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  #1  
Old 01-23-2008
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Barn door rudder design

OK,

Though I'm definitely a "if it works, don't fix it" kind of guy, I have to wonder about the barn-door style rudder on my Allied Princess, and if it wouldn't be more effective to round off the trailing edge as opposed to having the full-length-bottom-90-degree corner original design. Any arm-chair (or, real!) naval engineers want to comment? I feel having the 90 degree corner swung hard over creates way too much turbulence to be effective (experienced at 40+ knots), plus having the bottom edge at the same depth as the keel invites getting it bogged down or, worse, damaged/pinned in a grounding. Many full keel models have this type of rudder, and just as many have a more-rounded profile trailing edge instead.

What do you think? Did Mr. Edmunds make a blunder on the size/shape of the rudder, or was he right on target?

Cary Stotland
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Old 01-23-2008
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The rounded design is older.. Rawson's changed around 1977 from rounded to squared off. The newer rudder enables better sailing at slow speed. Hal Roth also upgraded his first "Whisper" with "a more modern shape" recomended by the designer.
Don't know the particulars, but if designers changed from round to square there must be a reason. My guess is a rounded design has more drag, and is less efficient. Its trailing edge is much longer than a square rudder, with less surface area.

Last edited by sailboy21; 01-23-2008 at 01:32 PM.
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Old 01-23-2008
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... and today rudders are back to "rounded" elliptical shapes, I believe to minimize vortice losses off the square tip. (harking back to the Spitfire fighter's wing design) btw- strictly armchair talking here....

Rounding off the tip of your rudder will cost you effective area. It may avoid some of the cavitation you feel at extreme conditions, but the loss of maneuverability in more typical conditions may not be worth it.

You do need a real designer's take on this.
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Old 01-23-2008
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Faster: are you confusing spade rudders with a nice foil shape to the huge glass/wood/metal sheets some of us have hanging off the end of our keels? I think those are two different universes! The only high volume full keel production boats I know of (Island Packets and some Pacific Seacraft) have a squared off barndoor. I think the only reason designs like the BCC have a nice round rudder is purely aesthetics. Not that I am at all an NA or anything.. This subject has come up a lot with Rawson owners. A whole lot of pre 1977 owners express interest in converting their round rudders to the modern square ones, and a bunch have done just that, even some spade conversions! Never heard of anyone going the other way. Would be good to hear the informed answer to this question. Designers & Naval Architechs???
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Old 01-23-2008
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Quote:
Originally Posted by sailboy21 View Post
Faster: are you confusing spade rudders with a nice foil shape to the huge glass/wood/metal sheets some of us have hanging off the end of our keels? ....... Never heard of anyone going the other way. Would be good to hear the informed answer to this question. Designers & Naval Architechs???
True enough, ellipticals are spades.

My post was intended to suggest that he leave well enough alone wrt to his squared off version

Agree (and already said) we need a pro opinion.
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Last edited by Faster; 01-23-2008 at 10:11 PM.
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Old 01-23-2008
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I would guess that squaring the rudder provides more rudder surface "further down" which keeps the rudder effective when heeled. First time I sailed a Laser28 and it was hard over, you could feel the rudder loss and the boat would round right up because of it.

Still guessing--if you were already "keel deep" with a rounded rudder, you wouldn't want to make it any deeper (unless you worked at Beneteau, ahem) so your choice would be to square it up, or extend it further aft (more expensive to start with).
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Old 01-24-2008
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The rudder on the Allied Princess is what at the time was called a 'Constellation Rudder'. It was developed by S & S through a lot of tank testing and was found to require less drag for any given rotating force. The idea was that more of the blade operated in free flowing water rather than in the highly turbulent layer near the surface. Another factor that came into play is that boats of this era, like the Princess, tended to sail at higher heel angles than boat that preceeded that era, or modern boats for that matter. At these large heel angles more of the rudder is operating near the surface and so ventilation of the blade was more likely to occur when heeled. The Constellation rudder placed more of the rudder blade away from the surface.

Of course at sharp rudder angles, these blades were more prone to producing larger tip vortexes, but on a long keel boat like the Princess the tip vortex of the rudder was fairly minimal part of the overall drag and not worth considering.

