Try contacting Marcus Towell (forum handle Marcus_W_Towell ) using the Forum PM mailing service. Chris Dean and he designed and build the Blade F18 sails and they seems to do very well. He could well help you out. Otherwise why just not contact Glenn Ashby himself ? Otherwise contact Phill Brander, he used to make his own sails as well and therefor may provide you with some helpful comments.

I personally agree with Rolf. It would be a nice experiment if they sail was about to be disguarded anyway. But I'm not sure whether it is an easy mod. Mostly because I fully expect that you'll need to restitch the seams up to, which may be a difficult job to do on a radial sail. And because the leech round for a squaretop sail apparently must be more straight to get the sail to work well. And the standard Taipan mainsail has quite a curved leech. On the other hand maybe if you straighten out the top 3rd of the leech you maybe able to get pretty close to what you need. Maybe all the curvature adjustment that you'll need can be forced into the top by carefully arc-ing the leech there and putting the right battens in. It will be quite a project though with a series of tryouts and intermediate recutting.

The area difference between the standard Taipan 4.9 mainsail and the new F16 compliant mainsails is just enough to simply add some 400 mm to the squaretop and have a triangle of extra cloth run down to about 2.5 mtr along the leech (= top 1/3 of mainsail).

Personally I like my F16 mainsail. I've only sailed with the standard Taipan mainsails on other peoples boats so my experience there is limited, but I have a feeling that the entlarged head on my mainsail really works. Also I got one of the first custom F16 mainsails (build 2003) on my boat and just as with Eric Poulsens boat I got quite a pronounced leech round, the newer mains have a far straighter leech by now. So yes maybe a standard Taipan main can indeed by recut and entlarge while still scoring improvements even though they may be less than getting a completely new main. Also my experience is that improvements are mostly in the control and handling of the sail. Straight line speed in stable conditions seems to be rather comparable to the standard Taipan main.

I do think however that downwind sailing WITHOUT a spi (something I almost never do anymore) is really helped by a large squaretop. It really seems to help induce a large twist along the leech allowing you to optimize drive all along the mainsail. Without it you always seem to either stall the top or undersheet the bottom.

Wouter