How sailboats sail

Steve Darwin · Post by **Steve Darwin** » Dec 5th, '08, 12:12

I found this article "The Physics of Sailing" in the February 2008 issue of "Physics Today" and thought it might be of interest to readers of this board.

The link is to the magazine. Enter "sailing" in the search box to access the full text and figures.

http://www.physicstoday.org/

John Vigor · Post by **John Vigor** » Dec 5th, '08, 13:51

Well, there. What did I tell you? I was right after all about locking your prop and the helicopter theory.

I am very gratified to have my theory confirmed by no less a personage than Byron Anderson, a sailor and nuclear physicist who is chairman of the physics department at Kent State University.

Buried in his wonderful dissertation on the physics of sailing is this equation:

R = (LÏ…)/(Âµ/Ï

mgphl52 · Post by **mgphl52** » Dec 5th, '08, 14:08

Hmmm, that must mean that my knot meter has some unworldly virus since every time I attempt to prove John correct by locking the prop, KAYLA responds by slowing down. Or, just maybe, there really is something about the fact the air can be compressed and water cannot!

-michael (who will be sailing tomorrow, foot-loose & free-wheeling, as always)

Duncan · Post by **Duncan** » Dec 7th, '08, 17:38

John Vigor wrote:...vortical vertices...

That would be a bit like the turbulent tips of the ears of a wascawwy wabbit as he wuns away?

darmoose · Post by **darmoose** » Dec 8th, '08, 16:45

While never wishing to detract from anyones cleverness, wit, or endearing charm, nevertheless, I think it must be true what they say about the excessive rain in the great northwest and soggy grey matter syndrome.

I heard a quote recently that said that "if you have a theory that won't stand up in the practical world, what you have is a bad theory"

Meanwhile, what i have for all the coolaid afficianados is this little challenge.

One million "Boat Units" to the one who can reach down to the shaft of a sail boat, under sail, and freewheeling its propeller through the water at any constant speed above 3 or 4 knots, and slow (or stop) the RPMS of that shaft by releasing (or relaxing) your grip, thereby proving that the slower spinning (or stopped) propeller creates less resistance than the previously freewheeling propeller.

Now I think I will go back to sleep, and hope this silliness (as entertaining as it can be) self dissapates.

Merry Christmas to all

Darrell

M. R. Bober · Post by **M. R. Bober** » Dec 8th, '08, 17:52

darmoose wrote:While never wishing to detract from anyones cleverness, wit, or endearing charm, nevertheless, I think it must be true what they say about the excessive rain in the great northwest and soggy grey matter syndrome.

I heard a quote recently that said that "if you have a theory that won't stand up in the practical world, what you have is a bad theory"

Meanwhile, what i have for all the coolaid afficianados is this little challenge.

One million "Boat Units" to the one who can reach down to the shaft of a sail boat, under sail, and freewheeling its propeller through the water at any constant speed above 3 or 4 knots, and slow (or stop) the RPMS of that shaft by releasing (or relaxing) your grip, thereby proving that the slower spinning (or stopped) propeller creates less resistance than the previously freewheeling propeller.

Now I think I will go back to sleep, and hope this silliness (as entertaining as it can be) self dissapates.

Merry Christmas to all

Darrell

Indeed, another beautiful hypothesis slain by a single ugly fact.

Mitchell Bober
Sunny Lancaster (where my head is spinning, but at least I'm no drag.), VA

Bill Cochrane · Post by **Bill Cochrane** » Dec 8th, '08, 18:48

One million "Boat Units" to the one who can reach down to the shaft of a sail boat, under sail, and freewheeling its propeller through the water at any constant speed above 3 or 4 knots, and slow (or stop) the RPMS of that shaft by releasing (or relaxing) your grip, thereby proving that the slower spinning (or stopped) propeller creates less resistance than the previously freewheeling propeller.

I will completely agree that releasing/relaxing my grip won't stop anything.

Grabbing the shaft and squeezing hard, on the other hand, might or might not. But I doubt it, otherwise prop driven alternators wouldn't work very well.

What grabbing said shaft does to boat speed is another matter. But I think we've had that discussion....

(Easy to tell when most of the boats are laid up for the winter, isn't it????)

M. R. Bober · Post by **M. R. Bober** » Dec 8th, '08, 20:03

Bill Cochrane wrote:
One million "Boat Units" to the one who can reach down to the shaft of a sail boat, under sail, and freewheeling its propeller through the water at any constant speed above 3 or 4 knots, and slow (or stop) the RPMS of that shaft by releasing (or relaxing) your grip, thereby proving that the slower spinning (or stopped) propeller creates less resistance than the previously freewheeling propeller.
I will completely agree that releasing/relaxing my grip won't stop anything.

