Planing Instability

Discussion in 'Hydrodynamics and Aerodynamics' started by Tad, Sep 30, 2013.

  1. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    I've got a puzzlah.....

    28' Aluminum planing boat, VP 315HP DP outdrive, floating as shown in the drawing she weighs around 15,600 pounds. Apparently that's with 600 pounds of lead in the engine well to make her work slightly better. From the lines the boat looks fine, perhaps a bit narrow but nothing weird in the basic hull. She is massively overweight, apparently the original design was at 9000 pounds.

    Now her performance....From about 18.1 knots (2700 RPM), if the throttle is advanced further she will heel to starboard about 8-10 degrees and trim flattens out(nose drops). As speed increases she then goes into a hard port turn and will not come out of it without chopping the throttle. This seems to be classic instability where she runs in equilibrium up to 18 knots, then drops to a lower trim angle where the bow is immersed and pulls her into a turn.

    The question is how to fix this? I've suggested a new hull, that's not happening due to budget. The owner likes the boat alot and the weight can't easily be reduced. It's a tremendously overbuilt and overloaded boat. Small sponsons were added aft outside the chines, they are 5.5" wide and 12' long, that did nothing and may well be driving the nose down. I'm thinking about big lifting strakes forward tapering to nothing aft......

    BelLCG.JPG

    Bellines.JPG
     
    1 person likes this.
  2. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Have you checked the hull symmetry first ?

    Perhaps it has inbuilt twists that create bad water flow at speed.
     
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Tad

    I don't envy you. These are never simple fixes.
    Looking at the lines, it does appear to be rather a deep Vee for such a low L/D ratio boat. Can you reduce the deadrise some, to be below 10-15 degrees at midhsips and around 5 degrees at the transom? Thus just changing the bottom 2/3 of the boat?

    Or, taking your suggestion, have you considered making the chines like small outriggers, rather like a Boston Whaler?
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    And it has exhibited this same behaviour since Day One ? Puzzling is an apt comment, just looking at that boat and where the static waterline is, I am surprised the nose ever comes down under power.
     
  5. dskira

    dskira Previous Member

    A small keel/skeg coming from the middle of the hull to approximately 10 inch at the transom.
    Probably checking the propeller. It's an I/O if I understand well checking if the drive is really vertical.
    Adding some sponsoon after the transom, both side of the drive making the hull longer can also give more stability at speed. Doing in aluminum is easy
    Adding length aft to the hull, putting a shaft between the engine and the drive, will also make the hull longer, and more stable.
    The hull seams short for the general configuration.
    Did you talk with the owner to make a model of the hull and try different scenario?
    The freeboard seams high, is the center of gravity low enough?
    I don't see any cure which will not cost a lot, since every changes should be tried on the water.
    I don't know, but getting rid of the added weight inside the hull, will be the cheapest way to start to check what's wrong, as finding its CG.
    Any way good luck, you will find the solution I am sure.
     
  6. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    I'd be looking to the deadrise too, planing on one surface and then the resultant flow at higher speed induces the turn. But you'll need to model test a free running large model before you put them to the expense of adding to and fairing in the new bottom. Unless they understand it's all on a wing and a prayer.

    Sometimes surprisingly you can get a partial solution by swapping the drives L-R if they are opposite hand. It can make a considerable difference.....sometimes !
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    It really does sound as if there is a hook in the bottom aft, on the starboard side only. If not, it is one for Ripley.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    DP=Duoprop ? Should be neutral torque ?
     
  9. Tad
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    Tad Boat Designer

    That's possible, I'll certainly be checking the hull with a laser if I ever get it in a shed. I have measured the hull against the drawing and checked the bottom is flat and reasonably straight, which it appears to be.
     
  10. Tad
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    Tad Boat Designer

    Hey John thanks.....I've thought of a lot of stuff. My original idea was just as you suggest, low deadrise sponsons starting at 1/2 beam, with negative deadrise chine flats, increasing beam by a foot and adding planng surface and beam forward. But that's a really ugly (because it has to come a long way up the topsides) solution. And it's heavy, adding more drag. The boat has a full and fancy paint job.....

    I thought of making her a catamaran, but nooo....don't think that will do it either.....
     
  11. Tad
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    Tad Boat Designer

    I don't really know, the present owner was duped into her and has been trying to get it sorted for a few years. There are some small, short lifting strakes right under the bow, down very low, that look like they may be part of a previous fix attempt. I'm temped to have those cut off. She also has a bow thruster (fixed tunnel perhaps 8").
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    How about fitting some tabs, if that doesn't stop it laying over, it is a lost cause.
     
  13. Tad
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    Tad Boat Designer

    Excellent thought on making a model, sometimes the simplest solution is right there......

    I would love to build a new longer hull, but I think longer forward is the answer. It feels like longer aft will push the nose down more......

    CG is really high, although the boat was comfortable and predicable to me at anything up to 18 knots.

    Right on getting rid of weight, but where to start, Daniel this thing is absolutely packed tight with stuff. Full built up liner. And it's all foamed in place, I looked and could not find the water tank, I know it's there, but I could not see it.
     
  14. Tad
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    Tad Boat Designer

    If it's there it's really tiny, I couldn't find it with a 7' aluminum straight edge.
     

  15. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    I think so....
     
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