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How many clamps?

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GarF

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I'm about to peel off the gunwales from a mirror dinghy and re-glue them. The old glue - probably polyester resin or aerolite has debonded in places and I suspect the only reason they aren't looser is the screws every six inches or so.

Anyhow, off they must come so they can be properly bonded with epoxy. I'm planning to use the existing screws for alignment only since I'm not convinced they will get the gunwale to conform completely to the curve by themselves, and they'll be left in for the sake of authenticity and to save a lot of filling (I think they're probably superfluous with an epoxy bond).

The question is how many clamps to apply along the 11 foot length of the gunwale? Bear in mind that the curve along that length is about 5 inches or so deep, but the piece (about 25mm x 12mm) has been in place for about 30 years!
G
 

Beau

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Should it be epoxied on? I know gunwales on some boats are put on with a more flexible adhesive like Sikaflex but cant remember our old Mirror 10 she is long gone.
 

GarF

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Yep the gunwales are structural (one section either side of the upper edge of the 5mm ply side panel. The stiffer I can make them, the better the hull resists rig tension, I've half a mind to hide some glass fibre in the joint while I'm about it.

MikeG-- one every 4 inches? Or as many as I can acquire/fit...
Cheers
G
 

Beau

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Not sure you will need quite that many. Epoxy can be structural and gap filling with the right additives. It's not like it's a piece of laminated furniture where you are looking to avoid glue lines. If you dont have enough clamps you could improvise. If you cut some C shapes out of some ply that had an opening a little over the width of your gunwales you could hook them over and tap wooden wedges in to clamp things up.
 

deema

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They should be screwed on only. The really top guys used to hollow out the inside to reduce weight! They were at the front of the fleet at the worlds.
 

GarF

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The inside of the screws?! Seriously though, it's more practical to get a haircut before a big event, maybe cut your toenails a bit shorter than usual. The front end(which doesn't get any load) was just about still stuck down once all the screws were removed. The fortunately the ply is only a bit discoloured where the bond was gone. The whole hull should be stiffer once it's all stuck back together.
 

GarF

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Beau":37p43bxu said:
Not sure you will need quite that many. Epoxy can be structural and gap filling with the right additives. It's not like it's a piece of laminated furniture where you are looking to avoid glue lines. If you dont have enough clamps you could improvise. If you cut some C shapes out of some ply that had an opening a little over the width of your gunwales you could hook them over and tap wooden wedges in to clamp things up.

Of course! That was the reason I kept all those scraps under the bench, honest.
 

shed9

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Call into your nearest chandlery and ask for the right adhesive and the right advice in application and how many clamps they would use. You will;

A: Get the right glue without any doubt and....
B: Get advice from people who do this for a living.
 

custard

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With Epoxy you only need cramps to make the workpiece conform to the desired curve, it's not like PVA where you also need the cramps to provide sufficient pressure to achieve a glue bond.

Consequently you might find you don't need that many cramps at all, even the screws on their own might suffice.

The smart thing to do would be to have a dry glue-up first and use that to determine your cramp arrangement. That will also allow you to have everything prepared in advance and not subject yourself to high pressure glue-up stress!
 

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