Bendability of Robbins Ply for Rolled-in Decks

[Ed]

Curious if anyone has any ideas or thoughts on this.

You will see in picture below:



That this 'Flying Moth' has really nice rolled-in decks.

The original ply was around 4mm and a 3 veneer full-mahogany board like Robbins Super Elite or Super Elite plus, but of course I don't know how bendy it was or anything else about the ply or how it was fitted.

Kevin wants to replace them with 4mm, and most probably with that super-expensive Robbins Super Elite and I was wondering if anyone has any thoughts or experience with bending either the Super Elite or Super Elite plus.

I know that Chris did Spritefull with Elite and had some issues with splitting on the grain (as have I in high impact areas on Jollyboat). But what thickness was that and how easy was it to bend. This bend is really quite tight with a radius of 6-8ins or so. I would imagine steaming would help, but anyone got any experiences they would like to share or advice they would like to give?

Looking forwards to hearing more from anyone on this with experience of bending Robbins ply or fitting rolled-in decks.

cheers

eib

[JimC]

Later this year I hope to have extensive experience in that direction, although mainly with 3 and 2mm. I'm certainly going to try steaming it. Andy Bloodaxe boats says the trick is to staple it in place after steaming and then let it dry thoroughly before going back and gluing it in place. Shall find out how it goes at my skill level...

[Ed]

Yes, that makes sense.

I was reckoning on clamping at gunwale, using steam to push it down into place with wood struts off board-case and then leaving it to take up that shape before removing and refitting with glue.

Just concerned with how easy/hard it will be to bend the 4mm ply through such a tight radius.

eib

[Obscured by clouds]

how about using veneer and laying 3 layers down on the diagonal? I know Robbins do building veneers and I have some nice mahogany stuff from a chap who was building a moth. used a bit when I was stiffening Unit2/7 and it bent in easily, as long as it was on the digonal.

[Rupert]

The bilges of the Pegasus are a shallower curve than that, and Uffa had to get special bendy ply made up, where the plys weren't at 90 degrees to each other.

I've seen bendy ply for sale somewhere more recently, but not marine grade (or pretty) I seem to remember.

[JimC]

The trouble with special bendy ply is that its done with the ply layup, which means it stays bendy, not what you want.

If money is no object then two layers of 2mm ply would surely do the trick, but that's not available in the pretty stuff. It looks like just a 2 dimensional bend though. I should have thought steaming or whatever would be the first place to go.

[roger]

I also have had issues with rolled decks, with splitting, my first Hornet, 140, I used a very thin ply from Robbins I think it was three ml but could have been two. and also with Keto although that is also a problem with the epoxy splitting and damp getting in but always on the curved decks.(why I dont like epoxy coating any more)

Shoestring on the other hand has her original early fifties ply rolled decks which are a much heavier/thicker ply.
When Chris and I re glued her we worked from underneath as we didnt dare remove the decks and there was no clue as to how the ply had been bent around the frames except that it was a very good job and painted all through with I suspect a lead based paint .

I asked on the Hornet forum a while back if anyone could remember how they achieved those lovely curves that were so common on the Doe built and other Horents but there doesnt seem to be any knowledge there.

The pic of the flying Moth is very similar to the Hornet with those frames etc. Shoestring also had buoyancy right through but also had an addition of some bags under the aft rolled decks, I guess to stop her going nose up in the event of her filling with water. Also some evidence of bungie to hold hatcheds in place but this could have been a later addition.

[JimC]

If you want a real nightmare the 11plus has side tanks with a similar curve that have a D section and so roll again underneath to meet the topsides rather than the floor. Great for avoiding those "stamp on the floor" tank leaks, but I have absolutely no idea how you could replace the ply. Building plans suggest that fabricating them was about the first job building the boat.

[Pat]

I saw some stuff at Avon Ply which had rows of cuts down the back to make it roll like lllllllllllllllllllllllllllllllllllllll all fairly close together. Probably no use whatsoever for decks but it did roll round a curve.

[Ed]

Hi Pat,

yes, I have seen this....and also the bendy ply is available from Robbins, but as Jim says it doesn't offer much structural strength....and as Rupe says, its dead ugly too.

It really has to be either one of the Robbins mahogany sheets, Super Elite or Super Elite plus which are both 5 ply

or our stock standard Robbins Elite, which is a 3 ply gaboon ply.

We know the Elite will be hard, but possible to bend into shape and fear that cracking will be an issue in time.

But the question is really about the 5ply mahogany sheet. First will it bend and how will we do so, and secondly will it crack at later date. I don't know and that is kind of the knub of the question.

Fwiw, I think it will be hard to bend, but possible with some steam. I hope it will be less likely to crack, but who knows. At nigh on £100 a sheet it would be good to make the right choice!

eib

[bornagainmothie]

The rolled side decks on this boat look to be a similar radius to the Europa Moth, which I'm sure were only bent 4mm ply over a few frames and stringers. It was only when the Europe rules dictated a flat top and vertical sides either side of the strict radius that builders had to resort to cold moulding with veneers. The Flying Moth frames seem to have quite a soft radius to allow for the natural bend of a sheet of ply.
Its true that a 5-ply will be stiffer but the outer veneer is much thinner on the sapele (tiger) and its much closer grain than the gaboon used for 3-ply so would be less prone to splitting.

Can you try just a small strip of each ply over the frames and see how hard it is to bend around?

There is always the option of fitting 3mm ply then vacuum bagging or even stapling a mahogany veneer over the top.

Pleased to see another Moth being saved

[Ed]

Thanks Lyndon,

that's good to know.

I would think the interior of this is very similar to the Europas, certainly looks like it.

I am coming around to thinking it is just a case of buying the wood and going for it.

cheers

Ed

[Rupert]

Have you a piece of old gash 4mm ply you can try with first? I know the bending might not be identical, but it would certainly tell you if it was going to be impossible.

[chris]

I have done two merlins with rolled sides like this.507 and 36,
507 has the paler super elite 4mm which is 3ply. I think the quality of that particular sheet was not as good as another I had which has not shown such a problem with splitting of the top veneer.
36 was done with the dark one probably gaboon. and that was 5 ply.
I did use steam and found it did very little. My method was to lay a wet cloth over and use a old domestic iron. working down a line following the greatest curve. The advantage of this method is that the steam is superheated and forced in under pressure and easier than setting up a large steam box. but the disadvantage is you work on a limited area. I had the ply supported along the side edges on 4x4" so that I could force it down to the floor as you go. It did something but not a lot. Don't forget that steaming you need to compress the inside rather than stretch the outside. When I glued it down I clamped the bottom edge first and let that set before pulling it round and gluing the rest. Used a couple of bars across so that the frame didn't spring apart.
When we did 36 there was no way the gaboon ply was going to bend to shape . It may have been thicker too but it was stiff. we did put a lot of cuts along the inside to help but I seem to remember in the end I took an electric plane and thinned down where the curve was greatest.
If I was doing it again I would consider two layers of very thin ply. Don't forget that Iska and gently's ply is all only 3/32" !

[JimC]

It occurs to me that if the external veneer was at 90 degrees to the bend, eg across the boat, then splitting over time ought to be rather more unlikely. Of course that's just my theory and I could easily be wrong!... If anything it ought to be stronger too. There was a fashion for tiger striped sapele set cross wise/vertically for a while in Cherubs, and it could look very attractive.