and it is far more tolerant to grain run-out and other defects, and since I use very dry salvaged lumber the moisture content is not an issue since the boiling puts the moisture back in.

Traditional coamings use just a steam bent hoop that is only about 1.25" tall with a 3 or 4 cleats to hold on the skirt. I have made a number of different styles, plywood ovals with the top one larger than the lower one to make a lip, steam bent oak, laminated spruce. Most of the fiberglass factory kayaks have coaming lips much bigger than necessary (sometimes making it difficult to remove the skirt quickly). I like mine steam bent oak of .2" think and about 7/8" tall, with a 3/8 x 1/4 strip glued to the upper edge to make a lip. I have an egg shaped particle board mold to warp the it around to get the shape I want. I have found that lightweight coaming that just "float" on the skin work the best, they are simple and light and have never been a problem. Though it is possible to simply size it so when it is all stitched (or clamped) into the skin, to lash it to the mastic or foward frame, aft frame and to the gunwales on either side to hold it solidly in place. I think once you try out a traditional one you will find making it "solid" is not necessary, it work just fine (you have to stop thinking scaled down big boat, that is the way "white-man" thinks, think like a native builder, light and simple is better).

Also your frame weight seems a bit high, most of mine have completed weights around 20 lbs (8 oz nylon with 4+ layers polyurethane finish) including seat and deck rigging. My daughter's Greenland type made of doug fir, 19" x15' lol weighs only 16 lbs. complete The heviest one I have built weight 35 lbs, and it had sealed bulkheads, deck hatches, foot pedals, heavy frame mounted coaming and other "white-man" accessories on it. Perhaps you could lighten up some of parts?

Before you skin it wrap it with plastic and duct tape and go paddle it around. If you want to make any changes there is no point in wasting a good skin, the "Saran Wrap test" is a good way to try it out before it is done. I have used 4-5 layers of commercial grade Saran wrap followed with several spiral wraps of clear packing tape. I have also used that heavy duty white shrink wrap they deliver on new boats, it is tough and self sticks once heated. I used a heat gun to soften and shrink it up, makes a good cheap test skin.

I am not sure you would gain anything by buying the books that shows you how to skin a kayak. I own 4 or 5 books and found none have complete enough information to be useful. When you get to that stage I can outline here what I do and you can give it a try. There are several options you can uses, some I have developed that use no hand stitching with the idea it would be used in a production SOF kayak that work great.

Good luck.

 

Thanks Petros. I will take it all into consideration. one reason my boat has a few extra pounds is that I built it with spruce rather than cedar gunwales, and spruce bow and stern as well. I used cedar for stringers.
Next boat will be about an inch shorter and that will save some weight. I use 5 stringers, and may go to 3. I can lighten up on the deck beams and save some weight. My ribs are .25 x 1 1/8" @ 8" centers. Ash. I can go down to 3/4" and save a bit I may laminate the gunwales for more consistant curves and size them 1 1/2" high and 1 1/4" thick. That way I can have open mortises for the ribs and I'll add the extra meat (1/2") to the inside--- I can do this because the gunwales don't need spreaders to hold their shape.
All in all, it's not hard to shave the weight down 20% I think. I do see that most SOF kayaks at 17 ft (and mine's nearly 24" wide) are about 35 lbs.
A 22" wide kayak a couple of inches lower with lighter scantlings would be a lot lighter, maybe 10 lbs. I do like a rugged boat though.

 

I weighed the boat tonight. 34 lbs fully framed. I wonder about the weight of the skin. Maybe you, Petros, know the answer to that.
i have done some research too, checking out scantlings of stringers, ribs, etc..
Turns out the ribs are spot on typical, and ash is very commonly used. The gunwales are spruce, as I mentioned. This particular part need not be spruce, which is heavier than my native cedar that grows here in Maine. I just had a tree cut from my sawywr's land, 18 ft lengths with few knots. Already very light a few days after cutting, these will make gunwales for future boats. I'm going to guess that the native white cedar comes in at about 22-23 lbs per cu. ft., and the spruce maybe 28. Maybe a 25% difference. This amounts to 2 lbs if my math is correct. Another pound could be saved by using cedar to do the stem and stern.
These are areas where I know I'm overbuilt. I don't believe it's worthwhile to thin or narrow the ribs because they account for such a small weight to begin with. I see smaller keelsons and chines being typical, such as a keelson of 5/8" by 3/4" while mine is 3/4" x 1". Ditto for the chines (and I've got two chines per side).
I'm using wrc for the stringers, since it's available clear and native cedar is not. You can have knots in the gunwales and stems but not in stringers.
Tonight I cut out a coaming form and so I'm ready to do the coaming.
I will take Petros word that the coaming can be strongly enough supported by a combination of fabric tension and minor framing support.