Off-Center rudder

Discussion in 'Boat Design' started by northcoastnobes, Jan 29, 2008.

  1. northcoastnobes
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    northcoastnobes Junior Member

    I'm building a tri out of a Sharpie and Hobie hulls. The transom on the Sharpie is quite narrow and I want to place a motor as well as the rudder on the transom. I've considered placing the Rudder off-center to allow room for the motor. Does anyone have any advice on mounting the rudder off-center?

    Dave
     
  2. SeaSpark
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    SeaSpark -

    Off center motor

    I'd put the rudder at the transom and the motor off center.

    The other way round you will have no prop wash around the rudder at all when maneuvering under power. When you mount the engine at the extreme end of the hull chances are big the prop will be lifted out of the water when pitching.

    Having the engine at the side off a narrow hull does not have many disadvantages especially on a tri. Benefit is that engine and controls are easy to reach.

    Cat-Mono > tri conversions usually are received skeptically a picture would be nice.
     
  3. rwatson
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    rwatson Senior Member

    Dont put the motor outside the hull line though - when underway the motor gets covered by the waves, and makes a lot of splashing.
    I had a small trimaran with a motor mounted on the side - easy to reach but constantly getting covered from waves.
    If I did it again I would put the motor in the centre of the stern, and do twin rudders either side of the stern. Since you are using a sharpie centre hull, this should work ok as the stern will be fairly wide.
    production boats like the macgregor26 use this technique, and it works well.
     
  4. northcoastnobes
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    northcoastnobes Junior Member

    Thanks for both these replies. I had intended to place both the motor and rudder on the transom, relatively equally spaced from the centre (I'm a symmetry freak). The transom is only 740mm at its widest point, so I don't have the option of centering the rudder and placing the motor off-centre. Neither do I think I have room for a second rudder if I centre the motor.
     
  5. SeaSpark
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    SeaSpark -

    Picture this:
     

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  6. SeaSpark
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    SeaSpark -

    A more complex but better solution is to mount the outboard in a well in front of the rudder. If you want the best sailing performance, there are several solutions to seal the hole in the bottom flush when the motor is pulled up. The well prevents water entering your boat so this seal is not really necessary, it is just to reduce drag.

    The engine is fully protected, reachable and the setup is symmetric.
     

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  7. messabout
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    messabout Senior Member

    I have a small sharpie that has a fairly narrow transom. I put the rudder in the center and offset the motor mount. When motoring I lock the motor in straight ahead position and use the rudder for steering. The tiller has a hiking stick which lets me take a various positions in the boat. That is a convenience. The spacing between the two mounts took note of the possibility of not having the rudder foul the prop. The reason for choosing the center location for the rudder is that the rudder will more nearly stay in the water when the boat is heeled. In your case that is not a serious factor. I have had no problems whatever with the offset layout. I have had other boats with offset rudders, again no problem.
     
  8. northcoastnobes
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    northcoastnobes Junior Member

    Many thanks for these thoughtful replies. I'm building the boat in two phases - the first to do as little as possible in order to get it operational and to test it. I want to make sure I''m pouring too much effort into a dog. The second phase, if the first is successful, is to create a more elaborate cockpit and cabin layout. I'm inclined to go with the motor and rudder mounted on the stern in the test phase and to move to the motor-well with a centralised rudder in the second phase.

    Again, many thanks for your comments.

    dave
     
  9. northcoastnobes
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    northcoastnobes Junior Member

    Whoops, should have read it before pressing post. I meant to say "I want to make sure I''m NOT pouring too much effort into a dog".
     
  10. Pericles
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    Pericles Senior Member

    Very Freudian, that slip. :D :D :D What have you got against dogs? On second thoughts, don't answer that. :D

    Pericles
     
  11. northcoastnobes
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    northcoastnobes Junior Member

    Actually, I love dogs. I just don't want a boat that behaves like one!
     
  12. rwatson
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    rwatson Senior Member

    All boats behave like dogs - every boat has its severe performance limitations. Most 40ft catamarans cant go up creeks like a canoe, and most canoes cant cross oceans.
    Its an injustice!!!
    I guess its just the degree of dogginess you are prepared to put up with Versus the amount of money you are willing to pay measured against the risk to your health.
    Just have FUN creating.
    PS the dumping fees for failed boats are very high. Just leave it at a beach after taking the registration numbers off. The kids will have a ball!
     

  13. northcoastnobes
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    northcoastnobes Junior Member

    Much appreciate the advice and dose of reality and humour!
     
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