RE: new vs. old slalom shapes
It's exactly what I'm saying with my comments above ;
" it's the control and longitudinal stability that are (very real) limiting factors in exploiting this"
- if you can't maintain longitudinal stability, the board will pitch and the planing surface will vary significantly in length (and this area and A/R) as the width over the length of the variance won't change significantly.
Then, as you comment, we have the practical reason for longer narrow board, however the reason for my comment above was to explain the difference between what is theoretically (??) efficent and what works in practice.
This might not seem a big deal, but if you really look inside what is happening with this current trend of new (short wide) vs old (long narrow) slalom shapes, then A/R actually does play an important part (but again, not in isolation) but without a steadier overall trim/ride
(fore/aft , longitudinal stability), any theoretical gain in "efficency" can be swallowed up in practice.
But we have a better trim, we have a planshape offering more efficent (A/R) planing area and we exploit that within practical gains.
Within speed boards (something I have close association with) the trend is again driven by the same theory; when you really study closely the design of the new modern speed boards, you can see a very distinct trend to move more surface(planing) area aft (again, within reason and practical limitations of control ). Yes, the boards are still "narrow" and small but the area is moving back (A/R inreasing) and the boards are working better.
Interesting to note that true speed boards used in superflat* water actually suffer less longitudinal stability / trim issues than (say) a hi wind slalom operated at much slower (relative) speed in rough open water. Light wind slalom (or FW) boards usually operating in "flatter" conditions (due to lighter winds, less chop) also suffer less longitudinal stability / trim issues than the hi wind slalom at speed in rough open water ; So you can see the trend is that the two "extremes" (lighter wind and speed) can explore and exploit the A/R efficency easier/further than the hi wind slalom design.
Looking (in finer detail) across the iSonic product range, you can see the development of this trend vs board size (or intended usage range) subtly but clearly in combination with other design factors
(like the ability to still be able to turn a slalom board at speed).
Hopefully this helps explain a bit more of what is really going on in both theory and practice with the newer boards.
Cheers ~ Ian
*keeping it simple for the discussion but noting at hi speed conditions, the water is never superflat due to surface chop, but the nature of this chop (+ subsequent trim) issues is quite different to open water hi wind chop.