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Jacques, you may recall that I've been interested in building a ST21 twin cockpit runabout, subject to medical clearance following cancer treatment. I've just now got that clearance, so I want to settle a few things in my mind before ordering a CNC kit once you've got your op behind you.

Following your previous advice, I'll be building a compounding deck using your Jetabout strip planking method.

If each deck beam is made with the same radius and with tops aligned to the hulls sheer line, will that produce an aesthetically pleasing compounding deck on the ST21 hull?

All the best with the op.

Either the same radius or follow the sheer.
The center line of the surface that contains the deck must be straight or have a slight camber.
From that center line, you decide how to draw the camber of the deck. Either put the ends at the sheer or keep a constant camber.
If you use a constant camber, start with the beam at max. width. The deck will end a little below the sheer at the bow and stern.
That will produce a cylinder which is easy to plank.
For the shape, you can use a plain arc or paraboles like this:



I told you this wasn't easy . . . that's why I did not want to design it but as long as you keep the center line straight, it will look aesthetically pleasant.

If I’m understanding this right, if I define the f’wd-aft centreline of the deck as a straight line, I can define the athwartships camber of the deck at any station as a parabola between this centreline and the sheer. Is this correct?

You better get cracking Michael if you are going to join The Bloody BIG Barra Boat at the Hobart Wooden Boat Festival in February 2017.

glossieblack wrote:If I’m understanding this right, if I define the f’wd-aft centreline of the deck as a straight line, I can define the athwartships camber of the deck at any station as a parabola between this centreline and the sheer. Is this correct?

Yes, either a parabola or an arc.
The plank in the middle is called the king plank but I sometimes name it master plank (French translation).

You can either
1. draw that curve at max beam and swipe it to either side to have a constant deck camber or
2. draw a new curve at every deck beam.

Method #1 gives a nicer deck but will not match the sheer, that means you will have small bulwarks fore and aft. I like it and it is easy to build. Look up the definiton of bulwarks.
Method #2 gives you a flush edge but is more difficult to build.

I prefer the flush edge look, so thanks for introducing me to Method #2. I now have a way forward, which is a relief.