Bending plywood panels

Hello all,

I've designed a boat on freeship that I intend to build with a friend of mine. Its a 10 metre (32.5 ft) hard chined displacement powerboat, displacement of close to 6 tons. We will be using epoxy composite stich and glue method of construction with marine plywood sheathed inside and out with fibreglass.

I've worked through the scantlings from Dave Gerr's Elements of Boat Strength, and the plywood panel thickness for the hull skin is 19mm (3/4").

My question: Has anyone had experience bending panels this thick?

Some pics attached to give you an idea of the curvature:

 

Except for the bottom, it looks like those are pretty easy curves to bend on the sides, but 3/4 inch doesn't bend very well at all. The experience I have is bending 1/4 inch built up to 3/4 with layers. 1/4 inch bends quite easily, but 3/4 doesn't bend at all unless you could steam it somehow or submerge it in hot water, but you might be risking seperation of the plys, which may or may not be a problem. With marine grade it shouldn't be.

 

3/4 inch ouch my 40 ft cat only uses 3/8 for the hull skin with stringers at 300 centres and bheads at 1.2m

 

Usually something that thick is built up of layers. I would say that 2 layers of 9 mm for the sides and 3 layers of 6mm for the bottom, but I would add an additional 6mm layer, just to be sure.

Steve

 

Here http://www.devlinboat.com/constructionsockeye45.htm , it is a bigger boat.
But they are building from a 1/2 thick to have some initial rigidity, then are stacking additional 1/4 layers up to the required thickness.

Other there http://boatplans-online.com/proddetail.php?prod=TW28 are saying you cannot ensure correct bonding between plywood layers amateur skills and tools, and are building from a single 1/2 plywood as core, and put enough biaxial fiberglass layers each side of the plywood, up to the required strength. The biaxial glass stratification can be correctly checked.

Now, it is YOUR boat.

 

Yet another one. http://www.emkaywoodcrafting.com/puffpage2.html

But the forward bottom looks more like cold molded with plywood instead of veneer than with true plywood sheets on frame.

 

Thanks everyone for the suggestions. The bottom has the most curvature, so perhaps 3 layers of 6mm, and a layer of 12mm and 6mm for the sides, as per Steve's suggestion - although I have already rounded up Dave Gerr's scantling number and some of the bulkead and stringer dimensions, so I don't think I will need an extra layer on the bottom. I don't think the curvature at the bow is so extreme that I will need to cold mold strips of ply, but I suppose that is always a fall back position.

Interesting that the guys at boat plans online build up their panel thickness with bi-axial glass - I was wondering how they acheived the required thickness.

In terms of laminating layers of plywood on top of each other, is the best method to drill small holes at regular intervals all over the plywood panel and squeeze until the thickened epoxy oozes out? ... and any suggestions on how to clamp the panel over the top of another already bent panel when laminating?

 

Brands01, you could vacuum bag the extra layers of ply to clamp em, its pretty hard to pull up whole sheets with just screws or staples & not get voids, & even with the vac bag I'de still stick in the vent holes as you suggest. Working with glue slathered big sheets isnt a fun thing without extra hands, you might be surprised at how fast you can hang up narrower (9-10") strips for a lot less strain & even stop for lunch. You might also want to check out the kurt hughes construction video- they do some massive epoxy ply layups on it with full sheets. I like that forward raised deck style of vessel, looks nice to me Regards from Jeff.

 

As fcfc said we start with 1/2" panels at Devlin and add layers of 1/4". On boats in the 30' range we go with 1" total thickness on the bottom panels and and 3/4" on the side panels.
http://www.devlinboat.com/constructiondevlincrown32.htm
I guess the thickness of the hull will determine how much internal structure you will need. At the link above, three of those bulkheads are OSB and will thrown out after roll over. This gives fairly large areas inside the hull that are uncluttered with full-size bulkheads. Structure will be added later with seating counters etc, and all these are glassed in as well which will add strength. I am just the builder, not the designer so I may not know as much as I think I do, but I just thought I would throw that out there and give you some more things to think about.
---Joel---

 

I did not pay attention to the stern when I first posted. I doubt even 1/2" will make that bend (even okoume plywood) On our fantails we stop the 1/2 panels near a rear bulkhead and go with 1/4". Even though the CAD gives us the panel loftings, they never seem to work properly so I always ignore them and bend the plywood around the stern

scribe and cut. It does take some work to get the bottom panels rigid enough to do this.
---Joel---

 

Thanks Waikikin for your complement on the hull - I like the raised deck style too - I haven't seen one modern boat of this style in Australia, so I thought I'd change that .

Thanks Joel for your advice. I have read devlin's book and noticed that you guys stack up the layers on your fantails - it was actually the fantails at Devlins that inspired me to do mine - I love the idea of being able to disguise the cines at the stern with a curve.

The angle at the stern is quite large. I have made a model of this hull from the developed plates on freeship, and we found that bending the panels at the stern wasn't difficult at all. We were using very thin model aeroplane plywood, but it occured to me that the larger the stern angle, the less the curvature is as it is spread out over a larger distance. Your experience will be much greater in this area than mine, and I would be very interested to hear your thoughts on it - whether you have noticed if it is easier or not to bend the panels at the stern of fantail boats if the stern angle is greater or less.

Either way, I think you are right that 1/2" is going to be difficult to bend around any substantial curve.

 

Nice boat
If you have the book from Gougeon Brothers, they mention some model scales and comparable model ply thickness. They used models to design "tortured plywood" hulls. Comparing your model and your model plywood with theirs will give you a hint.

 

One more, can you find the minimum radius in the back? Thickness = R/25 should be OK, but the pressure you need to bend it may be hard to maintain.

 

Brand01. As I think about it, we used 3/8 Okoume on the Sockeye at the stern. It is really a matter of getting a sheet of plywood on the boat and testing the bend. The species of wood makes a difference as well. We used BS 1088 Hydrotec (they call it "meranti") on the rest of the boat and I don't know if it would have worked in 1/4". The bend is not the only problem, the hardest part is getting the ends to lay flat if you are piecing it together with anything short of a full length scarfed panel.
---Joel---

 

Thanks again Joel, I really appreciate the input.

Just a thought: theoretically speaking, if you could join the two side panels together to make one long panel (joined at the stern end) - do you think this would work - assuming you had enough people to help hold it all in place? This would eliminate the problem of getting the ends to lie flat, but would probably need to use the 3/8" you mentioned.

Another question for you - in Devlin's boatshop, have you ever experiemented with fibreglass splices instead of scarf joints?