Wind-powered vehicle hits 126MPH, nabs world record

Tim was not too far off, although all the details were wrong. The best heading is actually slightly more than 90 degrees to the true wind. Also tacking (changing direction of the craft) is not necessary. Instead, constant trimming to the sail (wing in this case) is required. And the boat would not start "directly aligned with the wind" unless by that Tim meant perpendicular to the wind.

Any very fast sailing craft (that can sail at multiples of wind speed) will sail fastest on a beam reach (or very close to one) meaning that the true wind is from the side.

The process goes like this:

1. You start with the wing's chord in line with the wind (0 degrees angle of attack). The craft is aligned perpendicular to the wind. As you increase the wing's angle of attack (by sheeting in, sailors say) the wing will start to develop some lift. (If you are on ice the small amount of lift might not be enough to overcome friction of the blades, so you may need a little push.) This lift is directed forward, perpendicular to the wind. In a 30 mph wind the craft should begin to move easily.

2. As the craft moves, the apparent wind (from the vector addition of the true wind and the motion of the boat) appears to move forward. You sheet in more to maintain a fairly constant angle of attack of the wing with the wind. (In this particular craft, the setting of angle of attack is somewhat automatic, by virtue of the tail foil.)

3. The craft continues to accelerate, moving the apparent wind further forward, and you continue to sheet in to maintain an angle of attack.

4. The lift on the wing (which is directed not up -- as in an airplane -- but partly forward and partly to the side) is increasing with the square of the apparent wind speed. So, if you are now going 100 mph (and the true wind is 30 mph) the apparent wind is 104 mph from about 17 degrees off the bow. The lift has gone from (for example) 100 lb with the boat standing still (in 30 mph wind) to 1202 lb. An impressive amount of force. Unfortunately, lift is always perpendicular to the apparent wind, meaning that the lift vector is not mainly ahead, as you'd like, but mainly to the side. Only 359 lb is pushing the boat forward, and 1150 pounds is pushing to the side. Depending on tires, surface, etc. the craft could be starting to drift to leeward, provided it has the right leverages to avoid capsizing.

5. An unfortunate consequence of the high apparent windspeed is that drag has increased just as quickly as lift. Also, because the apparent wind has moved forward, the drag vector (which always aligns with the apparent wind) is well positioned to help keep the boat from going fast. In an idealized world, the ratio of yacht speed to windspeed is then set by the ratio of lift to drag. With tires being pushed sideways, rolling friction, fuselage drag, etc. this is not an ideal world, but in the most efficient iceboats a ratio of 8:1 is possible, in true winds of about 10 mph. This team, back in its Windjet days, was claiming about that ratio.

It is interesting to note that the absolute greatest thrust (lift aligned with craft's motion) available at a standstill was 100 lbs in this example. After the boat has accelerated to 100 mph, 359 lb is available. This starts to sound like a perpetual motion machine, doesn't it?

A surprising peculiarity of a very fast boat (even fast water boats like my Windrocket, which could do about 3 times wind speed -- if everything worked perfectly... which it never did) is that you never jibe to change tacks going generally downwind. (In jibing, the wind comes around from one side of the boat to the other, from behind. In tacking, it comes around from the front.) In very fast sailing craft, the wind is always from in front of the boat even when zig zagging down wind... unless the tack is poorly done and the boat slows. A fast boat cannot be sailed fast straight downwind so is typically sailed about 45 degrees to either side of straight downwind. The distance travelled is about 1.414 times as far as straight downwind, but the speed is 3 times as fast as the wind.

Sailplanes have 60:1 L:D ratios so a slightly better that 4:1 speed ratio seems like it could be improved. However, there is a balancing act between wing length (promoting high L:D) and capsizing moment, which increases with wing length. Loads of compromising and optimizing has to be done, and it is really nice to see that it has worked out for this team.