fore rudder?

to make quick turns in a 20ft slender trimaran could a fore and aft rudder be practical?

 

Having a rudder force at both ends will improve maneuvering, but it will come with other effects so it needs to be investigated throughly before deciding if it is the correct choice.

 

Wouldn't a small diameter thruster be a better solution?

 

Thrusters, generally, are only for maneuvering at speeds where rudders are not effective, and, generally, thrusters are not effective at vessel speeds where rudders are effective.

 

The problem with a fore rudder is that it will destablize the whole craft, so that means a larger rear fin/rudder is required to maintain directional stablity. That means more weight and drag (and complexity of course). So it can be done, but the "cost" is high.

One thing that might work is to have the fore rudder stowed up in the hull (or perhaps along side, folded back along side the gunwale), than only deploy it to make the maneuver. The momentary destablizing influence will make it turn that much more rapidly, which is what you want. Than once the move is complete, the fore rudder gets stowed back out out of the water. It would not take a very large fore rudder to have a large affect on the turn.

The problem of rapid rudder response for both cats and tris has long bother me, especially in catamarans. I have "botched" many tacks when we did not have the speed to bring the bow around back when I sailed them many years ago. Cost us a local race once. I do not think there is any easy way to solve the problem, a deployable front rudder would work but the complexity means a high probability of a malfunction at some point.

 

CBTFco( http://www.cbtfco.com/Home.html ) used twin, identical fore and aft rudders on fast monohulls with great success. They had normal steering(both foils turn opposite) and "collective" steering(both rudders turn the same direction-(largely eliminates) leeway.
The problem on a tri would be when the main hull lifts they'd be surface piercing foils with a chance of ventilation. If they were in the ama you'd need two on each side. On balance I don't think it would work well on a tri that flies the main hull but would probably work well on one that doesn't.
Good Luck!

 

Bolger also had a small sailboat with the rudder in front. I don't remember the details, but I think it worked fine. Search around for Bolger canard maybe you could find some info.

C.O.
http://whatsintheshop.blogspot.com/

 

Wouldn't a fore rudder create a lot of drag beyond just its size but by disrupting flow in the critical area between bow and bottom?

 

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Not if it was well designed-the CBTF system has been used on many successful ocean racers.

 

I have seen that with interest.

In the gallery photos, there are some small crafts that seem to a tri with forward amas that turn, might be a concept jet ski for all I know.

So, let me ask a dumb question...if such a set up had the rudders turned at the same degree syncronized, why wouldn't you be going more sideways than turning? Guess they are in opposite directions??

 

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On the CBTF boats the "collective" function is only used to zero out leeway ,not to achieve "negative leeway".

 

Thanks Doug, I caught the post above, originally missed it, now I'm with you...

 

They act as a keel on a keel less boat. Since tri are so wide, a wave can catch an arm turn it around unless it has some grip. Hydrofoil racer had the same thing.

 

Try it

Many fractional rigged sloops will sail backwards under just the main- a little "twitchy", but once you get them started, they will even go to weather. Try it sometime, it will give sort of a feel for a bow rudder, and give everyone at the clubhouse something new to talk about. In real world use, I think it is hard to protect a bow rudder from damage even if it does work better than aft. B

 

I don't see it. The rudder provides lift besides steerage. It is always generating drag.
So if your original design has a 3 foot sqft rudder areas and you add another 1 foot you have double your underwater drag. If it is in an area experiencing turbulence because of chop your drag number can go through the roof. If it anything but straight to the flow of water, it is cavitating. More drag.