Strip-Building on a Cylinder Mold

Ancient Kayaker,

While living in Japan I got ahold of some 1mm plywood to make a 24" model. That was the nicest stuff to work with but it was really expensive and only available in small (model) sizes.

I really wish the US had thin ply available, so many ideas would be very easy to make.

Gonzo's cylinder mold could have been made directly from 3 ply 1.5mm without cutting it at all. Wouldn't have worked for your adaptation to the process (to not cut it).

Might as well wish to be young again.

Marc

 

Keep it simple guys. You bend the plywood, a single layer, over the molds and laminate one skin of fiberglass; done.

 

Probably aircraft-quality ply, lovely stuff. Other folk write of difficulty finding thin torture-tolerant ply that's flat enough to use with vacuum bagging. Not so much of a problem as boat size increases though. I find it in US and Canadian model boat kits so it exists in NA.

Now there's an idea; I hadn't thought of using a single layer; it would certainly reduce cost. All the CM references I have found - including Kurt's own site - specify 2 or 3 layers.

Kurt is very open about the limitations of CM, including the problems of hull shape unpredictability and non-repeatability, and mentions strip-building as a method that avoids these problems but is significantly slower; I hope to combine its advantages with the speed of CM.


Gonzo: is it practical to apply the glass to the outside of the ply while it is on the mold - before it is joined to the second half of the hull? Also is it necessary to glass the inside?

 

Gonzo,

Have you done this? How do you hold the ply down while you laminate? A single ply has lots of resistance to being bent if you use the final thickness and won't be close to the radius you planned - spring back you know.

Have you done this? A little more than a one liner might help me understand.

Marc

PS, even the 1mm I mentioned above was done as 2 plys of plywood to match the contour I wanted, I.E., 2 ply total thickness.

 

If you want a tight curve, use bending plywood. Three ply will spring back some when you release it from the mold so it take a bit of experimenting. I made many of them for interiors, cabin sides, fish/bait tanks, etc. The plywood should be a bit wider than the final panel so it can be screwed to the mold.

 

Gonzo,

Bending plywood doesn't really have any strength in all directions, thats why it bends easy. No thanks for structure. Perhaps if I want to sail or paddle a fish tank I'll think about it.
I guess I could get plywood wider than the mold, if I was doing cylinder molding which is not what we are talking about. Then again I don't think you answered the question about spring back. If I screw down the ply to the mold, fiber glass one side, then release the screws, will I have the mold shape? Actually you answered that, it will take some experimenting. Any guidelines? I make enough mistakes on my own without having to make multiple molds until I get the desired curvature.

Of course that won't work with Ancient Kayakers modification of Cylinder molding. Perhaps we should call it Ancient Cyclinder distortion molding. ACDM - not much good as a catchy name. Maybe CylStrip Mold - CSM. I think I'll just stick with watching Ancient Guy doing the work.

Marc

 

The wood is mainly a core. Strip planking doesn't have the same strength in all directions either. Actually, it is basically identical to bending plywood. Spring back is not much of an issue since the method requires distorting the panel and then fiberglassing the inside.

 

Progress Report

I loaded some strips onto the mold and bent it today to see how they behaved. Not surprisingly strips around the curve of the bilge tend to lift off the mold during bending, but 66% of them - the ones at the sheer and bottom - stayed put which is a good sign at this stage.

Please note this was a heads-up not not a proper test; the strips used were discards which need to be replaced with clean correctly-dimensioned stock and the bindings were just pieces of string which also need to be replaced by a proper binding system that applies more pressure to the strips. It was worth doing as the test results suggested some other needed improvements to the mold.

 

How can you glass the inside of the ply while it is on the mold? A single layer of ply will spring back flat if you remove it without stabilizing it. That is why at least one more ply layer is needed. Even with 2 layers there will be about 25% springback; Kurt Hughes probably adjusts the mold radius to accommodate it.

If you glass the outside of a single layer of plywood the glass will be under considerable compression after removal from the mold, with a good chance of delamination problems down the road IMHO. With multi-ply layers there is no need to glass in order to hold the bend and glass is not needed until after hull assembly at which time it has its final shape. IMHO that is a far better way to go. I would like to drop this side issue.

 

What's in a Name?

CylStrip . . . hmmm. Lets see if it works before we give it a name. If it doesn't we can call it silly strip . . .

 

Wow, not many people in this discussion have experience with cylinder molding.

1: It's done with 3mm ply bent one direction only - the bendy direction. You can use any flexible ply like okoume, doorskin lauan, etc...

2: You don't screw anything to the cylinder mold. Gravity holds the 3mm panels in place very nicely on a well constructed mold.

3: You don't glass a thing while it's on the mold. You pull it off the mold, join the halves and pour and tape your keel. You then pull the bow together and make the shape of the sheer area with a deck flange. After you have the final shape IN WOOD, you then put a skin of glass on that.



One question for ancient kayaker...

Why use the cylinder mold? It's not a final form mold. It produces a partially curved panel that must then be worked into the true hull shape.

Why not just use a real or true mold/form and strip plank over that?

It would be much more accurate, since you'd arrive at the boat's lines immediately.

 

- I don’t believe gravity is sufficient to hold anything on any kind of mold except dust.

- True. I was addressing post #32 that claimed CM could be done with a single layer of ply and the subsequent discussions.

- I am trying to combine the building speed of CM with the advantages of strip-building, which are exact as-designed hull dimensions, repeatable hull dimensions (usable in class boats), beamier hulls etc. It may be an easier, perhaps even cheaper method too. I am still getting the bugs out, but that’s because it’s new.

 

AK: My post wasn't directed at you, except that last question. It was directed at all the misinformation on this thread.

 

I have done lots of hardtops on a cylindrical mold. I glass one side of the plywood, then take it off and glass the inside on the back of the mold which is female. There is no delamination or any other problem. I think you guys really complicate a very simple method.

 

No, Gonzo. You're reading it wrong. It's not a "cylindrical mold", it's a very specific process called "Cylinder Molding."

Kurt Hughes invented it.

http://www.multihulldesigns.com/pdf/cm33.pdf