High-Speed Sailing

I've been mulling over Sailrocket's last flip and I realized that I left out a key diagram when I posted the force diagrams for Sailien on 12 Dec 2008. Here it is and here's the key: The long blue line represents the airfoil (viewed edge on). The attached blue arrow represents the "lift" the airfoil is generating. The small black angle in the lower left represents Sailien's leeward foils or Sailrocket's lee planing pod (disregard its attached red arrow). The green line represents the windward hydrofoil and its attached green arrow represents its "lift" due to the water's flow. The dashed black line shows how the lift from the airfoil acts against the center of effort of the hydrofoil. The black arrow represents the force of gravity and its center. Disregard the blue dot, blue arrow and the red dot. Refer to the earlier diagrams and discussion for more info.

Now the fun begins: note that the windward hydrofoil is not parallel to the airfoil and its lift arrow is not in line with the dashed line. This is the situation Sailrocket encountered when the foil mount failed, causing the hydrofoil to become more vertical. The bold red arrow represents the resultant force from the airfoil's lift and from the hydrofoil's lift. I believe this is the force that launched the nose of Sailrocket. Once the nose was up, the angle of attack on the cross-arm wing was great enough to overcome the down-force generated by the flaps, and Sailrocket took off.

If you look carefully at Sailrocket's video of the last flip you can see the craft hit a small wave; the nose bumps up and drops back down. That wave did not launch the boat; was there a second wave? I cannot tell from the video, but unless the second wave was larger than the first, it would not have launched the boat either. I suspect the launch was not wave related but was totally due to the hydrofoil coming loose, changing its angle and thereby forcing the nose up.

Did the foil break loose because the load (center of effort) shifted lower on the foil because the wave bounced the top section out of the water? No, if the top section of the foil came out of the water, the total force the foil was generating would have lessened, decreasing the force on the mounting point. The only way the foil load could have increased, would have been if the velocity increased or the angle of attack increased and I believe that inertia rules that out (considering the short time span between the first wave and the launch).

If the first wave had any bearing on the launch, I suspect that the shock load from hitting it caused the already over-stressed foil mount to fail. The foil mount may not have been strong enough to begin with or it may have been damaged previously and that damage went undetected.

One more point I'd like to make. Many think that the inclined airfoil is creating an up-force on the craft and that the inclined hydrofoil is creating a down-force on the craft. In fact there is no up-force or down-force from those two lift forces. As long as the lift forces are in alignment, they cancel out and the craft does not "feel" them (except as a tension between the two foils). What counts is the resultant between those two lift forces, it should be zero, but if the alignment goes out, the resultant increases and the direction it points is the direction the parts are going to move. To fully understand what's going on you must account for all the forces involved and how they interrelate. This is a lot easier than most think since we only need to track the major forces (which I've been describing).

Bob