High-Speed Sailing

Now that Sailrocket is back in action, I thought a look at pitch control might be of interest. A sailboat has a normal pitching moment that tries to push the bow down. This is caused by the driving force, which is above the water, opposed by the water's drag on all the parts contacting the water. I make mention of this a bit in my posts and sketches of 12 - 13 Dec 08. Even though the sketches are for Sailien, the problem is the same for all sailboats. Windsurfers solve the pitching problem by standing on the tail of their surfboard, small cat sailors solve it by shifting their weight aft on the windward hull, and larger craft provide some means to keep the bow up (more buoyant bows, etc.).

Sailrocket has an airfoil shaped cross-arm connecting the main hull to the lee planing pod which is designed to provide some life using surface-effect. They are also using a flap on the outboard end to provide lift to the lee pod. Now as the velocity increases, the lift increases and they generated enough lift to overcome the weight of the craft and flip. A new addition to the cross-arm is a flap next to the main hull that will be providing some down-force to keep the main hull on the water. (Refer to their website for photos of this.)

There have been suggestions that automatic controls should be used to control lift on the cross-arm and that appears to be the best solution, although it will be more complex and expensive to do. Ideally the cross-arm lift should be limited to a pre-determined amount, which is never exceeded. Probably the best example we have of this is Trifoiler, which has automatic pitch and roll control provided by the two surface sensors (skis). This system could be adapted to Sailrocket's cross-arm, but then the arm would have to rotate some to adjust angle of attack -- you see how this starts to get a bit complex. I prefer not to have sensors projecting out from the craft and have been exploring other ways to control any aerodynamic lift on Sailien.

One other thing to note for Sailrocket is the new curved main foil. They reported that the main foil had been flexing so the curved foil should handle that. Now when the load on the foil increases, it will flex straighter which will be correct.

If all goes well Sailrocket should be able to capture the outright record this time out. Good luck to the Sailrocket team.

Bob