Bending plywood

[Deering]

I have a design of a small boat that will require me to bend plywood around a 5 inch radius along the length of the bilge, about 14 feet. Only bending in one dimension - no compounding in this area.

I'm looking at using 4 mm okoume. Would like to do this with scarfed sheet plywood and not get into torturing it or strip planking if possible. Once bent into shape I'll glass/epoxy it.

Is this plan feasible?

[Bergalia]

Quite feasible, Deering - with certain provisos. The main one being having a friendly panel-beater workshop handy, with the various jogs and presses for forming curves in sheet metal.
With such tools available, and a fair bit of experiment, I've been able to 'press' some 'extreme' curves in ply without too much splintering or disintegration.
Assuming you are using marine quality it needs either a good soaking (48 hours at least) or steaming (8-10 hours) and then will prove reasonably maleable. However a 14 foot length may prove something of a challenge. As you are going to glass the finished product why not apply it in shorter (scarfed) lengths to make the task easier.
It may well be possible, with a half dozen willing hands, to use a more 'primitive' method. Use a sturdy piece of pipe with an external radius slightly less than the desired 5 inches (allowing for the ply to 'spring') as a former. Have about 100 metres of strong cord or rope in handy lengths (10 metres) laid along side it, then steam or soak the ply as mentioned before, lay it over the pipe and then have your 'crew' lash it as tightly as possible making sure there is no slippage from the true line.
Walk away and for the next few days sit in a deck chair sipping beer, discussing the weather, or reading back issues on Boat Design Forum.
Or - you could change your design. Good luck.

[Deering]

Thanks Bergalia,

I like your idea of scarfing in place. I'm worried that a pre-scarfed panel won't bend consistently across the scarf - it'll be unfair in that area.

Alas, I do not have access to a sheet metal shop. I'll have to jig it myself. But your pipe suggestion is giving me some ideas.

I've heard of using a clothes iron steamer to locally steam it as it's being bent.

I don't have suitable marine ply available locally, so I have to have it shipped in. My worry is that I'll find that 4 mm can't be bent that far after I've spent the cash to ship it up. Maybe 3 mm is the way to go.

[LP]

Here's a thought. Connect the pipe to a source of boiling water and wrap the pipe with wet rags.

[Bergalia]

LP is on the right track with the steam box. A length of piping (the right diametre for your planking (6inch) poly or cast iron (old drain pipe is ideal); attached to a non-electric kettle. The kettle on an open fire box (bucket with holes punched in) and spout attached to main pipe via hose (firmly wedged and packed in place.) Wrap timber to be steamed in old cloth )cotton ideal) and light fire (with kettle full). Keep topping up kettle until timber is too hot to handle.
You'll find every small boatbuilding yard in the highlands is similarly equipped. And being Scots they later make tea with the unused water....

The steam iron idea is good and could be used to keep timber 'hot' while the team lash it in place. But watch out for their fingers.

Save your money. If you are going to glass over the final product ordinary 'exterior' ply will be sufficient. However - make sure you cut your strip ACROSS the face grain. A 14 foot plank with grain running from left to right. Grain running from one short edge to the next. In other words - hold your plank upright and grain should run from east to west, not north to south.
Cut the wrong way and you'll end up with a bucket full of hot, sticky splinters.
The internal sheets will have grain running the full length of the plank and will take the bend with ease.

Again, good luck.