Planing Instability

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

  1. Tad
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    Tad Boat Designer

    Some pictures

    The starboard bow, note the added sponson starts a bit aft of the forward strap.

    Bellexbow.jpg

    The Port Bow with a better view of the forward lifting strake. It ends just aft of the forward strap.

    Bellexbow2.jpg

    And the stern

    Bellexstern.jpg
     
  2. Tad
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    Tad Boat Designer


    Thanks Mike, I have perused all those and we do indeed have "Bad Behavior", but now my focus is on how to "fix" it, or even if that's possible. My second proposal to the owner was to disassemble the interior and cut away the hull, saving the transom, deck, house, and cockpit. Then build a new hull (with increased beam) with a blunter bow and more running surface. But that is about $150k, added to the amount already invested (substantial) it makes little sense.
     
  3. Tad
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    Tad Boat Designer

    Nice one Will....any experience with these on large and heavy boats?
     
  4. Tad
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    Tad Boat Designer

    Apparently on the "sea trial" of this vessel, there were "Just by chance" 8 people (Large men) aboard the boat. 5-6 of those stayed out in the cockpit. No evidence of bad behavior was observed....but...I have no idea who operated the boat, or what the top speed was. Brokers are not to be believed.....

    Thus the weight added aft, it helps a bit (allowing slightly more speed before the onset of spinout). One thought was to stack the weight on the swim step. But again that's silly, adding weight to an overweight vessel.
     
  5. Tad
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    Tad Boat Designer

    Thanks for that....unfortunately it seems we are going to pursue some "cheap solution", ie <$50k. A decent sized model appeals, then we could try various lifting strakes or steps......
     
  6. Tad
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    Tad Boat Designer

    Thanks, yes, that was my thought after reading Lou's piece. This boat does not quite fit in the "Likely" region. But she's so overweight that there is almost nothing to compare her with. The Bertram 28' was about the closest thing I could find. In plan view the two hulls are within a few inches of each other, but the Bertram is only 12,000 pounds......
     
  7. Tad
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    Tad Boat Designer

    That's interesting and it twigged a memory of the Blount/Codega fix for some USCG boats(?)......a series (perhaps 3 spaced 8-10") of diagonal lifting strakes either side of the bow...anybody recall this?
     
  8. Tad
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    Tad Boat Designer

    Yes, that's my feeling, get the bow up and keep it up. Or perhaps break that flow over the lee bow as she rolls down (by adding strakes or interceptors).
     
  9. DCockey
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    DCockey Senior Member

    From the website (underline added):
    With a StingRay Classic installed, boats get up on plane 50% faster, while bow rise is also reduced.

    Sounds like this would be the opposite way to go if the problem is that the bow is too low at speed.
     
  10. JSL
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    JSL Senior Member

    The paper I mentioned shows transverse steps (wedges)
     
  11. Willallison
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    Willallison Senior Member

    I've found they both and don't work on both that are relatively light as well as relatively heavy... so....:confused:
    None that are as so over-laden as this one though

    I do know of a heavy local boat (25ft...bit like a Bertie 25) that displayed all sorts of strange behavior and the foil all but fixed it.
    Of course, you would be adding lift aft, which in this particular case may not be helpful, but at such a low cost it is surely worth a try before resorting to more drastic measures
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Sometimes it is a culmination of all the little things. Although I'm sure you've though of many already. But, in this pic, the stem bar looks too bluff/thick. In the past to correct "odd issues" we have had to grind down the stem bar to a razor edge. Even had to add a stainless steel strip (razor sharp) to the bow of a composite boat, which first alerted us to these niggly minor issues. It is a standard feature on our boats now.

    It may make zero difference, but it wont hurt.
     
  13. Tad
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    Tad Boat Designer

    I see we can get one at West Marine for $60....the instal will be something, I guess a diver could do it. I'll recommend trying it.....perhaps with the leg trimmed out a bit it will add a tiny lift to the whole thing......
     
  14. Willallison
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    Willallison Senior Member


  15. PAR
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    PAR Yacht Designer/Builder

    Of the ventilation plate foils, I've found the delta shape products work best on higher speed craft, while the wing style better on heavier and slower craft.
     
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