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Author
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Topic: More About Rudders
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SuperPiper
Member
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posted September 08, 2005 06:33 AM
  
I just read that rudders typically make up 6 - 10% of the lateral underwater area of a sailboat. This was presented more as an observation than a hydrodynamic rule. So, I had AutoCad calculate the underwater areas for my Sandpiper model. For 2 different assumptions, the rudder areas calculated to be: 13.0% and 16.0%.
http://tinypic.com/dh5yte.jpg These approximations would imply that our rudders are all oversized. They may create more drag than would be required for the amount of lift they provide. On the other hand, a large rudder makes a boat less likely to broach going downwind. And, I am convinced that the Sandpiper rudder is sharing the side load with the keel: the tiller can be heavy even when it is perfectly centred. Making the rudder smaller could possibly increase the amount of weather helm. This calculation may be implying that the keel is too small and not that the rudder is too big. Do you guys find this interesting? IP: 199.243.115.165 |
Darcy
Member
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posted September 08, 2005 09:22 AM
This is really interesting. We're looking at the same issue from another perspective. I had a chance to helm a Shark last Thursday. It's a beautifully tuned boat with a slightly larger rudder as I had mentioned in other posts. The helm was much lighter than Shortwaves'. It had "slight" weather helm but not too much.Shortwave has much more weather helm. We're tweeking it by tilting the highly raked mast forward; trying different angles. We hope to achieve a lighter more balanced helm while still keeping "some" weather helm. It would be neat at the next Rendezvous to perhaps switch boats in some fashion while cruising to our destination. We would all get an idea of different tuning approaches then. Each Owner might also pick up some good advice on how to make their boat go faster from the other Skippers. D'Arcy, Shortwave IP: 65.92.99.242 |
elmet3
Member
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posted September 08, 2005 06:36 PM
That is interesting. I had to decide how large to make my rudder. I elected to make it slightly less area than the standard, hoping that the extra lift due to aerodynamic shape would compensate for smaller area. I wondered what fraction of wetted area might comprise the rudder, but assumed it would be much less than 10% (did not calculate) and assumed that it was a very small part of overall drag. Now I can see how altered rudder might affect speed. IP: 67.107.33.106 |
SuperPiper
Member
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posted September 08, 2005 07:46 PM
So, what if Elmet3 loaned his aerodynamic rudder to D'Arcy and Lynda? We would have a new generation: Sonicpiper, Rocketpiper . . .IP: 67.71.20.89 |
SuperPiper
Member
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posted September 08, 2005 08:30 PM
Xandpiper . . .IP: 67.71.20.89 |
elmet3
Member
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posted September 08, 2005 09:11 PM
Xandpiper!!!!! That's good. IP: 69.220.224.155 |
Darcy
Member
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posted September 08, 2005 11:14 PM
We'd have to install double trapeze to keep Shortwave flat. Then I could add transom eyes for water skiing.D'Arcy, Shortwave IP: 65.92.99.242 |