Anyway, if you chose to do a more traditionally profiled rudder you would need more surface area than the rudder that you have and so would probably see no gain in speed.

Respectfully,
Jeff
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Old 01-24-2008
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Quote:
Originally Posted by Jeff_H View Post
The rudder on the Allied Princess is what at the time was called a 'Constellation Rudder'. It was developed by S & S through a lot of tank testing and was found to require less drag for any given rotating force. The idea was that more of the blade operated in free flowing water rather than in the highly turbulent layer near the surface. Another factor that came into play is that boats of this era, like the Princess, tended to sail at higher heel angles than boat that preceeded that era, or modern boats for that matter. At these large heel angles more of the rudder is operating near the surface and so ventilation of the blade was more likely to occur when heeled. The Constellation rudder placed more of the rudder blade away from the surface.

Of course at sharp rudder angles, these blades were more prone to producing larger tip vortexes, but on a long keel boat like the Princess the tip vortex of the rudder was fairly minimal part of the overall drag and not worth considering.

Anyway, if you chose to do a more traditionally profiled rudder you would need more surface area than the rudder that you have and so would probably see no gain in speed.

Respectfully,
Jeff
Good discussion. The Allied Princess rudder looks very similar to what Ron Rawson replaced his original Garden designed rudder with.

My Rudder (1977 and later):

Earlier:


Allied:

Last edited by sailboy21; 01-24-2008 at 03:16 AM.
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Old 01-24-2008
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Quote:
Originally Posted by Faster View Post
... and today rudders are back to "rounded" elliptical shapes, I believe to minimize vortice losses off the square tip. (harking back to the Spitfire fighter's wing design) btw- strictly armchair talking here....

Rounding off the tip of your rudder will cost you effective area. It may avoid some of the cavitation you feel at extreme conditions, but the loss of maneuverability in more typical conditions may not be worth it.

You do need a real designer's take on this.
I have the other armchair (and thanks for a comfortable design decision)...

If there is one thing is mis-understood is the hydro-fluid dynamics are not necessarily correlated to air - dynamics...

Its the density and the fact that air and water have very differnet effect. For instance water is elliptical in shape..and temps actually elognstate (sp) water molecules - example - fill a glass with water and then add drop by drop... you will notice that the water will bend and statically hold to the point the overhang goes over the rim of the glass...


The newer designs of rudders are longer - and have less width and are rounded - some actually equipped with fins to create vortexes...inj the belief those vortices disturb the friction capability of water (ref the glass example above)...


Under sail - yes - you will get better perf - handling wise at lower rpms - nothing out there substantiates that it actually has an effect and is all hearsay at this point...
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Old 01-24-2008
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Jody,

I think your description of winglets does fit my understanding of the purpose of winglets relative to tip vortex generation. To explain my understanding of tip vortex generation, foils (whether we are talking about keels, rudders or sails, and whether they are operating in a compressible fluid like air or an incompressible fluid like water) have a high pressure side and a low pressure side. Lift is generated by the suction created as the fluid flows over the foil. As you approach the tip of the foil, some of the low and high pressure 'leaks' over the tip of the foil and where the high and low pressures meet, a highly turbulent area exists that leaves the tip as a vortex. The force of dragging this vortex through the water adds considerably to the drag of the foil.

The larger the tip of the foil in relationship to its span, the greater the tip vortex and the greater the drag that is produced at any given speed and side load. This one of the main reasons is why long keels are not as fast or do not go to weather as well as higher aspect ratio, shorter tip length keels.

While it is true that the loading on an underwater foil is ellptical, that does not mean that the keel itself needs to be elliptical to be efficient.

Winglets do two things, they are intended to keep pressure from leaking from the high pressure to the low pressure side of the tip (end plate effect) making the ajdacent area of the keel more effective. They also reduce the size of the tip and so reduce the size of the tip vortex. In keels they are typically made of lead which lowers the vertical center of gravity adding to the stability of the boat as well.

Winglets do not work all that well on rudders since the rotation of the rudder places the winglets across the flow adding enormous drag for little or no benefit. Instead designers of modern rudders use very high aspect ratio foils with pointed tips intended to minimize the tip vortex generated.

As far as I know surface tension has little or nothing to do with the winglets.

Respectfully,
Jeff
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