The International Canoe

The sliding seat Andy built for TT which I use on 257 is 480mm. There are ergonomic disadvantages to the wide plank, although most would be minimised if I were better at committing to throwing myself straight onto the plank instead of powering up slowly. I would say it's appreciably more difficult to get the trim forward and bow down coming out of the tack.
OTOH it's good being able to fine tune trim without moving the carriage. I hardly ever sail with feet each side of plank, indeed tend to use ladder even when feet inboard of gunwale. Feet behind plank really trimmed a bit far aft in light.
I tend to think I like the narrow planks when v light, wider one in breeze, but bear in mind I don't sail in serious waves.

Hi Perham
I see that your leading rail is 68, whats the trailing rail depth? I think the lift from bouncing on a wave will come from the angle of the bottom of the seat, my 400mm seat is 90mm at trailing, 50mm at leading, the bottom surface has a 30mm wide flat at the trailing then straight line to the leading edge. It seems to bounce ok on the waves. I have considered a curve fore aft on the underside but don't really want to build it that way. It seems from the replies here that wide is good when windy and narrow good when feet either side.
I will try and take some measurements at the nationals.

Trailing rail on GBR 311 slide is 86mm - so overall it has a significantly shallower angle of attack than yours; also, I suspect, than the initial wedge on Steve Clark's design. Maybe this was a trade-off against the relatively high sheerline of the Morrison 1 designs - perhaps Phil R or Rob M have comments on this?

Perham
GBR311

Sliding seats at Pwllheli. Width varied from 380/385 to 495. The most common was the 385 of the Rob Michael built in a wedge style, then some at 400, mine and a few others again in a wedge style. The C12 is generally a V style, 420 wide and the widest was Arne's at 495 again in a V style. Generally we had moderate wind strengths with quite a confused sea on the first day, I think a longish swell with some chop on the top of that. I felt reasonably comfortable on the 400 wide seat, I understand from Chris Hampe who was using a new c12 seat that the scallop on the end of that worked well, the end of mine being flat.

Alistair