rudder size

End plates on rudders can be a two edged sword.

Sure they reduce the energy sapping tip vortex---but when the back end of the boat pitches up and down the abrupt reversals of AOA can create lots of induced drag which may be worse than the tip vortex on an elliptical planform.

 

The whole package does need to be considered. Different applications for different hulls, the rudder hardware also needs designing for vertical loads.

 

Yes. The whole subject is more complex than most people recognise.

 

Simple sailors sail slow shapes sliding sideways .....repeat as fast as you want to go...

 

You might want to look at T-foils, which seem to serve much the same purpose, but have been demonstrated to be faster than rudders not so equipped on some cats. Pitching is an energy robber, and apparently the T-foil equipped boats pitch substantially less..

 

T foils would have to be big if they are going to help prevent the boat pitching.
You couldnt fit them on the bottom of the rudders because that would load up the rudder pintles somthing horrid
The continuous load reversals would eventually lead to fatigue failure and the loss of the rudder.
A rigidly attached T foil under the transom is entirely a different matter.

 

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Paddy, you're right about the rudder having to be very much reinforced but rudder T-foils have been and are being used on a number of modern trimarans and catamarans from beachcats on up to include both Hydropteres and the third Hydroptere maxi.
Pictures: Martin Fischers new experimental tri and the retractable T-foil experimented with on Gitana, Stealth beachcat rudder foils, Hydroptere .ch with retractable rudder t-foils:
click on image-

 

As Doug said, they are being used successfully, though there is some controversy. I can't remember where I saw the thread, but someone with a beach-type cat was promoting their use, and caught a lot of flak from folks who though it would cause more drag than it would save. I might have thought the same thing, but the guy went out and proved the boat was consistently faster with the T-foils even though the skin area on the foils went up considerably. It was the impression of observers that the boat was pitching noticeably less as it went through the chop.

Long ago I owned a Wharram and though I loved that old boat, it was a pitching *****. In light air and leftover chop, it pitched so badly that it kicked all the shape out of the sails and you couldn't get anywhere. Anyway, it made me a believer in designing hulls to be as resistant to pitching as possible, within whatever design constraints were primary. That was a major concern for me when I drew Slider's hulls, and she is very resistant to pitching for her size.