Post
by IvanV » Thu Jul 01, 2021 10:15 am
We need to be careful about the distinction between the component of the sailboat's velocity in the direction of the wind exceeding wind speed, and the sailboat speed exceeding the component of the wind's velocity in the direction of the sailboat. Clearly many posts demonstrate an understanding of his distinction, but occasionally people have got a bit mixed up about these.
There is no trouble with the latter, as a sailboat sailing at right angles to the wind demonstrates. The problems I describe in the next paragraph do not occur.
There are two issues when the sailboat tries to travel at a speed such that its component of speed in the direction of the wind exceeds the windspeed. First, at this speed the sail experiences wind resistance rather than wind assistance, whatever the shape of the sail: the sail is travelling through the air, in the direction of the wind, at a positive velocity relative to the air's motion in that direction. So, whatever the shape of the sail, it is experiencing wind resistance. Second, when this happens, a traditional sail cannot fill, and so loses efficiency in harvesting even the momentum from the wind that it was harvesting at a lower speed.
As philip correctly states, neither of these issues relates to energetic impossibility, but rather to the inefficiency of the mechanism of extracting energy from the wind. So a rigid-sailed sailboat does not suffer the problem of its sail failing to fill, which removes that inefficiency issue. The boat is extracting energy from the wind to overcome various air and wind resistances, and so there is no reason it cannot additionally overcome the air resistance of travelling such that its component of speed in the direction of the wind exceeds the windspeed. All we need is a mechanism for extracting energy from the wind that allows it to exceed that. There is no reason we cannot continue to harvest energy from the wind at right angles to the direction of travel with a suitable energy extraction device.
It is no more problematic than the existence of a glider. A glider has to overcome air resistance in its direction of travel, and typically considerably exceeds wind speed in its direction of travel. I think a glider has a minimum speed relative to the air to avoid stalling: so that would imply it must travel faster than the component of the wind in its direction of travel. A glider harvests energy from the vertical component of air motion, but is designed to do so only when the vertical component is in an upwards direction. So a glider can carry on flying indefinitely, but only when there is sufficient upwards vertical motion in the air for it to harvest energy from. This can come from travelling near a steep hill where the hill deflects the wind upwards. Or it can come from natural convection of the air which exists only in suitable weather conditions, and also depends on suitable ground conditions to generate the convection. Otherwise the glider comes back to earth before very long.