Are multiple 1/8" plywood strips as flexible, strong & durable as 1/4" bending wood strips

/Good luck, but every change you make will result in additional changes you don't anticipate.
You're not going to get any shrinkage on a tarp.
Just try an isolated piece to start.

Making a straight keel is foolish. It's just going to make the boat harder to turn.
The longerons will bend into place with no heat, steam or any other method.
Just fasten them and bend.

 

You may be right about the straight keel. But we all need to make our own mistakes.

I feel that a lot of the problems I've had with sea kayaks has related to too much rocker, because the waterline wasn't anywhere close to full length at my weight, and the bow and stern literally bounces off the waves while pitching forward and back over 1-2' high waves of the wrong wavelength. Also, I need a little extra speed to keep up with stronger paddlers. Despite the claims by someone here that damping prevents resonant pitching, I still don't think you get much pitch damping if a large part of the length stays out of the water.

Though I admit Yost's designs would have a pretty long waterline if I just make the boat a bit thinner, to match my weight.

I am willing to lean or edge to turn - maybe that will be enough, because one side of the boat effectively becomes the new keel. Of course, leaning far loses a lot of speed. But hopefully, once I am headed in the right direction, an unrockered hull will stay fairly straight even through breaking waves. At least that is my hope.

Plus, I have no space for a strongback. Though I suppose I could temporarily manage 2 or 3 shipping palettes (free at some stores), and clamp to them instead of a strongback to force the keel to bend in a controlled fashion.

I am worried that a completely straight keel, combined with using only pine wood, might not be strong or durable enough. Maybe an arch is stronger. But I'm not sure if that is true of wood.

 

You could laminate a keel in any curvature you like.

 

Actually, for me to laminate anything makes no sense. Lamination adds places where a poorly made glue joint or badly adhered paint-over-glue spot could allow water into the wood. Cutting and painting plywood is easier and more me-proof - which should be my first concern.

The only changes to Yost's designs I should make are those that simplify construction (possibly using straight instead of bent keel and top stringers, using pre-made palettes instead of making a strongback), use local sources for rapid initial assembly (PVC tarps instead of Coverlight, hardware store PVC cement instead of HH-66), or which adapt the design to my shape, size, and weight, so I get as full length a waterline as possible. I may also drill holes in the stem plates so I can tie down the ends of the boat onto my car securely - though I'm not sure the glue joints are strong enough for that. (Presumably adding lash joints could strengthen the plywood/wood glue joints, but I haven't thought of a way to strengthen the stringer/stem plate joints.)

 

Don't even pretend to start construction without a strongback!!

Clamping a few pallets together won't provide a suffently true surface to measure off of and align things to. You will make a snake not a kayak.

If you don't have room for a proper strongback, then you don't have room to make a proper boat!

 

Agree with Blueknarr.

 

Oh.

I played a bit with making a kayak shape from PVC pipe. I made cross pieces and cockpit coaming by bending polyethylene tubing and duct tape. I bent the pipe around the cross pieces, and taped them in place. The bending force of the pipe, forced it into a specific symmetric shape. The result was an almost flat rocker, imposed on the "keel" by the weight on the flat floor, though I'm not sure the keel would have stayed flat in the water. I have assumed it is the placement of the stringers on the stem plates that forces the keel into a specific rocker curvature. (I didn't use stem plates, just taped the ends together. Since the gunwwales were bent more than the other tubes, the ends of the other tubes stuck out a bit from the gunwale tubes.)

Admittedly, that was on a flat floor. Not as well defined as a strongback, because I couldn't clamp to it, and nothing but gravity forced the keel straight, but it seemed to work, and if you guys hadn't pointed out that PVC is a miserable structural material, that would probably break in use, I would have had a crude boat built weeks ago, built in a few hours, with the addition of some glue, and maybe lashing, and tarp for skin. Perhaps one could be used as a mold for fiberglass and epoxy - but that's way beyond my abilities.

That forced symmetry isn't true for wood strips? Why? Are wood strips from a single "clear" board not uniform enough in bending tension to create symmetry, even if they are cut to the same length and width, and attached symmetrically?

Or is it the tension of the skin that would bend it out of true? In other words, if the stapling of the skin to the wood wasn't done in such a way as to create uniform tension, perhaps the result would bend... But that would be true anyway, with a strongback, once the boat was removed from the strongback...

I rent a basement room in a house, with some shared space. The entrance door doesn't have a long enough run outside before hitting the concrete wall that lines the outside stairs to take an assembled boat out. I can slide a board in through the door over the wall. There is a yard, but I can't set anything up there for more than a few days. The room is long enough to do a preliminary assembly, but the pieces have to be taken apart to go outside. (I'm allowed to store kayaks in the yard, always my main housing criteria.) Too bad a folding kayak looks too difficult for a first boatbuilding attempt by someone without any experience.

Perhaps I could make the strongback out of long 2x4's, like Yost, but all pieces cut near the middle, but not cut all at the same length, so with two extra bolts in the center, the combination would be straight. I could temporally put the assembled strongback in the back yard for a few days while painting, glueing and/or lashing, and while the paint and glue dry and set.

It's obviously a lot easier to make a boat for people who have dedicated well ventilated space and properly aligned exit door to do it right. A garage would be perfect.

 

basically what he said. Plywood's big value is shear strength. Cut it into strips and it is weak in many ways. A solid strip would survive impacts much better than a strip of ply, for example.

