How Wings, Sails, Keels, and Rudders Really Work.

[Bill Roosa]

Which way

OK so the sails get pushed to leeward and the boat goes to leeward but the keel resists. That would make the LRF on the keel by the water a force directed to windward. Hence the moment or couple (of forces), sails to leeward and keel to windward. That makes the boat heel over to leeward.
That is the "drag it sideways through the water" version and did not account for the force that the keel produces by virtue of being drug forward through the water.
So now the keel is going forward and a little to leeward with the low pressure side would be to windward and the high pressure side to leeward. That would produce a LRF directed to windward.

I was assuming that we where actually trying to build (discuss) a keel that would provide some force to leeward, there by helping the ballast reduce heel. Making the boat lighter (less ballast required) and faster...... What you are describing is a keel that only produces forces that actually make the heeling greater. That would require a heavier (or deeper) keel to keep heel at reasonable levels. That is the way my keel works now and you are trying to increase the negative aspects of it????!!!

So, besides the trash collection feature, how is this a better sailboat keel?

 

[Bill Roosa]

more which way

Or are you trying to say that by making the keel more streamlined and increasing the aspect ratio the parasitic drag and wetted surface will be reduced while the induced drag will increase but not to the extent that parasitic drag was reduced.? All this adding up to less overall drag and a faster boat.

There are better ways to reduce parasitic drag that leave me with a keel to put all kinds of useful things like fuel, water, .... in.

 

[tsmwebb]

I was assuming that we where actually trying to build (discuss) a keel that would provide some force to leeward, there by helping the ballast reduce heel. Making the boat lighter (less ballast required) and faster...... What you are describing is a keel that only produces forces that actually make the heeling greater. That would require a heavier (or deeper) keel to keep heel at reasonable levels. That is the way my keel works now and you are trying to increase the negative aspects of it????!!!

No, I think this tread is simply trying to discuss how keels, rudders and sails as we know them work. Indeed, the lift created by conventional keels does make healing greater and does require something to balance it out (like ballast). However, you're also missing an important point. A keel that increases the forces on the boat to leeward will reduce the efficiency of the system. At some point such a keel would reduce a boat to drifting downwind. You can, however, use foils to reduce the heeling moment of a boat. The trick is to rotate the foils so that they are creating a righting moment but not producing force to leeward. Some advanced racing trimarans use foils on the leeward float for just this purpose. There were also links to a similar system designed for fast monohulls in the loop keel thread. These foils are oriented horizontally so that they lift up on the lee side or pull down on the windward side.

--Tom.

 

[Hermit Scott]

Tom Your boat is so nice it's sick! I wouldn't have any reservations about living aboard that. I think I am more of a catamaran or trawler kind of guy, but my wallet is more of a dinghy kind of wallet.

 

[tsmwebb]

Tom Your boat is so nice it's sick!

Well, I did send you to the designer's site and he makes a pile of money on each set of plans he sells... That said, we love the boat. And thanks for the kinds words!

--Tom.

 

[All U Get]

Just for grins

Hi guys,
I've followed the thread and mused "What if my fin keel could rotate to change the angle of attack"; or maybe 'I could change the shape by inflating a bladder on the leading edge of the fin"; how about "Mechanically altering the camber". Some of this research had been tried years ago, but it must have been too expensive. Just offering the thoughts.
All U Get

 

[Hermit Scott]

All You get, Now this will have 8 pages of posts!

 

[Ross in Tampa]

I have always thought of myself as at least semi-intelligent. I just have to say, you all have totally shattered my self image. I thought I had a basic grip on the fundamentals of sailing, and even have a dozen hours of flight training in a Piper Tomahawk. But, WOW, I am an idiot. This exchange of information is exactly why I pop in here every day.

Thanks, you all are truly a wealth of information. I feel like I have just been rushed through another college course.

 

[tsmwebb]

Hi guys,
I've followed the thread and mused "What if my fin keel could rotate to change the angle of attack";

Some dingy classes allow "jibing" boards and 12 meters and IACC boats may use two adjustable underwater surfaces one of which may be a trim tab on the keel. They certainly can work. In theory you don't want to reduce the leeway angle to zero. Indeed, the ideal angle of attack is variable and getting it wrong just slows you down... What these systems lack is something like tell-tails to help with trimming them...

--Tom.

 

[alangbaker]

Alan- https://www.kqed.org/quest/televisio...ics-of-sailing
Me and this engineer from NASA disagree with you. Don't get your underwear in a twist over it. Apparently there are engineers that disagree with him. This argument is really kind of silly. So there is a low pressure above the wing that causes it to rise and air being thrown down behind the wing that casues the wing to rise. Everyone wins yeah!

