Sunday, 25 April 2010

Take Your Partners

The Partner-thwart on the Coot is a curved affair which needs to be laminated up.
The plans say it should be laminated from four layers of mahogany, but hey Robbins plywood is the next best thing, isn't it?

Luckily there is quite a lot of plywood left spare on Alex Jordan's kit so it comes in useful for jobs like this.  

I thought I had nothing better to do than make up a ply/epoxy sandwich and bend it over a block of wood. mmmm more hurry less haste as they used to say. 

The main trouble with this method, apart from the fact that you get less of a curve and more of a kink in the ply, is that once its all dressed up and tapered to fit the gunwales, the upper surface slopes downwards towards the stem. See below.    

The reason for this is that I started off with a section of a cylinder which I tapered to fit the gunwales and so it tips forward.  Trust me.

The answer, I realised looking at it for a while, is to create a section of a cone.
Back to the Medium Density Fibre-board.

The amount of crown on the leading and aft edges needs to be the same but the circumference of the curve is different, because the distance between the gunwales is different.

So make up two cross sections and space them the correct distance apart. Cut some holes for your clamps and give it a dry run! Sorry that should have been wet run. 

 
To make it fit,  the ply has to bend in two directions at once, which it normally won't do so it has to be tortured into shape, and a good way of softening up the plywood is to pour boiling water over it. This softens the glue and allows it to bend both ways. 

Take your time and make sure you don't fracture the ply. It will eventually pull down to the shape you want, or at least fairly close.

 
Its not quite down to the marks here, it took extra clamps at the ends to pull the corners right down. Of course quite a lot of this will end up on the floor once it is cut to fit.

Here you can see the conical  section forming.




This is it after it had been glued up and left over night.

There is a bit of spring-back, not too much.


It took a lot of trimming/checking /trimming to get it to the right shape but I was quite happy with the way it turned out. Tomorrow I go shopping for a 2 3/4" core drill.

The acid test








1 comment:

michael b said...

Clever man, Graham!
Cheers,
doryman