 

Mitch,

Let me try to answer in an order that helps (I hope).
The keel on your PVC model would have bent upwards at the ends, with a skin and in the water. Just the fact that your weight is in the center and some of the water pressure is at the ends. If it was wood I'd guesstimate about 1" on each end.
Your stringers will bend the same if the dimensions are equal (unless there is something horribly wrong with one). Doesn't have to be from the same board, just the same type of material.
So the "forced symmetry" works just fine. The real problem is not bending them equally, then fastening them to the fore and aft plates. Taking that curvature out of the frame after assembly is a real PITA - ask me how I know, and that was with a strong back.
Getting the boat straight means keeping the fore and aft plates and the frames straight, and putting 6 sets of stringers on straight. The keel stringer can cause problems, but it is the easiest to check for straightness.
Before I suggested putting a single screw into each intersection of a stringer and the frames and end plates. The reason is that it is easy to look down the assembled frame, see any lack of straightness or unfairness, and correct by just re inserting a few screws. Do that as many times as required to get the frame you want.
This is before you make it permanent by lashing or epoxy or both.

The skin is normally not pulled very tight when it is put on. Even when you use a heat gun to get it taunt, there is not a lot of force. And I have never had it pulled out of shape.

If you can make a hard back in the basement, build the complete frame with screws only, and mark the positions of the stringers on the frames, then you can take it all apart.
Take it outside with a few days good weather and reassemble it by putting the screws back into the same holes. Should go back together with no issues. Now you can make the frame assembled permanent by your method of choice. Then take it off the hardback and disassemble it (the hardback).
Now you have something that looks like a kayak. Could you get by with storing it in the yard, with a tarp? Or even bare, the weather is not going to get to it for a few weeks.
When ever you are ready, you can set it up on sawhorses, to install the skin and paint.

You haven't mentioned the ring around the cockpit. Have you thought about how to do that?

 

Ummm. I guess PVC pipe is a lot easier to shape with than strips, because it is so symmetric, and so easy to bend. And I shaped it only by eye. Perhaps if I had taken measurements, it wasn't perfectly symmetric, and the boat wouldn't have glided true.

E.g. - if the cross pieces are tilted, rather than square across the keel, that could obviously make the two sides of the boat asymmetric, even if the keel were straight.

So, without a strongback, I would need to check its squareness, and also run a taut string along the keel to verify it was straight, left to right, and perhaps also take measurements along that centerline and of stringers horizontal and vertial distance from the centerline at many points along the length to verify symmetry of the gunwales and chines - a lot easier to do if there is a strongback, because I could just lay a large "square" across the strongback.

So I can see why a strongback simplifies things, at least if you can trust the 2x4s to be straight.

I see no point to sawhorses. I can lay it across 2x4s, on top of a tarp so the grass doesn't get painted - perhaps the same 2x4s that created the strongback. I don't mind getting down on my knees.

Actually, I was thinking of painting all the pieces before assembly, so water can't get in. Of course that means I would have to lash instead of glue - and all that epoxy, hardener, filler, and pumps that I bought will be a waste of money. I'm just not sure if hardware store 1 part polyurethane paint can take the bend. I could use a little epoxy to paint over the lashings, so they don't unravel, though of course that will kill some flexibility. If I understand right, epoxy won't stick to smooth surfaces like paint, but it should form a shell around the lashings.

(I just figured out that if I drill holes in the stem plates, before painting, and use a circular file to make indentations in the stringers, I can lash the stringers to the stem plates. I wonder if that could hold. I guess the indentation and holes should be circular, and everything should be rounded off, because sharp angles could initiate crack formation, plus sharp corners might cut into the kayak skin.)

By "ring around the cockpit" do you mean the cockpit coaming, or the structures underneath that support it? Yost gives a lot of options for making cockpit coaming, and it looks like he attaches them to the top stringers, directly on top of the cross pieces, that bear the weight when you climb in. I like his keyhole cockpit idea cut from plywood, because I find the small round cockpit on my current SOF hard and slow to get into and out of. And it is easier to rapidly attach a spray skirt to the sharp angles in the front - an important criteria for minimizing water entry during a re-enter and roll. I've never mastered attaching the spray skirt while still underwater on my current sea kayaks.

I understand what people said about plywood having to be thicker and heavier (it's also denser) than wood, but it is what Yost advocates for both cross pieces and coaming. I think Yost tried very hard to make things as easy as possible for first-timers, and I should take advantage of that.

Maybe I should take the time to order the Coverene skin and HH-66 vinyl cement for it that Yost advocates. All the PVC tarps I have found in hardware stores look too thin to look viable to my inexpert eye. Also, Yost's method of attaching deck rigging to the skin with cement, without screwing into the wood, so you don't have to re-waterproof the skin, must require an extra strong cement and skin.

AFAIK, Coverene doesn't heat shrink, to make it as taut as some skins. But - as I pointed out earlier, it maybe should sag a bit, like a suspension bridge, to support my weight.

 

You can also check the squareness by measuring the diagonals. Assuming none of your parts are warped, this is very precise.

 

In retrospect that makes much more sense. In fact by measuring all point-to-point distances that involve one side.

 

Good luck with diagonals.
I "think" there is too much stuff in the way if you use plywood frames.
Don't know exactly, its been a while.

I had trouble getting the frames lined up when I had a strongback. Then I had trouble when screwing in the stringers, like I said before.
BUT, a kayak will still work with a little distortion in the frame. How much is OK, who knows, and it probably depends upon the builder's bias.

 

Strike centerlines on each piece and you can measure the diagonals to those.

 

If this is a Yost design, the centerline is a hole.
You guys are barking up the wrong tree.
You'd need to check the diagonal while hanging on to two stringers while they are tensioned to go around the frames, so that you can fasten them down.

Ever built an SOF kayak?