Homebuilt Daggerboard

[john_nohj]

Hi there,

Im going to make my own dinghy daggerboard from a foam or ply core with triaxial fibreglass.
My question is should I tape the edges first and then glass each side, or glass the sides then sand down and tape the edges afterwards.

Going through it in my head I think id get a neater result taping first, but I think it may be stronger taping afterwards?

Can someone advise an amature of the best way forward please!

Thanks,
John_nohj

 

[William_H]

Foam or ply cored dagger board

I don't think it matters so much although I would start with a nice conforming leading and trailing edge. Lay leading edge with glass at 45 degrees to make bending easier. (I am not familiar with triaxial glass)
The trailing edge may best be done with glass laid square to board and simply sticking each side together further aft than the foam or ply core. It might be tricky to get the trailing edge straight. You might place a flat surface at an appropriate angle covered with cling wrap to form one side of the trailing edge under the board.
I would start with a pretty thin foam or ply and have a thick build up of glass. You might consider carbon fibre for the sides as you will want the whole board strong enough for you to stand on in righting the dinghy.
ie glass at least 3mm thick with thicker at sides.Sides should have weave square to length.
So given that your slot is probably what determines finished size start thin.
Plywood may be OK if it is strong enough with just a thin layer of glass. However this may be susceptible to delamination and damage. good luck olewill

 

[Searush]

Wot's wrong wiv glueing two bits of ply together to get the required width & shear strength (maximise the vertical grain)?

Dead quick & simple & the aquafoil profile looks superb as it exposes the alternate grain structures in broad & narrow lines.

 

[john_nohj]

Im planning on using 2 layers of 600g triaxial on each side.

This is actually going to be my second attempt now as the first attempt was kind of a practice and ended up a bit of a mess, but i have learnt how to do things differently now.

It is a hobie tiger catamaran daggerboard so i still have the other one to compare it against, and in my first attempt the 2 layers of 600g cloth seemed to make it stiff enough.

I havent got any experience of shaping foam though. Do you think id get a better result with a ply core?

 

[john_nohj]

Oh and searush, the ply by itself was no-where near stiff or strong enough. The daggerboard slot is only 25mm wide.

 

[Searush]

"Only" an inch? That's 2 chunks of 1/2" ply glued together with the long grain vertical on the outside of both pieces will be extremely stiff - unlikely to flex even when you stand on it like a plank.

My 14' Cheshire cat used 2x 3/8" marine for the 4'6" dagger boards & they only ever snapped once when the boat (some 400lbs + 2 crew) hit something sideways off a wave. But it's your boat & your choice, I'm just surprised you don't think 25mm of marine ply is stiff!

 

[john_nohj]

Well I have to admit I was using exterior ply instead of marine. Cheapskate I know!

The slot is 25mm at the widest point but most of the board is tapered so most of the board is a lot thinner than this. The dimensions of the board are 1600mm (L) x 25mm (W) x 260mm (D). And it is kind of a thin teardrop shape profile.

I dont know if it was just because I was using exterior ply but it certainly didnt seem up to the job without glass.

 

[electrosys]

I'd be inclined to KISS with plywood too.

I don't know where you're based, but builder's merchants such as Travis Perkins and Jewson are currently stocking 4mm Brazilian hardwood ply. What's neat about this stuff is that the plies are almost equal, and they haven't been mean with the phenolic glue. About £10 for an 8'x4' sheet.
I've just made up a commercial transom using this stuff by laminating multiple layers with epoxy - well pleased.
(The stuff from Wickes and B&Q is rubbish - thin outer veneers over a thick core)

BTW - the way to make the strongest daggerboard from plywood is to orient the plies at 45 degrees to the vertical - but you'll use more plywood that way (but you could always 'infill' the centre with offcuts, of course). But - the strongest daggerboard isn't always the safest - there's something to be said for the board breaking-off before the hull sustains damage.

 

[snowleopard]

I built one for my International 14 that did the job fairly well. I started off with 2 layers of PVC foam (Airex) and bonded them together with a thin laminate of biaxial cloth in the middle - wet one piece of foam, add the glass and wet out them place the other layer of foam on top and compress them together (either vacuum-bag on a table or sandwich between two planks with weights).

Shaping the foam was easy but tedious, I use a 115mm angle grinder with a 24 grit sanding disc. I used a ply template for the section and did it a little at a time checking frequently. If you gouge too deep you can always build up with a lightweight filler such as Fillite.

For the outer skin I clamped the board in a vertical position leading edge uppermost and draped a double width of glass over the leading edge. At the trailing edge I clamped it between a couple of pieces of wood wrapped in polythene to ensure there were no voids and the two skins bonded together. Once cured and trimmed a layer of Fillite and sanding with a longboard completed the job.

Mine was a tapered centreboard so much more complex to shape but the same principles apply.

When using triax I'm sure you know that you need to orient one set of strands along the board and the other two at +/-45°. If you can get them, a few tows of carbon fibre between the layers of glass will increase the stiffness. 2* 600 gsm is more than adequate.

A strip of Kevlar tape along leading and bottom edges will make it pretty bulletproof.