Prevent selfsteer

Discussion in 'Hydrodynamics and Aerodynamics' started by Janne Enlund, Jan 13, 2015.

  1. Janne Enlund
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    Janne Enlund Junior Member

    I have a double ender, model Saga 24. The problem is that she is designed as a motorsailer with a 600 kg ballast which means that she is rolling heavily even in moderate seas. I'm going to attach a pair of bilge keels the incoming spring to dampen the roll and there I gotten an idea. What if I slightly bend the keels in opposite direction to the hull shape. This would slightly add some drag but the more interesting question is if it would compensate the 'self-steer' that happens when she heels? She is now turning quite a bit even when people of normal weight are moving around.

    [​IMG]
     
  2. Manie B
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    Manie B Senior Member

  3. Janne Enlund
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    Janne Enlund Junior Member

    I'll do that!
    Nice project Manie!
     
  4. TANSL
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    TANSL Senior Member

    I think the most effective will increase slightly over the width of the bilge keels, if you think they are not good enough. The straight fins increase resistance but if you give them a certain curvature, also increase the turbulence in forward motion. Try that keels follow the flow lines of the water around the hull.
     
  5. Janne Enlund
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    Janne Enlund Junior Member

    You have a point there!
    I have had this idea myself earlier but never thought about turbulence. Would the turbulence be minimized if I keep the outer side straight and the inner slightly curved so that the fin thickness would be greatest at the middle?
     
  6. TANSL
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    TANSL Senior Member

    Bilge keels should have no straight section. As said before, should have the same form as the flow lines of the water around the hull. To make it easy, similar profile to the water line at the point of insertion of the keels.
    Or still better : if you have the boat body lines, draw a bilge diagonal and that's the line of the keel on the hull.
     
  7. Janne Enlund
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    Janne Enlund Junior Member

    I might have used the wrong term, right me if I’m wrong.
    Pictures of ‘bilge keel’ that I’m referring to:

    [​IMG]

    [​IMG]
     
  8. Janne Enlund
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    Janne Enlund Junior Member


    This could be accomplished with a laser that builders use to draw a straight level lines on walls. Wouldn’t this follow the lines of the hull and create a curved line if looking length wise of the hull?
     
  9. TANSL
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    TANSL Senior Member

    Although your pictures say otherwise, bilge keels usually have a length of about 2/3 the length of the boat and should not be straight.
    Forget all that. Take a flat bar or a bulb bar and adapt it as best as possible to the hull.
    Do something, more or less, as the picture below.
     

    Attached Files:

  10. Janne Enlund
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    Janne Enlund Junior Member

    Thank's for the input, much appreciated!
     
  11. Janne Enlund
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    Janne Enlund Junior Member

    After thinking and studying a while about fluid flow I came up with the conclusion that water in most cases takes the shortest path around an object. This gave me an idea on how to get the right curve on the bilge keel.
    1. Study the hull and find the best and most logical location for the keels.
    2. Measure the hull and calculate keel length to ~2/3 of hull length.
    3. Place one screw on the starting point of the keel and one screw at the end point. These points should be approximately at the same level as the waterline, but can also be at different levels based on the shape of the hull.
    4. Take a strong nylon line and stretch it between the screws. In theory this line should follow the shortest path between start and endpoint of the keel, which should correspond quit well with the flow line of the hull.

    I’ll think that this procedure only works when the hull shape is round like a sailing boat, not in any other cases. It is also uncertain at what levels the start point and endpoint should be placed, guessing and logic thinking is my friend here…
     
  12. HJS
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    HJS Member

    Delta wing and midship interceptor

    Attached illustration is a boat that won the Swedish Cruising Club, SXK, design contest on the future of long-distance motor boat.

    To suppress rolling and improve directional stability, the boat can be equipped with delta-shaped wing under the keel.

    In addition, the boat can be provided with interceptors amidship. Russian investigations have shown that the roll can be reduced to a quarter.

    I am convinced that a wing under the keel, far from the boat's roll center, is much more efficient than bilge keels.

    js
     

    Attached Files:


  13. Janne Enlund
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    Janne Enlund Junior Member

    I’m sure that this is a brilliant solution. The problem is that the archipelago in Finnish ostrobothnia is quit shallow and full of big underwater rocks. This makes everything extra attached deep down to the hull even more vulnerable to hitting ground. The idea has crossed my mind but I’m trying to keep the keel as thin as possible to minimize the risk of hitting something underwater. That’s why I’m opting for bilge keels positioned higher up on the hull.
    But having said that, how about placing a delta shaped wing higher up on the keel? Let’s say half way up, have you done any test with this type of configuration? A NACA-type of wing right in the center of flow would not cause as much drag as bilge keels. The cons of this location are that it would produce turbulence in front of the propeller.
    My boat was earlier equipped with flat sheet wings on her rudder. These where standard equipment on later Saga 24 and 27 models. I suspect that they were used to provide a slight lift.
     
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