View Full Version : Alternate material for SOF canoe frames and stems?

Canuck Bob
10-18-2016, 03:45 PM
I have an enviable stash of real primo quarter sawn sitka spruce. Bought for a homebuilt airplane project now cancelled. I also bought some 1X6X16' planks destined for a stripper project that someone else abandoned. It is for sof construction as detailed in a recent thread about hogging.

I checked the pricing with Canadian suppliers and decent marine plywood is very expensive delivered. Any suggestions to help supply locally or much cheaper?

What really interests me is making the frames and stems from the sitka spruce. I found one thread for this but the builder made his frames from many pieces of wood, well over a dozen! Any ideas or opinions for building stick frames for a sof canoe?

Gib Etheridge
10-18-2016, 04:36 PM
I guess the ply is for bulkheads? If so you can make the equivalent from spruce, either by laminating veneers yourself or by edge glueing and glassing both sides. It's been done before.

10-18-2016, 04:39 PM

Don't know anything about ply in Canada.
But I have considered laminated frames for a "fuselage" style SOF kayak.
Since the kayak needs a deck beam, I think the canoe would be much easier.

The Yost kayaks I have built used 1/2" ply with 1" width on the frame.
Since ply has much less strength than solid wood, I would guess a 1/2 x 1/2 laminated frame would work well, but you could always go bigger.
Any lamination will have some spring back which would distort the final shape.
I think 1/8 plies would have very little springback, there are calculators available - I've never actually used one.
You could also preform the strips using a heat gun so there would be even less springback. I use the technique on 1/4 strips, but do not need as much curvature as a canoe (all the strips are longitudinal).

There was a thread on this method several years ago, it seemed to work, but I can't find it. Sorry.

Actually, I used this technique to raise the cockpit frame at the top by using the bottom plywood section, and bonding in place while laminating the top. I thought it was really very little trouble once I thought it out.

Another suggestion. In the Yost fuselage kayak design, the ply frames are cut with notches on the outside to locate the stringers. I no longer do this, its more trouble getting the notches in the right place than I can stand. I just cut the ply to the geometry at the bottom of the notches. Then I mark the position for each stringer on each frame. When locating the stringer onto the frame, I drill a small hole thru the stringer into the frame and temporarily insert a deck screw.
If I find an unfairness in the stringer, I just pull the screw, adjust the stringer and reinsert. In this manner I can get the entire frame assembled, checking the overall fairness of the boat.
Then I glue stringers to frames, using the deck screw to maintain clamping and position. After the bonding I replace the screw with a bonded in wooden dowel.

Of course, most will just lace up the joints. I'm not personally convinced one way is better than the other. Bonded joints should provide a little more stiffness to the boat. Many would say that is not a good thing. :d

Good luck and I hope to see your choices and results.

10-18-2016, 04:58 PM
Any pictures of the design or specific parts you want to replace by spruce? (Never built a SOF before...)

Just thinking, since ply is generally used to be loaded in shear, what would stop you from building some sort of lattice truss?

Back in my school days, we once built a bridge out of nothing but 5mm square strips of wood and epoxy. The bridge spanned 2ft and could carry 280lb loaded in the center.

Canuck Bob
10-18-2016, 05:10 PM
upc..., thanks you validated my thoughts on the construction. Your method for eliminating the notches is exactly how Blandford designs his boats. I'm going with Yost methods and the simple methods I read about, stapled fabric, your method for attaching the stringers, Tremclad (Rustoleum) oil based paint, etc..

10-18-2016, 05:14 PM
If you really feel creative, have a look at the geodesic airframe structure of the Wellington (a bomber from WW2, I think).

It has diagonal stringers in addition to the fore-aft stringers. I imagine this could yield a very stiff structure.

10-18-2016, 06:57 PM
You could also look at geodesic airolite boats. http://gaboats.com/
Diagonal stringers would take most of the flex out of the boat. I could never decide on a simple way to assemble the boat or how to size the diagonals if they are solid (wood).

The typical SOF is basically about 9 stiffeners all bending together to give the stiffness.
The cloth has some, but very little benefit to the bending stiffness of the boat.

Stapled cloth edges is a nice simple way to attach the cloth.

Canuck Bob
10-18-2016, 07:21 PM
The wood was bought for a Cygnet Homebuilt Plane with geodesic wings. I'm sold on this structural design in wood. However the airolite boats are interesting but this is for daughter/dad building and fuselage frame is fine. I built a stripper Prospector to fine standards years ago. That would drive my daughters crazy. I paddle the 16' Prosector solo but it weighs too much for my current health. Transporting and handling a shorter 30# canoe must be a delight.

Canuck Bob
10-18-2016, 09:18 PM
Just realized many folks report using Baltic Birch over on Jeff's messing-about.com forum. I can get it locally and love working with it and use it as my go to plywood. I took for granted it wouldn't be moisture resistant, Jeff uses it that is good enough for me! Good varnishing or oiling the fuselage before covering should suffice nicely.

10-18-2016, 10:44 PM
I have used Baltic birch on a SOF kayak.
Structurally it worked fine, it is heavier than the more expensive Occume.
It is also very nice to work with, and what I used had absolutely no voids in the core.

