How sailboats sail

[wingreen]

The beauty of the TY,

Locking or not locking the prop....Why?

Just rease the motor and the debate is close.



Happy holidays to all

Marc

Motors? We don't need no stinkin' motors!

[Carl Thunberg]

As stimulating as this diversion into theoretical fluid dynamics is, did anyone notice the photograph in Figure 3 of the article that actually started this thread? I can't imagine the boat speed required to generate a mast top vortex like that! Seriously, take a close look at that photograph. There's a sailboat at the end of every one of those "clouds". That is simply awesome.

Seeing that photograph and watching how the Tacoma Narrows bridge was destroyed by vortex shedding shows how little we actually know about theoretical fluid dynamics. This bridge went through multiple levels of review by people with a lot of initials after their names, and it still failed. This should be an exercise in humility for all of us. Anyway, the video of the Tacoma Narrows bridge failure is here.

http://www.youtube.com/watch?v=P0Fi1VcbpAI

Stan - You ask a really good question and not stupid by any means.

[mgphl52]

Darrell,
You want simpler than a helicopter? How about maple seeds? They're half a propeller but they still illustrate the point. In any case you're suggesting that air and water have different physics. Where's the evidence of that?

Neil,

Have you every tried to compress water?

That pretty describes at least one major difference.

-michael (full-battened, foot-loose & free-wheeling)

PS: If I had a two-bladed prop and a quick-n-easy way to lock it vertical behind the deadwood, I would. Since I don't, I don't.

[winthrop fisher]

teach us some more john V........thank you ......winthrop

so how are you doing?

[Neil Gordon]

Have you every tried to compress water?

That pretty describes at least one major difference.

Sure, everytime I've ever gone off the high diving board and landed belly first.

I didn't say that air and water weren't different. I'm asking if the physics are different. In a different way, Stan asked the same thing.

Modern fin keels give the boat lift and they don't compress the water. Same for rudders... they turn boats because pressure on one side is more or less than on the other and vice versa. Any like an airfoil, turn it too far and it stalls, going from foil to brake.

[Hud Smith]

If you think of a rock sitting in a stream, it has a cone or pyramid of water upstream from the rock. The cone forms several seconds after the rock is dropped into the water. Once the cone has formed, the friction is water against water, not friction against the rock itself.
So maybe a slowly turning (freewheeling) prop creates more drag because it never has time to set up a situation where water is sliding across water- maybe the freewheeler acts like pumping the brakes.

[Oswego John]

Gentlemen, ladies, too,

A year ago I promised myself that I wouldn't get further (farther ?) enmeshed in the debate on locking/unlocking the prop.

I do feel that I should make mention of the interest I have in the subject. It amazes me when I read of the far reaching and thought provoking points of argument made, both pro and con.

Each point made or each question posed is truly ingenious in my way of thinking. How far can this discussion go? Every day it becomes more involved than the previous day.

Now, it looks like we are emerging into a new sphere of contents. We're getting technical to a higher degree. I love it, the clever ideas anew with each day that passes.

Buck up, lads. Keep the faith. Damn the torpedoes.

Or as some wag might phrase it, "Damn that helicopter"

O J

[Duncan]

... I promised myself that I wouldn't get further (farther ?) enmeshed in the debate on locking/unlocking the prop.

Well, me too, but my excuse is that The Vendee Virtual server is making me wait while it executes the 5 pm wind change...

I share the feeling that the discussion is interesting and thought-provoking. It's imperfect, illogical at times, and a bit comical. It sort of reminds me of how I trim sails.

I think there is a point which has been overlooked, though. It's been made, but it's been largely ignored. That's too bad, because it's pretty simple and it explains things in a way any simple-minded sailor like me can understand.

That point is that an unlocked prop spins, and that is the proof that it reduces drag.

("...connecting to vendeeglobe.com...please wait...")

1. A locked prop produces nothing but drag. If the locked prop wasn't there, dragging along, the boat would go faster, right?

2. The force acting upon the locked prop comes from the motion of the boat through the water. The Force acts in lots of magical and mysterious and (to me) incomprehensible ways. Nevertheless, it is clear, even to me, that the boat would go faster if the prop wasn't there.

3. As soon as you release the locked prop, it starts to spin. The force acting on the prop hasn't changed, BUT some of that force is obviously now devoted to spinning the prop. There is no more force acting on the boat, but some of that total force has now started the prop spinning, i.e. doing something other than slowing the boat down.

