Twin rudders good or bad ?

Discussion in 'Boat Design' started by frank smith, Apr 28, 2013.

  1. frank smith
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    frank smith Senior Member

    On a shall draft boat twin rudders would make sense in order to open up the transom for an outboard and also give additional steerage with out he need for a kick up rudder . What would be the negative effects of such an arrangement?

    Frank
     
  2. messabout
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    messabout Senior Member

    Frank I think that a shallow draft boat will be well advised to have kick up rudders. Shallow water and grounding go together all too often.

    Boats that sail with significant heel angles can profit with twin rudders particularly when the rudder shafts are angled. Some hot rod sailboats do use twins, inland lakes scows for example use angled twins along with angled dual centerboards. Twin rudders are standard on catamarans but not on trimarans. Twin engined powerboats use twin rudders so as to get the rudders into the prop slipstream. That is an advantage when maneuvering at slower speeds or stormy waters.

    Twin rudders add wetted surface and a certain amount of unnecessary drag when the boat is to be sailed or powered in a near upright attitude. Of course there are added complications in steering couplings.
     
  3. michael pierzga
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    michael pierzga Senior Member

    Everything good is a compromise.

    Twin rudders are exposed and prone to fouling.

    Twin rudders are twice as expensive.
     
  4. CDK
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    CDK retired engineer

    Lack of information makes your question hard to answer.

    The propulsion system I make for my own boat involves 4 small rudders and 2 props. The tie bars and shafts are submerged, only the inner starboard shaft is longer and carries the steering arm.
    The rudders are in close proximity of the props, so they actually deflect the water flow when the steering angle exceeds 15 degrees. Under 15 degrees the rudders do change the course of the boat, but the reaction is too slow for a docking or emergency maneuver.

    If your boat has twin rudders far apart and a prop in the center you will not be satisfied with their performance.
     
  5. fredrosse
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    fredrosse USACE Steam

    Twin Rudders on a Sidewheeler

    I have twin rudders on the sidewheeler, and they kick up if grounded, specifically to be in the wheel wash, and to minimize draft. The sidewheeler is difficult to dock, so the rudders can be folded up, and the flat bottom boat backed ashore. People can exit ashore without a dock, and nobody gets their feet wet.
     

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  6. rwatson
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    rwatson Senior Member

    Of course, there are some boats that take advantage of this configuration, to provide outboard power and/or sail as desired
     

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  7. PAR
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    PAR Yacht Designer/Builder

    The only advantage to twin rudders is, the ability to have them canted at an optimum angle, of course with the windward blade retracted. Most boats will have an ideal heel angle, so the boards are set at this. Some are adjustable (the cant angle), though this convolution isn't going to offer much more than a small fraction of a knot better ability.
     
  8. rwatson
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    rwatson Senior Member

    That should be "the only PERFORMANCE advantage" I think

    Outboard accommodation, water entry/exit, a built in spare etc are all good things to have.

    But back to performance - some designs of yachts benefit a lot from twin rudders - eg Volvo around the world contestants. With the wide transom, twin rudders make a lot of sense
     

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  9. PAR
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    PAR Yacht Designer/Builder

    Generally agreed on the addition of "performance", but I have an open transom, under belly rudder boat and the single rudder doesn't interfere with any shipboard or boarding operations. Redundancy is nice, but in the event of a failure, you're screwed on one tack with twins.
     
  10. Easy Rider
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    Easy Rider Senior Member

    Would a twin rudder arrangement w rudders just outboard of the prop wash result in greater efficiency due to usually not having prop wash pushing the rudders/boat "backwards"? While turning one rudder would be in the wash helping the turn in the usual way just not as much. I think larger and lower aspect rudders would probably be used eliminating some or much of the advantage but still the prop wash would not be hindering fwd progress.
     
  11. daiquiri
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    daiquiri Engineering and Design

    Depends on how you define efficiency. Rudder's task is to turn the boat, so they are efficient if that task is performed well. Putting them in the propeller's slipstream certainly increases the rudder drag (higher friction), but also greatly enhances their turning ability, or rudder authority. Which is what we need them for.
    A rudder placed out of the slipstream will certainly have a smaller friction drag, but will also be less effective as a device for steering the boat, and this degradation of steering performance would be quite dramatic at low speeds.
    Cheers
     
  12. michael pierzga
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    michael pierzga Senior Member

    The modern speedsters need twin rudders because of their extra beam aft. A single rudder would have to be located very far forward to stay wet on a beamy stern boat
     
  13. frank smith
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    frank smith Senior Member

    I was thinking primarily of a of large sharpie , (roll eyes ," not another wing nut thinking about sharpies"). Yes, twin rudders would not need to be pivoting , could be transom mounted , and allow an outboard to be mounted. I occurs to me that a swim platform could be built in front conforming to the bottom of the hull, and still have the simplicity of transom mounting. The sharpie I am envisioning would have a long shallow keel. and twin rudders would allow it to sit upright, when the tide went out. So the main question is how detrimental to performance would they be in that application. As PAR points out it would be simple to hang a rudder under the transom, but it would need to be pivoting or retractable. Thus adding to complexity.
     
  14. daiquiri
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    daiquiri Engineering and Design

    The "performance" is given also by these practical aspects of everyday usage, not just by the hydrodynamic aspects. Since you are talking about sharpie boat, long keel, beachable and with an outboard motor, you are evidently not interested into competitive sailing. So I see no problems with mounting a twin rudder on your sharpie. Many racing sailboats use it, so imo your sharpie can use it as well. ;)
     

  15. michael pierzga
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    michael pierzga Senior Member

    Its possible if you are willing to accept the complexity and clutter of all that gear on the transom.

    I dont think its worth it
     
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