Your link is broken, but it doesn't matter.

This is unalterable fact:

If there is no air displaced from its path -- if there is no change in momentum, then there is no lift. Period. Full stop.

Anyone who claims that an airfoil can create lift in direction x without causing a net movement of the air in direction -x is either simplifying the matter for the intended audience or just plain misinformed. I don't care what credentials they claim to have. They are simply wrong. That someone from NASA said it doesn't make it any more right.

For the air to create the force of lift on the airfoil, there must be a force created by the airfoil on the air; remember Newton? A force is defined as a change in momentum with respect to time. "F = ma" is actually formally defined as "F =dM/dT".

 

[alangbaker]

Alan- https://www.kqed.org/quest/televisio...ics-of-sailing
Me and this engineer from NASA disagree with you.

BTW, Scott: the NASA airfoil simulator link at that site disagrees with both you and the "engineer from NASA"...

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Play with this Java simulator and very quickly you will see that whatever the shape of the airfoil you create, the streamlines exit horizontally only when there is 0 lift.

 

[Hermit Scott]

Force is also defined as Pressure times area. Please do a free body diagram to illustrate your point.

 

[alangbaker]

This is really semantics but http://www.arvelgentry.com/origins_of_lift.htm does a reasonable job of showing why in a 2D world there is no net downward movement of air in the far field. It isn't as easy to read as Roger's stuff and what the heck all 3D foils do have a net downward movement... And, in the near field Roger is right even in 2d and that's good enough!

--Tom.

Sorry, Tom, but I don't see where that site suggests that. Would you mind quoting the text that you believe makes that statement?

 

[alangbaker]

Force is also defined as Pressure times area. Please do a free body diagram to illustrate your point.

There is no need to do so.

Agree or disagree with each of these statements:

There is a force upward on the wings from the air.

If there is a force upward on the wings, there must be a force downward on the air.

A unbalanced force on something must cause a change in its momentum.

I think you can complete the rest yourself (at least, I hope so).

 

[tsmwebb]

If there is no air displaced from its path -- if there is no change in momentum, then there is no lift. Period. Full stop.

Anyone who claims that an airfoil can create lift in direction x without causing a net movement of the air in direction -x is either simplifying the matter for the intended audience or just plain misinformed.

Yes, it is a matter of semantics. 2d foil section data is presented as infinite span and thus zero displacement. Any real foil with finite span will indeed displace air. Wing designers use wing theory to convert section data to finite aspect data (you might wiki or google Lanchester-Prandtl or lifting-line theory for examples) and are the intended audience. The displacement is accounted for by the wing theory as tip vortexes; the math works nicely that way.

--Tom.

 

[Hermit Scott]

Alan, "Agree or disagree with each of these statements:
There is a force upward on the wings from the air.
If there is a force upward on the wings, there must be a force downward on the air.
A unbalanced force on something must cause a change in its momentum."
All those statements are true. But I don't know what you mean by complete the rest yourself?

 

[alangbaker]

Yes, it is a matter of semantics. 2d foil section data is presented as infinite span and thus zero displacement. Any real foil with finite span will indeed displace air. Wing designers use wing theory to convert section data to finite aspect data (you might wiki or google Lanchester-Prandtl or lifting-line theory for examples) and are the intended audience. The displacement is accounted for by the wing theory as tip vortexes; the math works nicely that way.

--Tom.

Ah!

I see what you're getting at. That's not really 2D at all and the displacement is infinitesimal, I agree, in a theoretical, very 3D airfoil.

 

[alangbaker]

Alan, "Agree or disagree with each of these statements:
There is a force upward on the wings from the air.
If there is a force upward on the wings, there must be a force downward on the air.
A unbalanced force on something must cause a change in its momentum."
All those statements are true. But I don't know what you mean by complete the rest yourself?

I was afraid of that...

If there is an unbalanced for on the air, and you agree that changes its momentum, what must there be?

Displacement of the air in the direction opposite to the lift.

 

[Hermit Scott]

"an unbalanced force on something must cause a change in it's momentum"

How was I supposed to know you were talking about the air? You could have been talking about the wing.

Are you huffing gas or something?

 

[alangbaker]

"an unbalanced force on something must cause a change in it's momentum"

How was I supposed to know you were talking about the air? You could have been talking about the wing.

You mean other than my saying, "If there is no AIR moving downward, then there is no lift. PERIOD."

Are you huffing gas or something?

Nope. Are you always such a jerk just because someone shows you up?