Grabbing the shaft and squeezing hard, on the other hand, might or might not. But I doubt it, otherwise prop driven alternators wouldn't work very well.

What grabbing said shaft does to boat speed is another matter. But I think we've had that discussion....

(Easy to tell when most of the boats are laid up for the winter, isn't it????)

"It's like deja vu all over again." (popularly attributed to Yogi Berra)

RESPITE was hauled and blocked today. Vigor, prepare to repel boarders.

Mitchell Bober
Sunny Lancaster (where all of the water has gone out to sea), VA

drysuit2 · Post by **drysuit2** » Dec 9th, '08, 13:12

Wait. You mean to tell me it's not magic?

wingreen · Post by **wingreen** » Dec 9th, '08, 21:56

It's worth reading. Thanx for the cite.

wingreen · Post by **wingreen** » Dec 9th, '08, 22:05

M. R. Bober wrote:I heard a quote recently that said that "if you have a theory that won't stand up in the practical world, what you have is a bad theory"

One of the more amusing conversations I overheard lately was a college student talking to her professor (a friend) about her failing grades on math tests. She was very adamant and sincere in her belief that math was one of her best and most favorite subjects, it's just that she "doesn't test well".

When she left I told my friend that when I was a student we called that "being bad at math".

darmoose · Post by **darmoose** » Dec 11th, '08, 15:47

On 12/29/06 at 4:47pm you wrote...

"What my critics overlook, in their eagerness to conduct their own auto-da f nd have me publicly burned at the stake for my heliheresy, is the fact that a helicopters blades do not reverse in direction whether it is rising under power, or freewheeling in the autorotation mode. They spin exacly the same way. The very same thing thing happens to a sailboats propeller. It does not spin in reverse when you put the gear in neutral and the boat drags it through the water."

Heres the coo- day-ta question.

Do you realize, that while the helicopters blades do spin in the same direction when going from power to autorotation mode, the pitch of the blades is REVERSED from left hand to right hand (or visa versa) in order to provide the necessary resistance? Without this change in pitch, the helicopter would drop like a rock.

No propeller on a sailboat does this. Indeed, to simulate the helicopter, in autorotation mode, the propeller on our sailboat would have to spin in REVERSE (or REVERSE its pitch), like your helicopter.

Doesn't this error REVERSE your thinking?

Think about this and let it sink through, while you and we are all still around St. John. Get back to us at your leisure.

Darrell

mgphl52 · Post by **mgphl52** » Dec 12th, '08, 10:22

While John is so consumed in testing this theory, may I suggest that he also test the helicopter in the same environment, i.e. underwater.

-micheal, the full-battened-foot-loose-free-wheeler

M. R. Bober · Post by **M. R. Bober** » Dec 12th, '08, 11:21

wingreen wrote:
M. R. Bober wrote:I heard a quote recently that said that "if you have a theory that won't stand up in the practical world, what you have is a bad theory"
One of the more amusing conversations I overheard lately was a college student talking to her professor (a friend) about her failing grades on math tests. She was very adamant and sincere in her belief that math was one of her best and most favorite subjects, it's just that she "doesn't test well".

When she left I told my friend that when I was a student we called that "being bad at math".

Your attribution should go to darmoose. My quote was "Indeed, another beautiful hypothesis slain by a single ugly fact."

Mitchell Bober
Sunny Lancaster ( where accuracy and neatness both count), VA

Matt Cawthorne · Post by **Matt Cawthorne** » Dec 12th, '08, 13:17

No propeller on a sailboat does this. Indeed, to simulate the helicopter, in autorotation mode, the propeller on our sailboat would have to spin in REVERSE (or REVERSE its pitch), like your helicopter.

While in hover or level flight, the air flows down through the rotor. The pitch of the blades needs to deflect the air down and is set accordingly. While in autorotation, the air flows up through the rotor. The pitch is changed in order to maintain rotor rpm. In fact when this happens, the angle of attack for the airfoils remains positive and lift continues to be generated.

A sailboat propeller is designed to generate thrust while rotating in a direction according to the hand of the pitch. The angle of attack on the blades is affected by the direction the water is flowing through the propeller, the speed of the water and the speed of rotation. When power is applied, the prop spins faster and a positive angle of attack is achieved. When the boat is in neutral and there is some force slowing the prop down like friction, the prop slows down and the tangential velocity of the blade tips is decreased while the axial velocity is maintained by the sails. The vector that defines the angle of attack is reversed. The result is a reversal of the sign of the force.

The Cape Dory Board