Here is Medina sail plan showing the geometric centre as designed, in case someone would like to help Andrew with a proposed junk sail with forward raking mast.

If it were mine and if I were to chose SJR I think it would look like this

This shows a SJR sail of approximately the same area as the original rig, with the geometric centre (CoE) aligned with the original centre. The centre line of the mast is shown.

This sail area could be scaled up or down a little, depending on the hight of Andrew's new mast. (Note Andrew, the mast height must be at least as high as the top of the yard, and slightly more would be better. Also, the distance of the boom down to the deck, at the mast, needs to be at least 50% of the width of a panel (or slightly more would be better) in order for the spanned running parrel downhauls to function in the way Slieve specifies. The balance shown here is 33%. Mast rake on this rig  is not recommended.

Note, as this is a smallish vessel, the helm balance can be fine tuned by crew shifting weight from the rear of the cockpit to the front, so there is a tiny amount of wriggle room as to where the mast is placed.

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If Andrew decides to have a relatively high balanced contiguous sail, with some forward rake as suggested by David, there is possibly of even more wriggle room in mast placement, because the amount of forward rake can be varied a little at the design stage, and once that angle has been chosen and mast rake fixed, there is also the ability to move the sail forward or aft a little, to vary the balance a little if further adjustment is needed.

(Neither of these adjustments (mast rake, and fore/aft slinging of the sail) are possible with SJR.)

So there is something to think about.

Hopefully someone else will chime in with a proposed contigous sail plan to suit this boat.

Andrew wrote:

The only thing that is putting me off is that I don't want to move the mast forward much, because there's a lovely hatch there so I'm hoping with a SJR and balance of 35% it'll be OK. I can't ask Slieve because he's away on holiday I believe.

If you really can order a bare tube without any cutouts, then you've hit paydirt! Are there any more such tubes available for other JRA members to buy, or is this a one-off opportunity?

Weaverbird, a 23ft heavier, stiffer boat, has a 177mm dia x 3mm wall tapered tube from Nedal for a mast. This 165mm x 3mm tube from Norsk Hydro will be just fine for a 20ft boat.

This would be perfect - if it didn't have the 600mm cutout. Above and below that, there's plenty of usable length. So the question is: how do you make a strong joining piece? I think I'd try to work something out with a metre length of thick walled 6in tube, suitably padded out to fit into the 165mm tube.