That's interesting that even up on foils they were slower, but maybe the foils were too small and required too much angle of attack (and thus drag) to get it airborne? Of course if you go with longer foils you also incure more drag...it sure seems to work for the Moths, I wonder why not for the cats.
Weight is one big thing working against the catamaran....it does have twice the hulls...and if you think about it, if you're going to try and spend most of your time airborn, why have two hulls?
Next you have a LOT of power in the rig. The foils have to resist that power that tries to press the leeward hull into the water (and tries to lift the windward hull)..so the lifting force required by the foil increases dramatically because of the lever between the foil and the sail power (beam). As a result, having to lift more weight means the foil has to be larger (or at an extreme angle) and more inefficient. A moth foil doesn't have to deal with but a fraction of the leverage forces since it is mostly in column with the sail drive.
Here's a quick video from Steve from their first sail on Aethon. Hopefully more to follow. I hope to get done next week to look at the boat up close. Will see what time allows.
Steve Killing (The designer for the Canadian team, I think) wrote a 16 page article which I found somewhere on the net (I`m really good at finding stuff, and really bad at remembering where.) It was titled "THE 19th CHESAPEAKE SAILING YACHT SYMPOSIUM" - ANNAPOLIS, MARYLAND, MARCH 2009 - ALPHA AND ROCKER - Two design approaches that lead to the successful challenge for the 2007 International C-Class Championship.
It is a fantastic article, detailing almost all the aspects of the design of both boats (They are actually identical - one foiling, one not. They tested them both against eachother and chose the faster boat (non-foiling) to challenge Cogito. It is amazing how much info he is prepared to share, quoting Naca specs for the foils & daggerboards, Lift/drag charts, Data tracks, construction details, GPS Polars, and more. Some of his comments : "Two boats were designed for this challenge - the first was Alpha, a wingsail catamaran, which incorporated some new thinking on sail configuration and hull shape. This made it a successful evolutionary, but not revolutionary, step from the current state-of-the-art boats. Compared to Cogito, the main rival, she had a taller, narrower and thinner wing, lighter construction and more circular hull cross section. The second was Rocker, the bold hydrofoil catamaran, using hulls from the same mold as Alpha and an identical wingsail as the driving force. The daggerboard hydrofoils automatically controlled the ride height with trim tabs, while the control of the rudder elevators was left to the helmsman. Although she flew well, was very controllable, and was spectacular to watch, Rocker could not match the 20 knot plus speed of Alpha." and : "After this fairly careful experiment in foiling, we are comfortable concluding that hydrofoils don’t perform in this configuration on a C-Class catamaran. There is no doubt that fine-tuning our parameters could increase Rocker’s speed, but it will take more than fine tuning to make the required leap in performance. We encourage other teams to learn from our experiments and go forward to find the solution that will push hydrofoiling catamarans to the next level of performance."
I hope he doesn`t mind me quoting this here, but if you have a real interest in this I`d suggest trying to find a copy of the full article. I think it may have appeared on Sailing Anarchy or Boatdesign.net, but I seldom remember anything past last week Wednesday.
The cure for anything is salt water - sweat, tears, or the sea Isak Dinesen If a man is to be obsessed by something.... I suppose a boat is as good as anything... perhaps a bit better than most. E. B. White