Skin-on Frame Kayak Building Jig

Wooden Boat Magazine numbers 205 & 206 (Nov/Dec 2008 & Jan/Fen 2009 issues) carry a 2-part article for the "Solo Carry", SOF canoe. It uses steam bent ribs installed inside previously assembled battens if I recall correctly (I lost #205). Hopefully this is not too late to be of use.

 

I appreciate the interest, Rick. The "cradle mold" as I call it, is obviously very time consuming. When both stems and all stringers are caged in by the five molds and two end supports I will remove everything from the strongback piece by piece and copy each item, and that process will be timed, along with reassembly on the strongback.
The steaming of ribs (about 14) is one big unknown, but I plan to do them fast, first cutting succesive ribs to exact length and then swinging them into place (they are tenoned into the gunwales and initially I'd thought I'd have to do ribs first and then gunwales, but I am fairly certain it will be much faster and easier to hook them into the mortises while at a 45 degree angle and then rotate them down. The fit should be good enough that no clamps should be needed.
Some CNC framed SOFkayak designs (Dudly Dix kayaks,e.g.) go together in two days (classes are offered where it takes that long to complete the boat except for so paint and finish).
The problem is, plywood is used, and compared to stringers sprung under ash ribs, I believe the boats cannot take the same punishment. Ther frame is too brittle and won't flex when bashed by waves. If you really look at the two types of construction methods, you'll see what I mean.
The CNC machine is a money maker where SOF kayaks are concerned, and I think those who build from precut frames (kits) ought to know it ain't the same thing as ash or oak ribbed, and the kitbuilders obviously are aware of this.
Precut frames are fine where plywood and strip built are concerned, but they are monocoque and SOF is entirely dependant on the frame for strength.

 

I'll check out the WB issues. Thanks!

 

Alan, the stringers on all three of my kayaks are fastened with small wood screws. For quite a few years (like 1930's - 1960's) there were all sorts of 'build it yourself' plans published in different magazines for kayaks similar to mine. The one that my dad & I built was from a purchased kit (if you want to call it that) that was advertised in I think Popular Science.

 

Alan: I located WB # 205 finally. It shows that the hull is assembled upside-down around male forms; stiff (temporary) ribbands are bent over the frames then the steamed ribs are installed under/inside the ribbands, which are then replaced one at a time by more flexible stringers which form part of the hull.

Getting the ribs in place inside the mold requires a second person to help, and seems clumsy. I can see how using the ribbands then replacing them with the stringers tends to assure a fair hull but that seems an unnecessary step, too.

I much prefer your method of building right-side-up inside female forms.

As a further comment, in my experience allowing the wood to bend naturally rather than forcing it around an arbitrary shape produces the best canoe shape and is more likely to a fair hull.

I think you are going to have a great boat, but I doubt that your method is going to be very quick. If speed is of the essence, a marine ply skinned boat is probably the way to go.

 

I agree that allowing the wood to "find its own way" is a good way to produce an easily driven hull. I did ery little to disturb the run of stringers, even raising them slightly to compensate for gravity. so far it looks just right, with no annoying inconsistancies of fairness. Of course, designing this way, I can't get a good view because of the female molds, so what sighting I am doing is taking hours and hours of time just to be sure.

I'm not looking for doing this in the blivk of an eye, but I'm pretty sure I could produce a finished frame in two days, and probably quicker. Marine ply is quick, though the epoxy/glassing processes slow things down if they are part of the process. Then again, I believe it was you who builds without epoxy, preferring stringers/caulk?

 

I thought so. I see no other methods on the web except lashings. I am thinking of clench-nailing the ribs to the stringers, to speed things up.
I am thinking two clenched nails per joint. I think the lashings might be stronger though, so I'll have to do some tests.

 

QUOTE=alan white;265251] ... I believe it was you who builds without epoxy, preferring stringers/caulk? [/QUOTE]

Alan: guilty as charged! Worse, I try to eliminate as much as possible from the building process that doesn’t involve work on the hull itself. Not caulk though; if the joints fit properly so there are no gaps I use a quick-setting non-epoxy waterproof glue like Titebond III, but I use epoxy if I must. Lately I seem to spend far more time thinking about how to build than actually building, but I only have space for so many boats and I can’t bear to part with them...

I was thinking the first one might take a fair amount of time to get everything right. However, since you plan to produce several hulls from the same set of molds and strong-back it makes sense to make up a proper jig, and I think the female mold idea is a good one. Repeat hulls should be much faster, especially if you buy the wood already milled to size or if you have the equipment for that type of work. I imagine steaming the ribs can be quite fast with practice and organization.

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I think building the boat right side up is the way to go for SOF boats, especially if the joints are lashed, since that can easily be done from either side. I suspect the main reason to build a boat upside-down is cutting and bending large bits of wood around a form is easier and that step is usually followed by hours of sanding the outside. Neither is particularly relevant to a SOF. However, as you have discovered it’s harder to check for fairness from the inside: your mold must be very rigid to ensure that future hulls will be fair first time. Working inside a hull tends to be hard on the back - perhaps the mold can be tilted and raised for the lashing operation.

I also use ply forms with angle irons attached (shelf brackets are accurate and rigid) but I screw these to a table rather than clamping them to a strong-back. The table is handy for tools and there are never enough clamps.

The notches in the female molds may cause problems when it’s time to lift out the hull. Would it help if you make the notches wedge shaped and use turnbuckles or temporary lashing to hold the stringers? Electric cable ties are cheap and good for this, and there is a tool you can get that tightens them snugly.

