High-Speed Sailing

Now that we've examined how we can use a slower motion to produce a faster motion (post of 6 Sept. 08 "More on Sailing faster than the wind") and we've seen how the wind can "squeeze" the ice/land yacht against the surface it is moving on, let's see how this works in the water.

First let me point out that essentially the same thing is happening in the water as in the air, except that normally the air mass is moving (wind blowing) and the water is still. By the way, if the water is moving (river, etc.) and the air is still, you can sail just as well, the sailboat can't tell the difference.

If you take a canoe paddle, dip it in the water and move it very, very slowly, you will note that it takes very little effort to move the paddle through the water. You can push that paddle in any direction with little effort if the motion is slow enough. However if you push the paddle faster, you will notice that it becomes difficult to move the flat side forward but it's still easy to move the thin edge forward. This difference in the amount of effort it takes to move the paddle in those two directions is what we use to sail our boat in any direction, except straight down wind. The paddle is essentially the same as our hydrofoil (keel, centerboard, etc.) on our boat and it resists going sideways but is easy to move forward. This resistance to sideways motion is very slight at very low speed but becomes increasingly strong as we go faster. Note that this resistance to sideways motion is essentially the same as the resistance of the iceboat's runner and the land yacht's tire to sideways motion, and that the hydrofoil, the runner and the tire have low resistance to forward motion.

I want you to observe that if an ice/land yacht is not moving and you try to push it sideways it will have essentially the same resistance to sideways motion as when it is moving forward. On the other hand, the hydrofoil of a sailboat has almost no resistance to sideways motion when it's not moving, but as its speed forward increases, it's resistance to sideways motion keeps increasing. I alluded to the implication of this in my post of 11 May 08 "Hooking the Water".

As our sailboat's forward speed increases and it's resistance to sideways motion increases, our hydrofoil is "cutting" through the water (essentially the same as our airfoil through the air) and our boat is being squeezed between the wind's force on the airfoil (sail) and the water's force on the hydrofoil.

I hope I have done an adequate job of explaining this; I should include drawings and may develop this more fully at a later date. I suggest you do sketches to better grasp the relationships. Finally, please ask if any point is not clear, since I have tried to sketch this out as briefly as possible.

Bob