High-Speed Sailing

Last post we saw how a slower motion can produce a faster motion, but there can still appear to be some real complexity in trying to understand this in the world of sailing. Let's start on the ground, land-sailing so we don't have to deal with the complexity of the water.

Get a child's wagon and a sheet of plywood. Stand the plywood up on the wagon and attach it to form the sail. Orient the plywood at a 45 degree angle to the length of the wagon. Now with the wind coming from the side (90 deg. to the length of the wagon) your wagon will roll in line with the wheels because of the wind glancing off the plywood. The wheels resist going sideways but allow going forward. We can even get an idea of how fast our wagon will sail (roll). Let's ignore normal losses due to friction etc. and also disregard the headwind that we generate by moving forward (called induced wind). With a ten knot wind and no losses we would go forward at ten knots.

Let's look at this in more detail. We're sailing down the street at ten knots, ten feet ahead is an intersection and the wind is blowing at ten knots in line with the cross street (90 deg. to our motion). A child's balloon is floating in the wind coming along the cross street and it's ten feet from the intersection. Our wagon will arrive at the intersection at the same time as the balloon. When our paths cross, the balloon will barely touch the leading edge of our plywood sail and, as we continue forward, the balloon will cross our wagon, just barely touching the sail for the whole width of the sail. From our position on the wagon it looked like the balloon approached us at a 45 deg. angle, continued along our sail and departed at a 45 deg. angle. The motion that we saw the balloon make is the motion that we observe of the apparent wind. However if we were to observe this from above the intersection, we would see the balloon float along the cross street, graze the side of the sail and continue along the cross street, always moving in the same straight line. The wagon of course, continued straight along its course.

Naturally the above ignores the real world losses due to friction, aerodynamic drag, energy transfer from the balloon to sail (which will slow the balloon and alter its path somewhat) etc. but it is close enough for us to understand what is basically going on. The details are the complexities that fluid dynamics engineers have fun with. I will continue this.

Bob