...digression...(otherwise, the prop wouldn't be spinning, would it? It would just be sitting there, wondering why nobody cared about it, and passive-aggressively dragging along, perhaps moaning and whining a bit, but otherwise just slowing the boat down with its pathetic inability to make any positive contribution).

The spinning, in other words, means that less of the force acting on the prop is dragging the boat. There is no other explanation, since one thing hasn't changed (the force on the prop), and another has changed (the decrease in drag on the boat).

(For devotees of calculus, this would, of course, be the first derivative. In other words, as soon as you release the prop, the boat will speed up, and therefore the total force will increase. Here we are dealing with the instant of change.
Luckily for you (and not for your locked-prop-cult competitors) most of the energy generated by that increase in the Force on the prop will go toward uselessly spinning the prop even faster). This is the second derivative (the rate of change).

Here endeth the Second lesson, Glory be to Neptune, and May his Fortune smile on all Free-wheelers

Or as some wag might phrase it, "Damn that helicopter"

O J

http://www.rchelisite.com/what_is_an_autorotation.phP

The key thing here is that, to autorotate, you reverse pitch to spin the rotors faster. In other words, you reduce drag, by spinning the rotors faster. This enables you to, at the last moment, increase drag/produce lift just before you land.

This means that the sailing analogy is completely backwards, and proves the point why you should Free your Prop!

May the Force be with you.

[mgphl52]

http://www.rchelisite.com/what_is_an_autorotation.php

Duncan,

That is excellent! It should also put a huge damper on the helicopter analogy, one would think. Of course no one ever brought up the drastic difference between helicopter blades and those of say, a 3-bladed sailboat prop... but that, too, would require logic... and using logic ain't near as much fun when the boats on the hard for the winter.

But wait, KAYLA's still in the water - I think I'll go sailing Saturday...

-michael

[darmoose]

Duncan,

you are soooo right, as i have been saying for two years.

the only thing you have wrong is that the helicopter doesn't speed up its rotors to descend slower, it slows them, in order to comply with Newtons third law. I refer you to my other posting of this afternoon entitled "Helicopters and sailboats are the same after all...."

Free your props and speed up your boats

Darrell

[Neil Gordon]

It should also put a huge damper on the helicopter analogy, ...

You're kidding, right?

The helicopter analogy is right on with the spinning prop. The article says it quite nicely: "The principals of how an autorotation work are pretty simple. You simply swap potential energy (height of heli) for kinetic energy (speed of rotors)."

They call it "potential energy," while I call it "potential faster falling." They swap faster falling to blade spinning with the result that the helocopter falls more slowly even before the flare and lift part. Otherwise, the blades spin "for free," which doesn't fit with Newton's laws, at all. So if our props are spinning, where is the energy that spins them come from?

Putting aside locked vs. fixed, can we have a show of hands for how many think that the prop doesn't slow the boat one way or the other?

[Dalton]

Don't you think you should simply do what the transmission manufacturer recommends?

[Oswego John]

Dalton,

Please don't confuse the discussion with logic.

O J

[Duncan]

Ok, first another round of Sam Adams?

It should also put a huge damper on the helicopter analogy, ...

...The helicopter analogy is right on with the spinning prop. The article says it quite nicely: "The principals of how an autorotation work are pretty simple. You simply swap potential energy (height of heli) for kinetic energy (speed of rotors)."

OK, so far, so good. Reduce height, fall faster, speed up rotors: that's the trick.
The autorotation doesn't slow your descent continuously: what it does is "bank" the energy of your faster descent by speeding up the rotors. Less drag, more spin, just like our free-wheeling props.

On the helicopter, this means you can reverse pitch at the last minute, converting that built-up spin into lift.

Putting aside locked vs. fixed, can we have a show of hands for how many think that the prop doesn't slow the boat one way or the other?

Of course the prop slows you down - how in blazes could it speed you up? If it wasn't there, it wouldn't have any effect.

There's no free lunch - anything that gets in the way of the water flowing past the boat slows the boat down. If the prop is locked, it causes nothing but drag. If it is "allowed" to spin, at least some of that drag is released as spin.

It's just not that complicated.

[darmoose]

Duncan

once again you are right except where you say the helicopters blades are allowed to speed up to slow the descent. just like in the propeller on our boat, the slower the blades rotate, the more they want to slow the helicopter.

it is Newtons third law, see "helicopter and sailboats are the same after all..."

check it out

Darrell