Rudder design

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Quoddy

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Apr 1, 2009
241
Hunter 260 Maine
How important is the rudder design and position in obtaining good windward performance?
 

ghen

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Mar 15, 2009
104
2 216 St. Augustine
It is pretty vital. I depend on the rudder a lot a a significant part of total required lift in my designs. One reason is, when hand steering (correctly) the rudder is not forced out of a nominal angle of incidence range. The helmsan reduces angle when the foil is pressed out of the lift\drag bucket.
 
G

Guest

Rudder

Hi Glenn, would you please explain your last reply in lay language? There have been times (very few) when I sailed Belle-Vie in 25 plus winds, close hauled (about 35 degrees) and full sails. The weather helm was very strong, the rail was in the water and my blood was boiling.

The P42 is pretty stiff and does quite well in the 15 to 20 MPH apparent wind before I feel the need to reef. When the admiral is aboard I always keep the boat at or below 20 degrees heel and will usually pinch in order to keep her comfort level in that range.

Terry Cox
 
Oct 22, 2008
3,502
- Telstar 28 Buzzards Bay
Underwater appendages, like keels and rudders generate lift as water moves over them—which helps prevent leeway. This is particularly true of more modern fin keels and spade rudders, rather than older full keel designs. Keeping the boat's sail plan well balanced, with just a bit of weather helm allows the rudder to stay in the range where it is generating lift to windward instead of just generating drag. If the boat is unbalanced, either too much weather helm or lee helm, then the rudder doesn't generate lift as designed, and the boat's performance suffers.
 
Jan 1, 2009
371
Atlantic 42 Honolulu
... when hand steering (correctly) the rudder is not forced out of a nominal angle of incidence range. The helmsan reduces angle when the foil is pressed out of the lift\drag bucket.
Glenn,

Are you using custom rudder foil sections? Many rudders are still built using NACA four digit foils which have no drag bucket (but friendly stall characteristics). 63 and 64 series foil sections are popular, too and they have drag buckets that are pretty narrow. There are a couple of Eppler vertical stabilizer foils that I know have been used with some success on rudders. But, with those "cook-book" laminar sections unless the rudder is very deep (not always practical) or quite large or very lightly loaded they will operate very near the limits of the bucket to weather even in ideal condition. On a cruising boat that has bottom paint, sits in a slip gathering fouling, may be sailed somewhat out of balance by a helmsperson/autopilot with less than perfect anticipation and so on it seems a stretch to hope for laminar flow using published sections on traditional rudder plan forms on the wind. Or at least, I've operated on that theory. Has the state of the art passed me by (again) or maybe I've been wrong all along? Could you share some of your thoughts on rudder design for performance and cruising boats?

Thanks,

--Tom.
 

ghen

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Mar 15, 2009
104
2 216 St. Augustine
Terry,

When you are steering, you probably let the boat yaw to windward when the helms gathers pressure. If you had pulled the helm down, the angle of attack of the water to the rudder would increase. As that happens the drag goes up but the lift doesn't go up much. Similar to landing an airplane, the flaps are down creating high lift but high drag as well. Landing an airplane you want to slow down but in a sailboat we generally don't.

Tom,

I developed my own sections using a Vacanti software. All the NACA sections were developed in air tunnels and water density makes those irrelevent.
 
Jan 1, 2009
371
Atlantic 42 Honolulu
I developed my own sections using a Vacanti software. All the NACA sections were developed in air tunnels and water density makes those irrelevent.
Glenn,

Not sure the fluid is what makes those older sections irrelevant. Reynolds numbers are Reynolds numbers. A meter long foil going 3.5 m/s in salt water has an Re of 3.5x10^6 which is within the tested range of all the foils I listed. If there is an advantage to the NACA foils it is that they have been tunnel tested and used extensivly in the real world. Going right to production tooling from a virtual foil seems like a leap of faith to me.

--Tom.

PS. If you are comfortable designing section data you might be interested in http://web.mit.edu/drela/Public/web/xfoil/. The code is more modern than the Vacanti Foil stuff but the interface not so nice.

T.
 

ghen

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Mar 15, 2009
104
2 216 St. Augustine
With a lot of success designing race boats I feel pretty confident about it. It is the incompressability of water that makes the difference.
 
Jan 1, 2009
371
Atlantic 42 Honolulu
With a lot of success designing race boats I feel pretty confident about it. It is the incompressability of water that makes the difference.
Your success speaks for itself.

I'm sure it is my fault, but I think we're talking at cross purposes. At low mach numbers compressibility is not significant for foils working in air either. But, whatever the reason, if you're winning you've got fast foils.

Peace,

--Tom.
 
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