[ian0]

Kind of a dumb question but I'll ask anyway. If the wind is blowing in the exact same direction as the ship is travelling and you begin to retract the sail - do you recoup some of the energy you use during this retraction process through increased pulling power of the sail? Is the ship being pulled a little stronger during the retraction process or the exact same amount as if the line wasn't being retracted?

Im imagining something similar to when you'r flying a kite and begin to reign it in - it seems like it pulls you towards it.

[kirrent]

Newton's 3rd law. If you're applying a force to reel in the kite, that force is equally propelling you forward. Plus, the faster apparent air flow over the kite will produce more power, which is why sailboarders pump their sails while they race. In the real world, I've only ever pumped a kite like this to momentarily get some more energy into it to help with ram air inflation.

[murgindrag]

I don't think that's quite how the physics works out.

Speed of airflow is approximately constant. Extreme case is reeling it in at 1mph in a 200mph wind. Force is approximately fixed.

Consider that I have an object with a fixed force on it (a good approximation in that case). If I reel it in, I do work equal to force times distance. That work goes into heating up the air. The force on my boat is the same.

I'm not sure it's a problem we care about, but you know how we solve the problem? Collapse (or partially collapse) the kite before reeling it in.

[kirrent]

No the force is not the same. To actually reel the kite in you will need to pull on the lines with more force than during normal constant distance flight. It is this delta that I was implicitly referring to. The work done as you reel the kite in will largely go into kinetic energy for the boat. You're literally pulling on something in front of you.

Your wind figure is crazy large by a factor of 10. Nevertheless, my point wasn't on the magnitude of extra energy produced through apparent wind, it was referring to the fact that apparent will in general produce more apparent.

[murgindrag]

My point was that Newton's 3rd law isn't the right way to think about it. Newton's 3rd law means that if I throw a rock with 1 N-s of impulse, I will also gain 1 N-s in the opposite direction. It's a conservation law: "Equal and opposite reaction."

Yes, you do get pulled forward a little bit more, but you can reel in the kite with arbitrarily little "pulled forward a little bit more" (by pulling in slowly) or arbitrarily much (by jerking hard).

The force on me and on the kite is equal-and-opposite, but it was equal before I started reeling it in too.

[kirrent]

>The force on me and on the kite is equal-and-opposite, but it was equal before I started reeling it in too.

Yeah, of course. The force is always equal when objects aren't accelerating, so when you reel in the kite at a constant speed with an increased force, the force on your ship is increased as well. When you reel in with more force you reel in faster, which means that you can make your increase in force arbitrarily small, but in doing so you'll make you're increase in speed arbitrarily small and make your reeling time arbitrarily large. In doing so, you'll end up accelerating the ship by the same amount.

The work done in reeling in the kite is obviously larger than the kinetic energy imparted on the ship and is minimised by reeling in the kite arbitrarily slowly. That energy doesn't just go into fluid interactions but also the potential energy of having your foil back at the ship as well as the ship's kinetic energy.

All of that is a bit of window dressing. To reel in your kite you need to increase the pull or force on its lines. That increase will equivalently act on the ship. The ship is being pulled a little bit stronger in the retraction process just like the original commenter asked.

[riffraff]

There are designs for wind power generation based on kites that work like that.

You basically let the kite fall, then use its pull to generate energy and make it soar, then let it fall again..

[taneq]

Depends on the speed at which you reel it in but I can't see it being hugely significant. You would, however, recoup that energy when reeling the kite out again if the winch has regenerative braking (assuming the wind speed is constant).

If the wind speed isn't constant then you could save power by only reeling the kite in when the wind slacks off. If you felt especially proactive you could generate power by reeling it out during gusts then back in during lulls.