Deep V off plane wander. What are the forces causing it?

Discussion in 'Boat Design' started by cyclops2, Jan 21, 2012.

  1. Milehog
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    Milehog Clever Quip

    Has it always done this?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Only advice I can offer is, get one of the crew to lean out over the back of the boat ( without risking falling in, of course) to determine whether the drive leg is oscillating back and forth. If it not moving at all in relation to the boat, I have no idea. If it is, your problem is in the steering mechanism.
     
  3. eyschulman
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    eyschulman Senior Member

    I wonder what a fixed casett lifting rudder like board located on stern would do if lowered at low speeds?
     
  4. IMP-ish
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    IMP-ish powerboater

    > The bow swings P & S exactly 22 degrees back and forth no wheel movement

    I would also check for any play in the steering arm of the drive or the cable. Then watch the power steering cylinder hatch open while the problem occurs.
     
  5. cyclops2
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    cyclops2 Senior Member

    I will do those steering checks. I took my marinas word of O K.
    I mentioned to them that the wheel has always had a LOT of dead band or slop. No problem was their answer. Normal.

    It is a sealed Teleflex steering box. Always has been sloppy. If the box is sloppy, can the hydraulics drift back and forth in the sloppy range ?


    Rich
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    There's probably several factors contributing to this condition. Play in steering, high CoG, lack of immersed lateral area in the stern seem likely culprits. I suspect reducing play in steering would reduce the swing but not eliminate it. Have you got a lot of weight up high?
     
  7. cyclops2
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    cyclops2 Senior Member

    No, it is a standard bow rider.

    I would hate to think I need + 1000 pounds of ballast to idle straight.

    Rich
     
  8. maxstaylock
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    maxstaylock Junior Member

    Nothing wrong with your boat.

    This is standard behaviour for all small plaining hulls at less than hull speed, with outboards/outdrives.

    I have not a clue what causes it, but I have noticed it on all ribs I have been on, from 4.5m single outboard, up to 12m twin inboard, heavy displacement contra rotating.

    Putting weight (crew) forward at slow speed reduces the ultimate swing angle, but most people (without thinking about it) oscillate the wheel, at slow speed, then increase to hull speed as soon as wash/speed limits allow.

    Get used to it, it is perfectly normal. Putting permanent ballast in your bow will cause other problems. If you find you want to spend a lot of time at sub displacement speed, get a heavy displacement hull.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    He says it happens with hands off the wheel as well.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Hardly. Sounds to me like he has got excessive wear or play in the steering, but plenty of small planing craft below hull speed run straight as a die, less so the deep vees though.
     
  11. maxstaylock
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    maxstaylock Junior Member

    Sorry, yes, should have said all Deep V hulls, have no experience of other types of small plaining hulls.

    Try "All Deep V outboard/outdrive hulls oscillate at sub displacement speed, to some extent, regardless of steering system efficiency"
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    They don't oscillate in the way he described, like a pendulum though, without input from the driver.
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Too much play in the steering, combined with a faulty setting of the little trim fin on the drive leg? Adjust the fin according to manual, or set at a slightly biased angle, causing the servo valve to find a stable position.
     
  14. tom28571
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    tom28571 Senior Member

    There must be some dynamic undamped force to have the kind of oscillation the OP is talking about. I would suggest that the boat is very likely heeling a bit when it makes these steering excursions. That is, when it steers to starboard, it will heel to starboard near the end of that oscillation and do the opposite when it oscillates to port. That this is more common on deep V hulls reinforces this idea since deep V hulls are much less stable transversely than flatter hulls and heel more easily. When any boat heels, it unbalances the directional stability to some degree and a heel to starboard causes the boat to turn to port and vice-versa. Something must excite this oscillation and that might a be sloppy dead band in the steering or unsymmetrical wave action. Once started the oscillation will continue unless damped out. An immersed bow makes this situation worse and is the reason that increasing weight aft helps it. Its a kind of like chine walking that is more common on fast shallow V hulls.

    When the boatyard says that this oscillation is normal with this particular boat, they are probably right. Moving weight or ballast aft may help to correct it but is a poor choice to have to make since that will make the boat slower to get on plane and cause other handling issues. I don't know of a good solution to the problem but it is another reason I avoid Deep V monohulls unless their special characteristics are needed. Some larger ocean going boats have dynamic stabilization with foils, trim tabs or steering impulses to correct this problem.

    Many of us who are sailors may have used this heel/steering action to sail a boat, including tacking, without ever touching the tiller.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Tom, if heel sends the boat off course it will just keep on turning in that direction, surely ?
     
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