I find using lots of spring clamps over a long period can be fiddly and hard on the hands; if the stringers and ribs are constant thickness, what about cutting out a bunch of ply "pegs" to do the job?

I worry about pre-cutting the ribs to exact measurements. Wood usually yields more in compression than in tension when it is bent and steaming may make it worse so they may end up too short which is worse than too long. You might be wise to make the first batch over-length by a fixed amount and note how much has to be cut off at each location. Then you can pre-cut for subsequent hulls. Personally, I would probably make up a simple jig to cut the rib tops to length in one operation: no worries.

I think the plan to build and sell frames is a good one, but I don’t think applying the skin is very time-consuming. HOwever, it is probably the step that would worry your customers. Selling the covered boat for the buyer to paint may work better from a marketing perspective.

Good luck whatever you decide to do!

 

I'm on the run right now, but a lot of what you said makes sense. I built the strongback to be at what I felt was an ideal height for me, but we'll see how that relates to each process.
As far as the release from the molds goes, each stringer notch has a vertical cut above its widest point from centerline. The boat will simply lift out. Holding stringers in place is done by clamps (25 of them, but swing-away wooden hooks with camming ends would be a better speed-building feature). The gunwales will be held by either clamps or short screws from outside that won't show in the finished boat.
It really appears that assembly of the stringers, keelson/stems assy., and gunwales, ready for ribs, will be a fifteen minute procedure. Streamlining the following procedures should be similar.

 

Sorry, you already stated that and it can be seen for the picture but somehow I missed it.

What cross-section are the stringers?

I'm wondering if they may flex between the forms under pressure from the ribs, resulting in an unfair hull. If this proves to be the case you can deepen the slots and use stiffer ribbands to support the stringers until the hull is complete.

Probably though, there'll be enough support already from the gunnels and keel to prevent stringer distortion.

 

The stringers are 3/4" square. The keelson is 3/4" x 1" wide.
Regarding unfairness due to differences in support between molds and stringers alone---- The mold centers are at 24" spacing. If I space the ribs at 12" centers, you would think half would fall on the molds. However, in fact I can have two ribs between the molds, spaced the same distance from those molds. Then all ribs are subject to the same relative condition. Of course, it is most probable that even if a small difference did occur due to spacing half, for example, on the molds and half between, my guess is the boat would self-fair, once out of the cradle.
I am concerned with the bottom rounding, and I'm thinking of blocking up the keelson in six places, halfway between the molds. Very easy to do. The length of the ribs will be critical. If the keelson is blocked up, the correct length will ensure the stringers couldn't be forced out.

 

Terry, what do you think about using a bisquit joiner to make rib pockets (MORTISES) IN THE GUNWALES? It would aid in swinging them into place and I should be able to get about 3/4" depth. They then get pinned with a couple of 1/8" dowels, glued. The machine would have to be reset once to get the full 1/4" width.
The slots may not look as elegant but there wouldn't be any wracking force to worry about and if anything the cuts in the wood are less abrupt than straight-sided mortises, which I like. I like the speed advantage in any case. Some use a router, but that takes a dedicated setup with plunging and hassling with a bit that wants to walk unless the rig is held very steadily.

 

3/4" sq stringers should be stiff enough to stay fair I would think if supported by molds at 2' centers. I haven't much experience with steaming and none on boats, but if the ribs are uniform and steamed sufficiently to the same degree, the boat should, as you suggest, self-fair.

I agree I would not like to cut lots of miters but I'm not sure I understand what you have in mind with the biscuit joiner, without a picture anyway. I assume you will drop the gunnels in place over the rib ends after all the ribs are in place, or did you plan to put in the gunnels before the ribs? Either way, biscuit slots seems like a lot of huge cavities for this.

I can imagine a couple of methods. One would be to simply drill holes in the underside of the gunnels for the rib ends and round off the ribs to fit, possibly using a plug cutter. That would work best if the ribs were not too wide, and the gunnel would go on last.

Another would be to cut a full-length groove in the underside of the gunnels for the ribs and insert short battens between the ribs, the way balusters are fitted into the groove underneath a wood handrail. I would think that would be fast and easy; the battens could be cut to length on a chop saw very quickly. That would allow the ribs to be put in after the gunnels, but I would still put the ribs in first so I could cut the tops off to length in one pass. there's some pics here http://www.hammerzone.com/archives/decks/handrail/traditional/assembly.htm

Don't forget the photos as you progress!

 

I thought of a full length groove. The biscuit joiner cuts a slot in a second, so since those pieces are nearly 18 ft long, I'd rather clamp the gunwales and then move along with the biscuit joiner, dipping where marked. The ribs are 1/4" x 1 1/2". I'll round off the ends before steaming and the gunwales will already be in place. I'll insert the ribs into the gunwale slots hot, skooching them in and swinging the bottom into place. That way no clamping (or little clamping) will be needed. At least that's the plan.
I finally finished the last mold today and am working on the cutwater (stem)shapes. The stringers will land on hardwood wedges screwed atop (two)steps in the stems.
That way the stringers can be cut square instead of being cut at compound angles and lashed or screwed to the cheeks of the stems. It's quicker and stronger.
I'll take some pictures soon, as soon as there's something significant to record.

 

I didn't realise the ribs are 1-1/2" wide, your plan makes sense now.