Flanking rudders for high speed

Discussion in 'Boat Design' started by Frosty, Oct 19, 2011.

  1. Frosty

    Frosty Previous Member

    Ive seen many a flank rudder on slow speed stuff but what about high speed say 20kts.

    My idea is to put a spade rudder on each hull of my cat in the inside of both hulls, this means I can tie it mechanicaly and maintain track allignment.

    I would not think any performance attributes would be affected ???

    My present are twin one legged tunnel rudders shrouding the surface drive leg, they are huge and a devil to keep clean and are not used for manouvering.

    It would be a simple matter to rip out the rudder shaft bearing and move it forward and glass it in to the side, use both rams and connect mechanicaly.

    Whddya think.
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    For my tunnel drive project I've put small flank rudders on both sides of the props. At the time I thought it would reduce drag because the flow speed is lower than behind the rudders and that is certainly true, but because of that they are also less effective. At rudder angles under 15 degrees they do almost nothing, only when they reach the prop wash they start to behave like rudders.

    I can live with that because for manoevering playing with the throttles or putting one engine in reverse provides much more response than turning the wheel.
  3. Frosty

    Frosty Previous Member

    Thanks CDK you know there is nothing on the internet about this stuff , ive been looking all afternoon . Now you want **** then the internet is awash with it, what does that say about the human race eh. Did you have 4 rudders then?

    I did find something that said flanking rudders in front of the prop must be inside the diameter of the prop, for me thats not a good diea as I have 24 inch surface props.

    I cant see why that would make a difference though , the writer said the rudder would be usless but it was a model boat.

    You see the tunnels are almost flanks now but to the side. If I could move them forward about a foot I would have a reall nice neat unit.

    Any body know anything on this stuff.

    CDK,-- some one has just posted a pic of some kind of flanking rudder design on searching for fuel efficient power boat

  4. Frosty

    Frosty Previous Member

    Wow--looks like no one knows anything about flanking rudders. Its rudders that are infront of a prop and usually at each side, even if there is 2 shafts.

    I did find a site that said that the rudder should not be wider than the dia of the propellor and said going outside of the props dia would render the rudders usless.

    If the rudder is in front then the rudder is not affected by anything but flow from the vessels movement only.

    Why am I always doing stuff no one can help me with. Why dont I just buy a speed boat with a 100HP on the back and enjoy life.
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