AO is walking down the square-cube law just like we F16's did and so it should end up close to F18 performance.
I'll explain :
Ratio lengths
AO/F18 = 4.3/5.52 = 0.7790
F16/F18 = 5/5.52 = 0.9057
Displacement ratios when assuming all hulls are a downsize F18 hulls
A0/F18 = 0.7790^3 = 0.4727
F16/F18 = 0.9057^2 = 0.7429
Real combined weight ratios
AO/F18 = (76 + 80) kg/ (180 + 150) kg = 0.4727
F16/F18 = (107 + 140) kg / (180 + 150) kg = 0.7485
Are you noticing something here ?
The ratio's as good as identical ! Meaning that one could just scale down an F18 hull to get an F16 or AO hull. Physics models now predict that wetted surface area's and wavemaking drag (hull related drag) will be related roughly by the following ratio's
Length ratio^2 =>
OA/F18 = 0.7790^2 = 0.6068
F16/F18 = 0.9057^2 = 0.8203
The size of the rig determines the drag magnitude of the daggerboards and of course of the drag of the rig itself. These three main components (hull drag, daggerboard drag, rig drag) dominate the total drag of a catamaran. So if the size of the rigs are close to the ratio's given above for hull drag than all considered boats will have very comparable drive to drag ratio's. And this leads directly to the conclusion that all considered boats will have similar speed potential.
F16/F18 (rig) = (jib) 3.65/4.15 = 0.8795 (main) = 14.85/17 = 0.8736 (spi) = 17.5/21 = 0.8333 (They are all relatively close to 0.8203
AO/F18 (rig) = (rough combined jib + main) (13+0)/17+4.15) = 0.6146 (spi) 9/21 = 0.4286 (Drag ratio was 0.6068)
Note how the upwind area of the AO and F18 are very close to their respective drag ratio's. Only the spi ratio is significantly lower.
First conclusion is that the AO and F18 (as well F16) will have very comparable upwind speed as long as sea state and wind conditions don't hinder the short hulled AO to much. This simply because they all share the same "drive to drag" ratio. Downwind the AO is expected to be a little disadvantaged
So it is not at all surprising that the AO is achieving these results. Actually if you remember I wrote you that in an e-mail two years ago. You are walking down the length-area-volume path that the F16's have taken several years ago and the results once again underline the thruthfulness of this approach. Before the F16's , Frank Bethwaite wrote about this path and he called it the square-cube law. It is a very powerful design law in sailboats. Especially catamarans as they don't plane well.
The reason why this result surprises so many people is because for years designs made 14 and 16 footer without understanding a damn what makes these boats "tick". If you go shorter you have to lighter. If you go lighter you have to go shorter. Or else you'll risk overdosing on wetted surface area and kill performance.
The only drawback of this law is that pitching stability is reduced while keeping the same performance. The shorter boats tend to sail more "lively" and you'll have to move more about as a result on shorter hulled boats.
So in short on these physics models I predict that the AO F14 will end up with a rating close to the F18's and F16's as long as the AO design can keep the single drawback (increased pitching) within acceptable bounds. I for one think that the F16's have already pretty much maximized on that when looking at sustained periods of racing. At a certain point the human mind grows tired and concentration drops leading to delayed reactions and thus sub optimal speed in "maximized" designs. The 49-er skiff is a good example of that and I'm told their cup races are relatively short as a result.
Sorry about this all but the engineer in my wanted to have a say. I hope you all found this interesting reading
Wouter
