I like the foregoing answer. Of course, it is not clear what the questioner had in mind; if one is willing to ignore such things as the speed of light as a limit, and the mechanics of air resistance, and water resistance, and the mass of the vessel and so on, then in a light breeze, you could get as near as you like to infinity, on the theory that your limit is the tangent of your sail's angle to the wind. In practice there are dozens of formulae (you can find several on line) depending on what sort of boat, what sort of water, what sort of hull and sail etc you have in mind. It is all very well to speak of a function of wind speed, but hull speed simply is not a function of wind speed. At best you can say that for given circumstances, an increase in wind speed causes an increase in potential hull speed if nothing goes wrong, such as the wind causing waves. Let the helmsman sneeze, and your "function" changes.

This shows how important it is to put questions clearly. If you don't, they just degrade into nonsense.

But in practice, as the man says, "the subject is best studied from the deck of a small sailing boat".

It's a bit naughty to ignore water resistance.

The foils of a boat (keel, centreboard, rudder) provide directional stability and lateral resistance. Because they are flat, this is highly directional but not completely so.

If you completely ignore water resistance, then you have to allow the foils to exert no resistance either. So your boat just skims directly downwind like thistledown.

Similarly, it is wind resistance against the sails that provides motive power. You can't really say I will have air effects when it's pushing me, but not when it's slowing me.

As always, it's the engineering ingenuity of maximising the upside and minimising the downside that makes it so amazingly clever.

There is a class of "Asymmetric" rigged boats for which the fastest course downwind is to zigzag. The reaching (crosswind) effect is so powerful that it more than compensates for the extra distance covered. Many types exist, but google "Laser 2000" is a start.

It appears to be possible to combine an air and water propeller to make a boat that will (a) sail directly upwind, or (b) sail directly downwind faster than the wind. Don't shoot me, I'm only the messenger! Try google DDWFTTW (and hold onto your hat!)

Whoa! I hear 85% of statistics are made up on the spot!

I struggle with "59% of the kinetic energy of the wind". Obviously not of all the wind, e.g. that in Patagonia. I can believe this fact can be a true mathematical analysis of aerodynamic efficiency, but it still might mean either of two things:

(a) A turbine blade can extract at most 59% of the energy from the air that flows across the cross-section that the blade presents at any one time.

(b) A three-blade turbine can extract 59% of the energy from the air that flows across the disc that the blades can cover in one rotation.

Actually, even then I have my doubts. I don't think a boat sail actually extracts a high proportion of the absolute energy from the wind. What it does is to cause a large change in (directional) momentum of the airflow, which then exerts a reactive force on the boat.

This force is only converted to energy when the boat is moved through some distance. So in fact, the faster a boat moves in a given wind, the more energy it can extract from that wind, so the faster it goes .... (This is such a bizarre thought that I refuse to think about it further.)

Compare with an aircraft wing, which does not extract any energy from the air: it causes a change of momentum in the air, and just captures enough reaction to hold up the weight.

I can't figure why, given the capital costs of the turbine mast and machinery, they don't cram the hub with 21 blades to optimise the extraction rate. Think of those sheet-tin water pump windmills in Westerns, and scale them up.

The bits about apparent wind and chord angle for sails surely also apply to turbine blades. As they rotate, they should continuously move into virgin airstream moving at full speed.

I also can't figure why the mechanical drive at the hub. I believe some helicopter rotor blades are additionally driven by blade-tip vents and the turbojet exhaust is ducted to them through the blades. I would have thought wind turbine blades move fast enough that they could have venturi suction vents at tips and on low-pressure parts of the blade: centrifugal force should augment suction too. This could draw air up the mast and out through the blades, and use an air turbine near ground level to drive generation. This has to be as efficient, and much less engineering cost, than a mechanical linkage with gearbox etc. For a start, somewhere in there the blade rotation (maybe every ten seconds) has to be (variably) geared up 500 times to 50Hz to match the grid mains frequency.

But what would I know?

"If a man speaks, and no woman hears him, is he still Wrong?"