single rudder on a catamaran

What do you guys think about using a single centrally mounted rudder on a catamaran? Obviously this is only viable on a heavier cruiser which will never lift a hull out of the water. I know that some designers do this, Derek Kelsall has used a single rudder on quite a few cats, there has been a Grainger cat for sale in Sonora, Mexico which has a single rudder and 2 outboards which is the arrangement I would like to employ on an old classic Gemini I recently purchased. The current setup is daggerboard style rudders on each hull and a single outboard on a hinged fiberglass "bucket" (their term) on center, the motor is tied into the rudders with lines so it turns with the wheel. The Gemini is only 14ft beam with fat hulls so the c/l to c/l is only 9ft but I think having motors at that spread should still give decent close quarters maneuverability and I don't see this boat ever lifting a hull.
Does anybody see any reason not to go to a single rudder with a pair of 9.9 Yamaha high thrusts on sliding brackets on the hulls. The existing rudders have a chord of 11 1/2" and a depth of about 2ft so about 4ft2 with both down and my thoughts are that I would want to go deeper and keep about the same area, maybe 15" chord with about 4ft of draft angled fwd to get some balance and in a daggerboard style so it can be adjusted in depth or retracted completely while motoring or moored.
Anyone here have any actual experience with a single rudder?

Steve.

 

Single rudder is always less efficient because it intersects free surface. Say, for rudder under bottom 'screen effect' of the hull will increase effective aspect ratio and thus lift on the rudder 1.1-1.3 times. For rudder intersecting free surface, this coefficient is 0.6-0.8. In practice, this means more rudder area on same cat, thus more drag, etc.

 

I agree with Alik. Don't do it. Kelsall doesn't use the central rudder any more. And think what will happen when you sail over driftwood with nothing to protect the rudder!

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

I knew a bloke who flew 747s. He said they were great planes. I asked him why and he said "They have 4 engines - if there was a plane with 5 I would fly those" Redundancy of important parts is great.

cheers

Phil

 

Everything is a compromise, we all know that a rudder under the hull is more efficient than a transom hung rudder for example but is more likely to sustain damage as the transom hung one can usually able to kick up or retract so for this and shallow draft many if not most multis unless they have Lar keels use the less efficient transom hung option. Lar keels are less efficient than daggerboards but many cruising cats choose them for other reasons. While a center rudder may be less efficient than transom hung ones they obviously still work well enough otherwise no one would use them, Derek Kelsall does still use them on some boats, I did one of his KSS workshops last summer and the 42ft cat we were working on had a single rudder, it did have a fairing built down to about the waterline just like you do for a central mounted outboard motor. I don't see the central rudder with fairing being any more prone to damage than a transom rudder and less so than an underhung rudder. I don't think a central rudder would need to be bigger in area than the the 2 existing rudders but would be deeper and adjustable. While the redundancy of 2 rudders is great there are a few Trimarans and monohulls around that do ok with one. I used to own a Macgregor 36 cat with underhung rudders that were very damage prone and a single daggerboard in one hull that worked well.

Steve.