Float rudder size

I am considering converting from a single under-hull rudder to transom hung twin rudders on my tri's floats. I am pretty sure that drag will be less and control better. How do I calculate the size of the new blades?
B

 

Why wouldn't you use the same area as the original rudder? Just out of curiosity, why do you think drag would be less?

 

Doug, my rudder has about a 1.3x1 aspect ratio to keep its draft less than the draft of the hull. About 32" deep and 24" long at the top. It is also 3" thick at the top, angled back 10 degrees and doesn't even get much help from the hull since the hull form is quite V shaped back there. It has a lot of turbulence at any speed over 5kts. I am pretty sure a much thinner high aspect rudder hung from the float would have better control at speed, and unless it was over 6' deep, it would have less drag. I expect much less would provide plenty of control. I can probably "guess" and not be too far off, but if anyone has some experience that they could share, I would appreciate it.
The improved dagger board I built last winter has really helped on all points of sail, and I think better rudders would be the next step.
Bruce

 

Since it's a "high speed" rudder area can be less. The float rudders on the Multi50's look to be about 50% (or less) of the area of the main hull rudder, that might be a good place to start. Here is the Multi50 "Drekan" for comparison.

 

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Thanks, Bruce....

 

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Excellent point, Corley-though I've seen several tri's with no central rudder.

 

I'd think about 50 % of area too as at low speed both will be in the water. Same with deep off wind courses. If your current lower aspect rudder has enough area for slow speed sailing you might get away with a fudge factor and go less than 50% if a higher aspect ratio is used.
Couple thoughts....kick up? retracting? For beaching etc...
I played around with the idea for the Nicol but ultimately decided against it as I felt some strengthening of the aft ama section up to the beam would be required to handle the loads. There isn't a weight advantage as there are 2 rudder shafts and connecting linkage too. The easiest way for me to handle the conversion would have been a kick up ama stern, say for a canoe stern that extended from the original transoms. This would have let me use spades and have good bearing support between the keel with pivot hinges at the deck to kick up the whole section and retaining lines below.