Twisting/bending 6mm plywood

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Twisting/bending 6mm plywood

Post by PaulM » Wed Aug 03, 2011 3:42 pm

The thread title says it all really..........

In my Int. Moth days I found little trouble in getting 3mm and 4mm ply to take the shape I wanted, but 6mm is a different animal; I need to get a twist into the forward end of a bottom panel and I'm not forcing it down over a mould so can't do it in stages or screw it down. My first attempt at applying leverage found a weak point in the ply, but the epoxy in the repair should have set by now......

Various DIY and boatie forums and advice pages say various things about soaking and steaming - soak it in cold water? hot towels? pour boiling water over? combine all 3? apply hairdryer carefully? Other ways with which people have experience?

All advice gratefully received!


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Re: Twisting/bending 6mm plywood

Post by JimC » Wed Aug 03, 2011 3:54 pm

What are you working on? There might be some mileage in doing it in two layers of 3mm ply. Got to be quite common in the later days of wood Cherubs.

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Re: Twisting/bending 6mm plywood

Post by jpa_wfsc » Wed Aug 03, 2011 4:32 pm

I would call Robins - they seem to know what they are doing...

Perhaps you could then let us know?

Certainly its possible as our Skua is a 'tortured ply' singly chine hull shape home built from 6mm Marine ply. ... SZLpXc8bLA

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Re: Twisting/bending 6mm plywood

Post by PaulM » Wed Aug 03, 2011 10:13 pm

The 6mm is cut, so layers of 3mm aren't an option.

And I'm afraid it's not Robbins marine ply, so I can't really ask them...........anyway the theoretical torsional strength is only of academic interest at present.

So how can I get it pliable enough to twist/torture, without doing permanent damage? And if I soak/steam it, how long will it take to dry out?


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Re: Twisting/bending 6mm plywood

Post by PaulM » Sat Aug 06, 2011 11:42 am

So has anyone actual experience of soaking or steaming plywood, or even of slitting or scoring it to get a curve or twist into it?

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Re: Twisting/bending 6mm plywood

Post by roger » Sat Aug 06, 2011 12:51 pm

My limited experience. On a small patch repair Chris Barlow scored the back of 6ml ply to get it to bend into shape to fitthe rolled decks on my Hornet. It was only a patch about 4 inches square so not really the knowledge your looking for. The decks on said boat are very curved and about 6mm and I can only think they were made on a mould of some sort using steam or wetting (again Chris has used an iron before). The only time I have done curved/rolled decks in ply it was with 3ml and done it sections. ie glued the bottom section pulled over the frame and then glued the next bit. Even then I split one piece of ply and the result was not by any means perfect but that is probably my skill level.

A lot of the early Hornets had very curved decks like Demons boat so the knowledge must be out there somewhere. I will post the question on the Hornet forum and see if any of the older chaps can tell us.
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Re: Twisting/bending 6mm plywood

Post by STEVERESERVE » Mon Aug 08, 2011 9:25 am

try asking on the UKHBBR forum
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Re: Twisting/bending 6mm plywood

Post by billytwiglet » Tue Sep 20, 2011 11:11 am

If the decks are steeply curved and 6mm, they are probably pre-formed; ie layers of veneer layed up with adhesive on a former mould either male/female and maybe vacuum bagged.The finished component once set retains it's shape, cut to size then fixed in place.

You can try cutting lines through the bottom layer of the repair ply (or two) to allow it to bend-trial and error or get some constructional veneer and layer it up in situ. Ideally you should fix a thin layer to the inside of the curve, overlapped, and work up from this. Cut the top layer larger than the desired shape, lay it on top (grain in the right direction) and mark accurately with a fine scriber. Cut out the section with a new stanley blade and set the new piece in. Ideally level but in reality if it's slightly proud you can sand it back rather than if it's too low....
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Re: Twisting/bending 6mm plywood

Post by chris » Tue Sep 20, 2011 2:21 pm

I agree with what people are saying about it being preformed and built of layers.
If its a tight curve 6mm ply may simply not do it. If you do manage it it might be under so much stress internally that it will not give much strength unlike say two layers of 3mm boinded together. I did get some 4mm to bend round the side decks of my Merlin but within a couple of years the outer surface is showing splits along the grain (though I don't think the quality of ply was as good as it should have been as other less curved areas are splitting too.)

A suggestion with may be worth a go is to try some localised steaming with a soggy towel and a domestic iron. This will work well on solid timber upto about half an inch thick though I do not know how ply will like it. Try on some scrap just in case it simply undoes the glue.

It might also work to plane through the top layer of veneer in the side that won't show and then paint epoxy on it to put the strength back.

By the way Robbins do a very flexible ply if all else fails.

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Re: Twisting/bending 6mm plywood

Post by Obscured by clouds » Tue Sep 20, 2011 2:31 pm

Robbins sell building veneer which seems quite reasonable pricewise for this sort of work. You could build the two bottom veneers double diagonal in a cheaper veneer and use the expensive 'for show' stuff on the top, laid in the same direction as the first lay up.Use s/s staple to hold each layer down, and glue up with epoxy or *hoof and horn* if you prefer!

I bought some leftover mahogany from someone who was building a 10sq canoe, and used it in Unit2/7 to strengthen the hull where it had been damaged. It went in quite well, and fitted snugly the the curved inner hull, laid at 90* to the original

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Re: Twisting/bending 6mm plywood

Post by mole » Thu Sep 22, 2011 2:15 pm

Have you consider kerfing?

This involves cutting a series of saw slots along the line of the curve.

When assembling you can use a thixotropic epoxy adhesive in the slots to restore most of the original strength.

A similar technique also works well on honey comb or foam cored panels if you cut just the inside skin of the panel and can achieve almost and amount of curvature or single angle required.

Just put kerfing in google to find plenty of results for all sorts of applications

hope that helps


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