High-Speed Sailing

What has Sailrocket taught us?

01 November 2009

I am disappointed that Sailrocket fell short of their goal to break the outright record. However, they have been doing an outstanding job and Paul and all the crew are to be congratulated. They have been sailing extremely fast and there are very few craft of any type that can equal or exceed their speed. If you compare true wind speed to the sail-craft speed, only Innovation and Hydroptere are competition; the sailboarders and kite boarders require much more wind to post high speeds.

What have we learned from Sailrocket? The key factor they have demonstrated is that roll stability is achievable, with no ballast. By correctly positioning the airfoil and the windward hydrofoil while also eliminating any leeway resting device to leeward, roll stability is achievable under any wind strength. They have yet to conquer yaw totally, but they do have it under control within a narrow range (as Malcolm discussed in his recent blog post). They actually have pitch under control with the basic design, but the fixed airfoil on the cross-arm proved to be a bit of a problem.

Sailrocket gave us a great demonstration of why a fixed horizontal airfoil isn't the best choice for straight-line speed. It is superb for aerobatics and I'm sure that Paul's flips have done a lot to generate interest in speed-sailing. I would not be surprised if Sailrocket's antics and Paul's lively blog have been more valuable to the sponsors than the outright record done under "stealth" mode (such as Innovation). Public interest is more valuable than a record that few notice. The drama of the effort to reach a difficult goal will generate interest faster than anything.

Paul mentioned something in his last post that intrigued me. He said that Sailrocket seems to have a speed limit at about 52 knots. Isn't it true that the first time they hit 52 knots, Sailrocket flipped? The fix was to add flaps generating down-force to the cross-arm, to counter the up-force generated by it. I suspect there is enough extra drag there to slow the craft significantly, particularly at higher speeds.

Bob

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Paul | Flados@netzero.com

02 November 2009 23:58:39Z

I also noted the “speed limit”. They indicated it would go up to 52 and then additional wind did not help. With more than a few runs where this happened, I seriously doubt that it had anything to do with wing trim, or other issues with loss of “lift” from the wing. There is nothing about the boat planning features that should result in any sudden drag change at 53 knts. With the above, the obvious area of concern becomes the main foil. At 52 knts, the force generated by the wing and opposed by the foil is very high. An increase in wind speed does increase the wing force, but if the main foil is near cavitation, the increased force starts the formation of vapor pocket (this is not air entrainment cavitation) even without any boat speed increase. The vapor pockets increase foil “drag” and they would limit foil “lift”. As long as this is a gradual process that does not cascade into general loss of lift across the foil, the above theory matches what they reported.

Paul | flados@netzero.com

03 November 2009 00:16:08Z

(Continued from above) The fact that the original flight occurred at close to this speed does seem like more than a co-incidence. Normally the inclined foil is providing “lift” that is a force vector pointing at the center of wing. If you loose some of this foil “lift” without loosing any wing “lift” (force pointed at the foil) the forces are no longer balanced. The upward component of the wing force becomes larger than the downward component of the foil force. The net imbalance would tend to cause the nose of the boat to pull up and yaw toward the wing. With the low nose weight, a little bit a wave action and the magnitude of the wing/foil forces, the possibility of launch seems much more understandable. With the forward mounted crossbeam acting as an airfoil generating upward lift (due to increasing angle of attack as the attitude changed), the “launch” turned into much more than just a momentary “jump” or “bounce”.

Paul | Flados@netzero.com

03 November 2009 00:30:46Z

(continued) Foil cavitation due to vapor pockets is a function of fluid, speed, foil shape and foil surface area. A larger foil with a thinner profile would seem to be the theoretical solution. On the other hand, the other choice is intentionally transition over to “super0cavitation”. In this mode air is intentionally introduced across the low pressure surface and the “lift” from the surface becomes more like a planning action. The propellers on the really fast drag boats use this approach. The problem is that the transition to this mode needs to the “planned” and overall design of the foil becomes much more challenging. The various web discussions also tend to indicate that this mode is less efficient (lower lift to drag ratio). This is an area where those who do not have deep pockets are really at a disadvantage.

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wrote on

03 November 2009 05:54:48Z

Paul, good possibility. This is the problem Trifoiler had in the low 40 knots. Hydroptere does not appear to have encountered this problem, so far.

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