A Nice Little Conundrum - Page 7 - SailNet Community
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post #61 of 67 Old 06-14-2011 Thread Starter
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Looking back Ghris Gee our old Kiwi friend was first in with the correct answer. B wins and its all down to apparent wind.

Who the hell is Stuart Walker ?

Andrew B

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post #62 of 67 Old 06-14-2011
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If the race is over, can the winner now spring for the grog? Doesn't matter...boat A or B...start pouring...
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post #63 of 67 Old 06-16-2011 Thread Starter
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Original Question

Boats A and B are identical and sailed by equally skilled crew.

Both are same distance from finish line.

Both are in a ten knot current flowing in the same direction as the boats.

Boat A has ten knot true wind from astern, Boat B no wind.

What will be the result ? A B Tie

Why ?


Chris Gee's answer

A has no wind relative to the boat 10-10=0 so travels at 10 knots over the ground with the current.
B has no true wind but 10 knots apparent wind from ahead from the relative motion of the boat versus the air, against which it can tack at say 5 knots increasing the wind a bit. In any event it can make way relative to the current so must be travelling at more than 10 knots over the ground so will get there first.

The travelling with the current at current speed may not be spot on but all else is. B wins. It all comes down to apparent wind and if you don't get that then you just don't get it at all.

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post #64 of 67 Old 06-17-2011
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I suspect that if you're sailing at 5 knots, the added thrust of the trolling engine (which can only manage 1 knot in flat water) would be insignificant to the overall picture. As an example, given enough time you could probably make one knot with a paddle... could you add to your speed (at 5 knots) with that paddle????


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Originally Posted by Chris12345 View Post
Ok, so then please think about this for a minute, and let me know what you think:

1) Not A or B, but I, sail on my boat in some wind, no current, and I am making 5 knots.
Now I deploy one of these little tow generators, and power a trolling motor with it's power output. (The trolling motor normally pushes my boat at 1 knot.)
How much faster do I get with the extra push from the trolling motor?

2) How is the above scenario different from what you describe for Sailor B, energy-wise?

Thanks!!

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post #65 of 67 Old 06-17-2011
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Quote:
Originally Posted by Faster View Post
I suspect that if you're sailing at 5 knots, the added thrust of the trolling engine (which can only manage 1 knot in flat water) would be insignificant to the overall picture. As an example, given enough time you could probably make one knot with a paddle... could you add to your speed (at 5 knots) with that paddle????
I think Faster missed where you're powering the trolling motor by a towed generator.

Now you are talking perpetual motion. Overall that would be a loss since the system couldn't be 100% efficient.

And that has no relationship to Boat B. We're not talking square riggers here - the modern boat can sail toward the wind. If you want to take it to Physics 101, look up Bernoulli and wings. If you want a complicated discussion Google Arvel Gentry.

If you are looking for a discussion on the principles of conservation of energy and momentum, you may want to consider the energy is introduced by the current and wind.

.

Last edited by ottos; 06-17-2011 at 10:54 AM.
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post #66 of 67 Old 06-17-2011
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Quote:
Originally Posted by Chris12345 View Post
Ok, you got part one right: Every bit of energy that the trolling motor contributes to forward motion is derived, i.e. removed from just that forward motion, by the drag that the towing generator creates.
And more.
Quote:
Now for part two:
How is that different from TDW's boat with no true wind??
The water is pushing towards the finish, the air is offering resistance.
Increasing the air resistance for propulsion (i.e. sailing) is exactly the same as launching the little tow generator:
You are ignoring that the sail is a wing! If what you are saying was true no boat could sail above a beam reach (and no plane could fly)! See figures 6 & 7 here http://www.arvelgentry.com/magaz/How...Gives_Lift.pdf

Wind moving across sail causes lift (horizontally) which draws the boat forward.

And it is relative motion which counts - whether the boat is pushed through the air, or the convective forces in the atmosphere push air past the boat - it is the same.

Quote:
I think it's you guys who have been talking perpetual motion, is it?
If not, please explain part two to me, I don't see any difference:
What mechanism of energy conversion you use does not matter much, sails, a windmill that powers a propeller, etc., etc.: They all take slightly more energy out of the "wind energy", which is supplied by the current in the Boat B case, than they contribute. Even without friction/efficiency loss, they'd achieve a zero overall gain.
Right?
If wrong, where?

This is getting interesting.

[Hint:
You say: "... you may want to consider the energy is introduced by the current and wind."
There is no wind in TDW's riddle. Nor current in mine.
But you deny me use of my "apparent current", while you allow tdw's B use of his "apparent wind". No fair. ]
I think I'm done.
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post #67 of 67 Old 06-21-2011
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Quote:
Originally Posted by Chris12345 View Post
(a) There is no wind, only current. Which exerts a certain force on the (now sail-less) boat, whose amount is determined by current speed and hull shape. This force maintains the momentum of the drifting boat against air resistance.
The boat with apparent wind disagrees with your "no wind" claim. Is he wrong?

Quote:
But where from do you gain extra energy/momentum, that makes you faster than the original, sail-less drift?
The boat who feels apparent wind extracts energy from that apparent wind, which apparently has less energy afterwards.

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