Kayak design parameters.

My preference would be to go with 2 sheets and then I could come up with whatever else was needed from scraps or, at worst, a trip to the re-store.

As for the cockpit ring, I prefer to make them out of strips of wood steamed and then glued together, but that's just a personal preference. The boat I'm building now used a two part cockpit ring design with a 3/4" plywood base and a laminated wood ring that sits on top of it. The original intent was to have the skin pinched between the two rings, but I've since decided that I will probably use recovered doorskin for the upper surfaces rather than fabric.

I've attached a couple of photos showing what I did. The first shows the ring base being attached to the rest of the boat, and the second one shows the actual ring sitting on the base. The fabric skin was supposed to completely cover the base and then the ring would go over that. The pics also show my penchant for building things out of little bits of scrap

 

LP I really like the design, and the plate packing is impressive. I appreciate what you're going through here since I'm a cheapskate and kinda anal about minimum waste design. I designed a workbench once and counted the screws and it came to 106. redesigned it to use 98. Screws come in 100 packs.

Aesthetically, I have an aversion to transverse butt joints on ply boats. (stripes make em look fat) You're doomed to suffer one joint on each panel as it is unless you order 16' panels. The extra ones on the side deck would annoy me. Is it possible to get two longitudinal strips the full length of the deck and as wide as the side decks out of that middle bit? I'd rather look at any number of fore-and-aft joints on the deck vs a butt joint. A fan pattern of narrow wedges on the center of the fore and aft deck can be a very attractive and distinctive detail; and it uses up all those little wedge scraps that are inescapable in any plyboat project. Add a strip of contrasting hardwood to the mix. Just a thought. If not, give careful consideration to everything and anything you can do which minimizes the visibility of the butt joints. Get rid of the grain contrast, use semi-opaque stains, Fuax-paint, what ever. Maybe mount the paddle holders on the joints on one side. I saw one ocean kayak boat that had his deck bungies cross directly over a splice. I always draw the panel splices in when I sketch the boat on paper. there are practical as well as aesthetic reasons for doing this. Please don't take this as criticism in any way. You can build that boat exactly as you've laid it out and it will give great pleasure.

 

No offense taken. I wouldn't post here if I was going to be sensitive to constructive critisism.

I have to agree that I'm not a fan of these butt joints either, but(t) I don't see any way around them except to develope a full length panel, but I'll still have a joint, scarf, butt or otherwise. I could strip plank the deck and get rid of them all together. Tempting, butt a whole other ball game. I'm not going to piece together the forward or after decks as that defeats the whole purpose of using plywood. You've got me thinking though and an angled joint would improve appearances; angled out towards the stern.

 

You mentioned a question about bend radius. The Hardwood Plywood Mfg Assoc used to put out a good little guide with all this in it. And if the stuff is certified by HPMA, LLoyds, or anyone else, you should have zero trouble getting this info from the manufacturer. One other point- when you are dealing with a lot of curvature, you should be aware that a sheet will bend according to real world physics which is different from the curve of a NURB over the same span. Pointy ended tapered panels are also tricky because the pointy end can't transfer much moment to the broad section. Just stuff you figure out along the way.

I just noticed your response- Yes, I think a bit more "radial" and a bit less "transverse cut" could go a long way. Small boats like this can be taken in by the eye all at once; and we tend to judge them organically. Stuff that's ok on bigger boats can look odd on dinky ones. Theres also the matter of optical illusions caused by intersecting lines. Either you exploit them or they screw with you. There's no middle ground. I think you're in pretty good shape. Get cracking.

 

Either that or go completely the other way and use a puzzle joint and two different kinds of wood to accentuate the difference. Sometimes when you can't hide it it's best to call it a "feature".

 

- my own rule of thumb for bend radius is thickness x 100, slightly more with the face grain and less across it - perhaps a 20% spread for 3-ply but the difference is less as the number of laminations increase. That's fairly safe with a low probability of cracking; I've exceeded it many times but it gets chancy. PAR has a useful table that he may be willing to send you if you PM him.

Sometimes you can hide a joint under a bungee.

 

I think I'm more concerned with the effort and difficulty in bending the radius rather than whether or not it's physically possible. I have not used good marine ply in all of my efforts and the 3/16" ply that I was prototyping with was quite rigid. The 1/8" ply I used was either door skin or underlayment. It was easy to form in most areas except at the stem where there are a lot of conflicting forces involved. The thicker panel might prove to be too difficult in that area.

I have a plan with the 1/8" at the bow for a better shape there. On the little yak, the deck went flat for the last few inches so I'm going to try something to assist in maintaining the proper lateral curvature.

I've been looking at the butt joints on the little yak and I don't think they are all that obtrusive. Just a nice clean line. A scarf would produce an even wider line. I think their location on the narrow side panel reduces their prominence, too. Puzzle joints? Disregard. Sorry CT.

 

You find puzzle joints puzzling?

 

When bending wood into an arc of constant radius the amount of torque is constant but the force required increases considerably at the edge of the panel. If you flatten the curve at the edges these forces are reduced. Bending the wood over a fulcrum at the mid-point produces a curve using minimum force. A catenary provides constant pressure between the mold and sheet, a property I have used on occasion to simplify clamping arrangements.

The stiffness of a sheet material increases as the cube of the thickness. For a uniform material 3/16 is more than x3 as stiff as 1/8 material.

In the case of plywood stiffness is influenced to a lesser extent by the number and thickness of the veneers. For example 4 mm ply with a 2 mm core is about x1.5 as stiff as 4 mm ply with 3 equal thickness veneers, when the bend axis is parallel to the face grain.

 

I did some looking around and you can get 4x10', 12', 16', and 24' marine plywood in various thicknesses. CLC sells it, but as you might imagine, it's not cheap. 4mm 4x16 is about $190 a sheet.

When I was out paddling yesterday I hit a rock hard enough to leave bits of plastic hull floating in the water which got me to wondering about just how durable is marine plywood? It seems like if you scraped the top layer off, or even scratched it good, it would tend to soak up water like a sponge. Given that this kind of abuse is not uncommon with me, it does give me pause when considering building materials. When I was still planning to build a cedar strip boat next I planned to use ash or oak for the strips within about 6" of the keel for wear resistance.

 

- I dunno, it seems more logical to wonder about the plastic hull's durability to me. You can boil marine ply and it won't delaminate, wonder how many plastics would put up with that threatment.

 

If I'm kayaking in boiling water I probably have other things to worry about

Anyone have experience with marine ply under conditions of extreme abrasion?

 

Would having your trailer fail catastophically and dump your 16' strip built sailboat on the highway and dragging it until you could get stopped count?

Burning wood is not the smell you want coming from your woodenboat. Avoiding the gorey details, the portions of the boat theat hit the pavement that were sheathed in glass did great....considering. The glass was crushed and abraded, but it was not penetrated. Technically, not marine ply. There were other areas where the boat settled that did not fair as well.

 

Not real good if not sheathed. That's why people sheath 'em.

Chucking some graphite in the fill coats makes it nice and slippery too. Wont cut skin friction in the water, but will increase abrasion resistance.

 

OK, I'm convinced!