But ---- Birch has a significant tendency to rot.
At least seal the edges of the ply with epoxy. At the very least varnish the edges with several coats.

Or decide this is a bio-degradeable boat. Dry it as well as possible between uses and store indoors.
Outdoors the skin will condense moisture, draining it down to the gunwale which means it will be prone to rot since it stays wet longer.

Good luck.

Canuck Bob
10-19-2016, 12:16 PM
Storing indoors and our local climate is "Rockies rain shadow" dry. It will get sealed well and treated with respect. No epoxy though, after my stripper I am so sensitized that that poison will not grace my shop. In fact avoiding poisons and respirators, sanding, poly goops, and such, will dictate a bio-degradeable boat (nice description).

10-19-2016, 12:32 PM
Interesting, I have a friend who got so sensitized to cedar dust that I had to build the deck for his boat, but the epoxy did not affect him at all.
And I get "flu" like symptoms when I cut strips (without a heavy mask), but no issues with epoxy.

Canuck Bob
10-19-2016, 12:44 PM
Cedar can be irritating as well. I am glad to be working with this sitka. Still proper dust control, hearing protection, and filters are sensible precautions when working with tools. I just don't want to build in a makeshift hazmat suit!

MN Dave
10-19-2016, 12:51 PM
You could also look at geodesic airolite boats. http://gaboats.com/
Diagonal stringers would take most of the flex out of the boat. I could never decide on a simple way to assemble the boat or how to size the diagonals if they are solid (wood).

The typical SOF is basically about 9 stiffeners all bending together to give the stiffness.
The cloth has some, but very little benefit to the bending stiffness of the boat.

Stapled cloth edges is a nice simple way to attach the cloth.
The geodesic design is very good, but using the heavier ballistic Nylon or polyester cloth and urethane coating on this design is far more durable. The Kevlar diagonals are hard to get and keep tight and in my opinion more decorative than practical. The heavier cloth doesn't add that much weight and is very tough.

Sitka would be perfect for the stingers, gunnels and keel. Steam bent oak or ash ribs are good, but laminated Sitka should be fine. Springback is a minor difficulty that will be compensated for by the rest of the structure.

Titebond III should be good for the lamination if you don't want to poison yourself with epoxy. Some threads have indicated that the allergy can be specific to a certain brand. I think that the Duckworks novalac is far enough removed from the others chemically, but the experiment is probably something that you are wise to avoid.

If you go with the geodesic or similar design, I would try lashing the parts with braided polyester (https://www.wirecare.com/category/cable-organization/lacing-tape/braided-polyester-dacron-lacing-tape/lt2-s3-fc-nt-lt2-braided-polyester-lacing-tape-synthetic-rubber-finish.077.094-50lb-natural-1-500-ft-lacing-tube). It isn't slippery and is easier to handle. It is used in high reliability electronic applications and takes coatings well.

Final thought if you go with the plywood frames, soak the Baltic birch in a concentrated borax solution for several hours and dry. A quick rinse and wipe with a damp cloth will remove any residue from the surface. It will not affect the varnish. The borax prevents rot. Some will (always) argue, but borax is the material used for mold remediation. Plywood will make the boat faster to build, stiffer and heavier. Everything is a compromise.

The Wellington bomber construction is a bit too complex IMHO.:d
https://upload.wikimedia.org/wikipedia/commons/2/2c/Wellingtons_under_construction_WWII_IWM_CH_5980.jp g
By Royal Air Force official photographer - This is photograph CH 5980 from the collections of the Imperial War Museums., Public Domain, https://commons.wikimedia.org/w/index.php?curid=22039568

Gib Etheridge
10-19-2016, 01:46 PM
Best source of borax preservative.


10-19-2016, 02:32 PM
Spruce should work fine. We've used White Pine for stringers which are a bit stiffer than Western Red Cedar but work fine as well. For the plywood, even if you aren't looking at a very rot resistant material, you do want one that doesn't have voids. The Baltic Birch might work for that.

Most of the folks in my class use either a marine spar varnish or wipe on polyurethane on the frame before skinning. I'd think that would provide an excellent protection to the wood for a boat like this that typically gets stored indoors and doesn't sit in the water for days or weeks on end.

Canuck Bob
10-19-2016, 07:36 PM
My experience suggests that locking a strong synthetic fabric in a resin matrix does wonders for strength. The only one I'm considering is Dave Gentry's Premium PL application that he produced a video on, remember HD Chandlers or Lowes Boat Shop or Canadian Tire and Canoe! For decades these sof craft were covered in canvas, paint and varnish. Respected builders and designers still do it that way.

There is no plan to skimp on fabric. It will come from a respected SOF supplier and be top quality.

10-19-2016, 09:37 PM
Well, the PL Premium is more about abrasion resistance. The SOF fabric isn't really going to give you strength, the frame is the strength. The fabric provides shape, a waterproof envelope, and some resistance to mechanical/abrasion failures.

10-19-2016, 10:17 PM
If the inuit used drift wood I think that will be fine.