Single offcenter rudder for a cruising cat?

[slidercat]

How feasible is this?

Derek Kelsall uses single rudders on his smaller cruising cats, I understand, but I assume this is a rudder on the centerline of the boat.

What would be the downside to putting one big rudder on one hull instead of two smaller rudders on each hull?

I've wondered about this for various reasons, and though my intuitive reaction is that there would be a "good tack" and a "bad tack" intuitive reactions are not always correct. For example, intuitively most of us would want a daggerboard in each hull, but John Shuttleworth demonstrated using tank testing that a single big daggerboard in one hull was more efficient. That's the way I went with Slider, my little beachcruising cat, and it worked very well indeed.

Of course, a single offset rudder would only be feasible for a cruising cat that was intended to never fly a hull.

This would be for a cat that has a rig on centerline-- in other words, not a tacking proa.

I can think of a couple of advantages to such an offcenter rudder. For one, it would probably have a better lift/drag ratio than the sum of two smaller rudders-- I think this is probably why one daggerboard is better than two. (An analogy would be .the superiority of a sloop rig to a ketch rig, efficiency-wise.) Another advantage is that you could hang an outboard off the unused transom, and it would be protected from spray and already in the entrained water from the hull. It would be easier to build and cost less.

So what are the potential drawbacks? It occurs to me that beach cats and other racing cats flying a hull are already using an offset rudder. What problems are associated with steering while flying a hull?
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[John B]

I don't understand the comment about the single centreboard... surely cats sail with the windward board up anyway, equating to one board.

[JimConlin]

Bad idea.

Cats heel too, and when the boat is pressed the hardest is a VERY BAD TIME for the only rudder to be out of the water.

The usual crossbar rudder arrangement is easy to build and works well.

[James McMullen]

I think I remember Phil Bolger designing a cruising catamaran that had a rudder and DB and the rest of the rig on one side, and an outboard motor bracket and camp-cruising acommodations on the other side. Don't know if any were ever actually built.

[wizbang 13]

Are your 2 rudders canted (toed)in? What advantage is there to taking away one of the advantages of 2 rudders?

[slidercat]

Cruising cats heel very little. On another forum, someone mentioned that he was once out in a Great Barrier Express-- a very fast Malcolm Tennant design with a lot of sail area, and one of the rudders was broken. He said the boat sailed surprisingly well with just one rudder. He said it lost bite when the rudderless hull lifted, but a cruising cat should really never lift a hull.

wizbang, I don't understand your comment. No, the rudders are not toed in-- I'm unaware of any design in which they are. Sometimes boards are toed in. The performance advantage is that one large rudder would have more lift and less drag than two smaller rudders. There would also be several other advantages, mostly covered above.

This is probably the reason that a single daggerboard has been demonstrated to perform better than double daggerboards. This only applies to cruising cats. Cats that routinely lift a hull obviously need boards in both hulls.

Oddly enough, some monohulls have two rudders, and in a way, this makes more sense than two rudders on a cat designed to be sailed flat.

You know, it just struck me that I plan to build an RC model of the boat prior to construction-- maybe put the jib on a club. It would be pretty simple to put a GPS aboard and compare the tacks.

[JimConlin]

Some catamaran steering systems are designed so that the two rudders turn differently.
Google "Ackermann catamaran steering".

[slidercat]

Jim, that's right. Slider has Ackerman linkage on her rudders, and I think it helps her be as handy as she is. However, Ackerman is a solution to the problem of having two rudders with different turning radii. I don't think a single rudder would suffer that problem.

By the way, a rough sketch of the new boat:

[wizbang 13]

I learned to sail on Spronk cats in the 70's in the caribbean. ( Maho 62' and Taruma 47')Their rudders were toed in a bit, as the boat turned,(tacked),the inboard rudder had more angle and the outboard one did not act (as much) of a brake. I certainly have NOT kept up with multihull technology though. I would think for a cruising cat, 2 rudders adds saftey in redundency and this would be a step backwards ,for saftey not speed. But then, not a big deal at all for a small boat.Your little beach cruiser is slick as a whistle.

[slidercat]

James, the Bolger design was Singlehander Catamaran. Bolger was a brilliant designer, fearless about trying new stuff and he advanced the art and science of small boat building considerably, but he wasn't terrific at multihull design. That's a really odd design, and if one was ever built, I doubt it sailed very well. But you're right about the rudder and board on one side and outboard on the other. Bolger says confidently that the single rudder will work as well or better than two. If the rest of the design wasn't so weird, I'd take his word for it.

[slidercat]

Thanks, wizbang. I think you might be referring to Ackerman linkage, which addresses the different turning radii of two rudders. Put another way, the outside rudder in a turn is not turning as tightly as the inside rudder, and by moving the tiller bar pivot points a little inboard, you get differential rudder angles. Slider has that kind of linkage, but a cat with a single rudder wouldn't have to fix that problem.