How to avoid porpoising/slamming at high speed?

Discussion in 'Powerboats' started by Bala5278, Sep 20, 2018.

  1. Bala5278
    Joined: Feb 2018
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    Bala5278 Junior Member

    Dear All,
    I'm currently working on developing more high performance hull shapes(5.5 m, 7 m, 8.5 m) for the. My main concern is to avoid porpoising/slamming due to high speed (40 + knots) requirements. All these boats are designed as per category C (<2m waves).
    Can anyone guide me how to know if the hull shape will have porpoising effect at high speed, any particular calculations?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    That is a bit subjective, you can't avoid porpoising or slamming of some degree, just keep it within what is acceptable. These boats have what purpose ?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    For a very quick rough n ready guide, the graph below may assist you:

    upload_2018-9-20_17-43-57.png
     
  4. Bala5278
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    Bala5278 Junior Member

    Thank you,

    I understand its not possible to avoid it but I would only like to reduce them in the design stage. It will be used as a rescue boat(The purpose may change in the future). Its a prototype as well. Recently one of the boat was even rejected by the client due to more porpoising.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    A rescue boat ? Power catamarans, with a smattering of rigid hulled inflatables, seem to have just about all the market, where I live. But 40+ knots is pretty brisk for small boats on choppy water.
     
  6. Bala5278
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    Bala5278 Junior Member

    Rescue boat is not the only market, the idea is to build a high speed craft which also not so bumpy. Later the based on the client needs the purpose can change. rigid inflatable power catamarans are interesting too. Can you show some pics or links?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I meant power catamarans OR rigid hulled inflatable. Deep vee monohull gives a good ride, but as a rescue boat, less suitable than a catamaran, due to less stable platform. What will the boats be made from ?
     
  8. Bala5278
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    Bala5278 Junior Member

    The boat will be FRP. would you suggest any software to analyse the porpoising ?
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I suspect porpoising is less of a problem than slamming, porpoising is a nuisance, slamming hurts ! What is the actual cruise speed you are wanting ? I assume the 40+ knots is top speed.
     
  10. Bala5278
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    Bala5278 Junior Member

    cruising will be 20 knots. But 40 is max we are looking for. I'm sure at cruising speed there wont be slamming, its just at the high speed it matters! The problem arises when the customer wants 40 knots speed as well as less slamming.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    20 knots certainly isn't "high performance". Is there a requirement that the boats run on a single engine ?
     
  12. Bala5278
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    Bala5278 Junior Member

    no requirements as of now, since its a prototype the requirements are not fixed yet. Roughly 1 engine with 150 hp for 6 m boat is what we have in mind.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Power cats offer the best mix of performance characteristics, in my opinion. most people who go out of monos into cats, don't go back, but the catch is the twin engines requirement. But if you want a single engine, and ride quality is near the top of the list of requirements, you need a deep vee around 20 degrees transom deadrise. Such a boat as you describe, 6 metres 150 hp, will cruise comfortably at 25 knots, and top speed not far from your 40 knots. Porpoising should not be a problem if properly designed, and there is always the option of trim tabs, which are probably a good idea anyway, to maintain level running at all times.
     
  14. Bala5278
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    Bala5278 Junior Member

    Thank you for the inputs, would be great if you can suggest any similar boats for reference.
     

  15. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Trim, either from an out-board (engine) or trim tabs... oops, I'm thinking of chine hopping.

    I've driven a number of 733 Zodiac and an 850 for about 1000 missions.
    Up to 52 knots, max 7.5m waves, max twin 300hp, zero porpoising.
    Cruise: 30 knots.
    It was never a problem.
    Redistributing load would be my go-to fix, also diagnostic in nature.

    733 Zodiac copies are hugely common here, numerous.
    Coast Guard mostly, in fiberglass with hypalon pontoons.
    5 000 kg of buoyancy, self draining, a rescue boat.

    I've never had any Zodiac porpoise, even knock-offs (fakes).
    Although, the 650's did occasionally, it was minimal and transitionary.
     
    Last edited: Sep 20, 2018
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