Enhance directional stability

Discussion in 'Hydrodynamics and Aerodynamics' started by Janne Enlund, May 4, 2015.

  1. Janne Enlund
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    Janne Enlund Junior Member

    My grand old lady, a long keeled Saga 24 has directional stability as a drunk teenager. I can't let go of the wheel at any point or she turns on every roll.
    Can someone explain to me what the main factors are that have most impact on directional stability on a standard displacement hull? Are there any factor like center of gravity, center of buoyancy or length of keel that can be changed to correct the problem?
    I'm going to install an autopilot for now but this is not a real solution to the problem just a workaround. What I'm looking for is not a definitive solution to the problem just to get an overall understanding.
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    How's the waterline? She might swim a bit deep by her bow..
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

  4. Janne Enlund
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    Janne Enlund Junior Member

    Would a relocation of the rudder further aft have a positive impact on directional stability? Saga 24 is equipped with a rather large balanced rudder.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Do you have any pictures or dwgs of the hull showing under the waterline?
     
  6. daiquiri
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    daiquiri Engineering and Design

    Can you say a bit more about what exactly is happening?
    For example:
    1) If you hold the wheel, does the boat go straight in a calm sea? And how does it behave in waves?
    3) If you free the wheel, what happens in calm and in wavy seas?
    4) If persons get on one side of the boat, does the boat steer in the same or in the opposite direction? Is it happening with the fixed or free wheel?
    Please be sure of the correctness of your descriptions, because a wrong input will lead to a wrong assessment of the problem.
    Cheers
     
  7. daiquiri
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    daiquiri Engineering and Design

  8. Janne Enlund
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    Janne Enlund Junior Member

    1. It goes in a almost straight line in calm weather, only small corrections needed.
    2. Even small waves means that I must constantly compensate.
    3. In calm weather and everyone sits still the boat goes in a almoust straight line. In wavy seas she goes in every direction besides straight forward.
    4. She steers to the opposite direction.

    I have asked Saga Boats for drawings but they replied that they only have the moulds left, nothing else.
     
  9. PAR
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    PAR Yacht Designer/Builder

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    It's pretty tough to tell what's going on with the handling manners of your boat, though she should be more steady on the helm then you've described, judging by her general shape. It's probable the ends are being lifted by waves and contrary seas, which will push her around quite a bit. The wing on the rudder is a joke and should be removed. If the rudder is remade, try to fill the gap between the hull and the top of the blade as best as you can.
     
  10. Janne Enlund
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    Janne Enlund Junior Member

    Yes, the flaps are removed already. They are original mounted equipment by Saga on both 24 and 27 models.
    By filling the gap, do you mean making the rudder higher to follow the hulls curve or to make a fixed part attached to the hull shaped as the rudder profile?
     
  11. daiquiri
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    daiquiri Engineering and Design

    The first solution is better, as it will increase the rudder area too. You will feel a slightly increased resistance from the wheel due to the center of pressure of the rudder moved slightly aft, but nothing to worry about.

    That is one thing to do, but it will likely not resolve your problem. I think that the problem is related to the volume distribution of the hull and the position of the CoG (center of gravity). A possible cause is the CoG placed too far aft, which might be compatible with what you said in the other thread (http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/prevent-selfsteer-52318.html#post721173) :
    "The problem is that she is designed as a motorsailer with a 600 kg ballast"
    By removing the rig from a motorsailer (and it's weight) and altering the ballast, it is very easy to mess up the overall balance of the boat. you might try to put some ballast towards the bow and see what happens.
     
  12. Petros
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    Petros Senior Member

    Many boats are directionally unstable in variable seas.

    One thing you might do if you make a new rudder is move the hinge line aft some so side pressure on the rudder will not turn it as much. If you can put the hinge line right on the dynamic center (usually between 25 and 30 percent of the way back from the leading edge), it will not rotate at all in a side load, but it will possibly act squirrelly at higher speeds. You want the hinge line a little head of the dynamic center. It might mean the LE will be closer to the back of the prop, as long as there is adequate clearance this should not be a problem.

    There are other more drastic measures you can also try, but i would do the rudder first. like, cut out more of the keel towards the front of the boat (in the forward half), add some small auxiliary side keels slightly forward and on either side of the rudder protruding down at 30- 45 degree angle (angled outward). that it will have more of the directional control no matter what the condition of roll or pitch, so more of the extra keel surface is in the water at any given time.
     
  13. Janne Enlund
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    Janne Enlund Junior Member

    I have in mind to make the rudder more balanced by moving the rudderstock and bearing farther aft both on rudder and hull.
    She'll also get a thrust bearing between engine and axle to get rid of the last irritating vibrations which means that I get to push the propeller further aft to the position where it gets more hull clearance before I cut the axle to the right length. The newly made composite propeller axle pipe can then be mounted so that the cutlass bearing is positioned in the right place and hull fearing can be made better around the pipe.

    Max speed is about 6 knot so hopefully this is not the higher speed you are talking about?

    This is something that I have thought about as the bottom half of the hull is water damaged from the inside cause of her internal lump of concrete that has sucked up water since 1977. I’m going to make a mould of the hull and then cut away all the damaged parts. The problem when talking about cutting and reshaping the hull is that the bottom area of the keel is elliptic shaped and has a flat bottom about 25cm at the widest point. If I start to cut away then the reshaping will be a monstrous job to get the right form. If I opt to only a cut away a part of the forward part then will I end up with a flat surface angled upwards which is in my option not so good.
    Her hull shape is sadly a compromise done in the -70s oil crisis era to make a cheap running boat that could be sold both as a powerboat or a motorsailer. The only difference was if the customer liked to have a mast or not.
    You can have a point there! She was dipping her *** quite low last summer even thou all her tanks was full (she has a 150l water tank ¼ from the bow and 250l fuel amidships). Her waterline was well up in the air at the bow.
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  14. PAR
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    PAR Yacht Designer/Builder

    Well, there you have it - a trim issue and an easy enough one to check and fix.
     

  15. Janne Enlund
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    Janne Enlund Junior Member

    Thanks! This is what a forum is all about!

    So it's safe to assume that a heavier bow may result in better straight line handling?
    I'll have to pac a lot of sandbags that can easily be distributed around the boat to test different scenarios while at sea!
     
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