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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > I have a new blog entry on rudders
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Thread: I have a new blog entry on rudders Reply to Thread
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Topic Review (Newest First)
11-12-2011 06:56 PM
bobperry Generally you can use 10% "balance" on a spade rudder. If the boat is under 30' don't go above 10%.
11-12-2011 06:25 PM
RXBOT
balance

Bumping this thread to ask a ? about rudder balance. I have a 40 year old MORC style sloop. It was designed by C&C but is not a C&C boat. They put a rudder on it like a hockey stick. I would like to make (transom hung) a more proper foilshape. What percentage of the body can go forward of the pivot point as to the amount aft of the pivot point. Thanks to anyone who can shed some light on this for me.
10-27-2011 02:53 PM
bobperry Len:
You could be right but that technical paper seems pretty convincing to me. It could be a cases of "you see what you want to see" then invent the formulas needed to back up your theory. But in this case we have a very intelligent owner who is convinced the tubercles work and have improved the rudder performance when hard pressed.
10-27-2011 02:40 PM
Capt Len Why do tubernucles have to have a purpose? Ive got toe fungus and and can still gumboot dance. Ruffel's ruffel was thought to be caused by an injury when young but maybe mother nature tries different stuff now and then just like navel architects now and then.
10-13-2011 12:45 PM
ronspiker Bob,

I like your idea of watching what the racers do. That is the exact same reason car makers have racing teams. It gives them a chance to experiment with new designs and improvments. Many times they find that even though its not a good fit in the racing world its is a good fit in the real world. Even the things that work well in racing many times make it to the real world, like fuel injection, manual shifting (without a clutch). Keep learning.
10-13-2011 11:15 AM
bobperry Mid:
If you have ever done any racing you will know that one of the first things you do when you get to the starting area is to lock the prop even when it's a feathering or folding prop. I do know that spinning the prop while sailing can be hard on some transmissions.
Maybe that's in oder to insure that a blade is not down while racing.

Besides, spinning props are noisy.

I've seen "fences" or end plates on some rudders over the years but no vortex generators. If vortex generators were effective on rudders I would suspect we would be seeing them on high performance boats today. We are not. Theory is always fun but these ideas are not new and competitive racers are almost always willing to try so meting new looking for a slight advantage. I design primarily cruising boats but I always keep my eyes open to what is happening in the racing fleet. I could make the argument that paying attention to racing design trends is what lead to the venerable Valiant 40.
10-13-2011 10:31 AM
PalmettoSailor I was thinking of vortex generators more on the rudder than the keel. If they worked as the do on aircraft, it would seem the rudder would be more effective perhaps reducing the amount of deflection required thereby reducing drag.

My reference to the differences in water and air dynamics was informed by prop drag experiments done by Maine Sail. He showed a free wheeling prop has less drag than a stopped prop. In flying prop driven aircraft, we learn that a spinning prop creates more drag than a stopped prop (something on the order of 10% more glide distance). Maybe it has more to do with the vastly differnt forms of props, but I would have assumed the stopped prop would give less drag on a boat as well.
10-13-2011 10:15 AM
bobperry At Carter's office prior to my arrival tghey tried vortex generators on the bottom of the keel. As I recall the shape was like an itsy bitsy keel about 15% of the cdhord backl from the leading edge on the bottom of the tip. This little fin stuck down about 4 to 6".

I don't know if it worked. I do know that it would have been considered by any rule as part of the keel and therefore an addition to measured draft. So even if it did work, which I doubt, it would have not worked better than simply extending the keel fin down another 6" for the same rating. But it was a novel idea. Carter had a lot of novel ideas.
10-13-2011 02:42 AM
SlowButSteady
Quote:
Originally Posted by chrisncate View Post
Back in the late 80's Stuart surfboards did some boards with dimpled bottoms (like a golf ball) claiming the dimples reduced drag. Never caught on, neat looking boards though.
It probably didn't catch on because it would only lower drag at very slow speeds, if at all. For a two-meter board in seawater drag would only be lowered (relative to a smooth board) at velocities between about 1 cm/sec and 30 cm/sec (surfers tend to travel at 5 to 10 m/sec, probably higher on really big waves). Above about 30 cm/sec drag on a dimpled board would be higher than drag on a smooth board. And all that would only happen if the board were acting like what is known as a "bluff body", which it shouldn't (i.e., it should act as a stream-lined body).


Quote:
Originally Posted by chrisncate View Post
I wonder why more sailing designs (boat bottoms, rudders etc) don't borrow from the surfing industry.
Because surfboards are designed to plane, while sailboats (with the exception of a few small dinghies) only truly plane for short bursts, if at all.


Quote:
Originally Posted by chrisncate View Post
I have often wondered about making my Alberg a tri fin for surfing in the ocean...
Good luck with that.
10-12-2011 10:01 PM
chrisncate Back in the late 80's Stuart surfboards did some boards with dimpled bottoms (like a golf ball) claiming the dimples reduced drag. Never caught on, neat looking boards though.

I wonder why more sailing designs (boat bottoms, rudders etc) don't borrow from the surfing industry. I have often wondered about making my Alberg a tri fin for surfing in the ocean...
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