Maintaining a level hull during WOT acceleration.

Discussion in 'Boat Design' started by cyclops2, Sep 27, 2010.

  1. cyclops2
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    cyclops2 Senior Member

    The WW II Scott-Paine PTB had this fantastic abitity. Is this a result of the 10' beam & 60' WL. Or the % of the hull area that is actually in the water at rest.
     
  2. Anytec1210
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    Anytec1210 Junior Member

    Are you talking about the PT9 here? Scott - Paine made several designs.

    Lovly boats anyhow. All of them - The Elco 70 (US version of PT9)

    [​IMG]
     
  3. cyclops2
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    cyclops2 Senior Member

    I really do not know.
    There are so many variants in the 60' X 13' size class. As I recall it had the low , long center cabin & 2 Dewandre plexiglass turrets.

    I can post a picture of it within a few days. I am on the road & my PTB files are at home. I ask this question, as I am building a 20' X 4', 2 seat Gentlemans Runabout with a constant 27 degree deep V bottom. It has a vertical bow to prevent climbing over safe sized waves. It should simply split the wave apart & continue on, in a very boring & dry ride untill waves are higher than the bow.
    Speed & fuel consumption are of no importance. A deep spade rudder is. Hopefully we will start with about a 12" X 36" rudder & slowly shorten it as we increase speed & the sudden hard over turns. The goal is to be able to snap the wheel over suddenly at maximum speed & bank very comfortably into the turns. The " G " force will push us straight down into the seat cushions.
     
  4. daiquiri
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    daiquiri Engineering and Design

    The axe bow (if that's what you intend) will not be enough, the behaviour you have described will very much depend on the overall vessel's mass and on the longitudinal mass distribution.
    Besides, the waves do not just split apart. The water will also want to swell up and go below the hull. The water incompressibility and the law of conservation of mass are the culprits. So it might be a more leveled ride in the chops, but it will also be a wet one too. ;)
     
  5. cyclops2
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    cyclops2 Senior Member

    I have a 2002 19' Chaparrel bowrider with a 5.0 L V8. It is 8' wide.
    The boat off of plane is a wandering dog. It wanders from port to starboard constantly during a 20 second duty cycle. All the time. I have been told by dealers that is a very common tradeoff problem with modern " stubby " hull designs. Needless to say it bounces up and down with each 1' wave. I know of no boats that have full keels or full length constant deep V designs, to have such bad directional control.

    As to wetness. I simply slow down. Or not take the boat out if waves are too much.
    The Chap is outstanding in 4' waves & 6 people. Tilt the drive in & bring the bow up to a attack angle of 35 degrees. Very dry ride compared to 40' old woody cruisers in the boat club. The Chaps bow is about 7' high in that rough water trim condition.
     
  6. MoonlightShadow
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    MoonlightShadow Junior Member

    Interesting question. I bet the answer is mega complicated. Have a Ribcraft 4.8m with 60hp outboard which stays perfectly stable accelerating under WOT.

    And another one, (Scandinavian built) that does not. Every now and then under WOT from rest, it starts to chine bounce. They are identical length to beam ratio.
     
  7. anthony goodson
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    anthony goodson Senior Member

    And the relative heights of the collars on the hulls are ?
     
  8. MoonlightShadow
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    MoonlightShadow Junior Member

    Ah. You are provoking my thought now. The RibCraft sits with the tubes on both sides just kissing the water on both sides, while the other one lies to one side or the other, as you move about in it.
     
  9. cyclops2
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    cyclops2 Senior Member

    Can some " stubby " V hulls chine walk while off plane in gear? 5 to 10 mph.
    16' X 8' at the waterline.
     
  10. Willallison
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    Willallison Senior Member

    As daiquiri points out, there's much more to attaining a smooth ride that having a vertical stem...
    But much more concerning, if you are really planning on building a boat are some of your other comments. A conventional planing hull will typically run at between around 3 and 6 degrees trim when on the plane. I find it very difficult to believe that you are operaing with 35 degrees of trim... apart from anything else it would be impossible to see over the bow!
    Further, how you would manage to do so by trimming the leg in is beyond me.

    And no - boats don't chine walk at displacement speeds - it is a dynamic instability problem, and there is little if any dynamic lift when going slow.
     
  11. MoonlightShadow
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    MoonlightShadow Junior Member

    In theory, shouldn't the tubes of a RIB not be touching the water when properly trimmed on the plane? I know it doesn't actually work out like that, but is that not the theory?

    Therefore, chine hopping is a hull moulding/balance issue?

    What actually causes it?
     
  12. cyclops2
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    cyclops2 Senior Member

    All good points about the 35 degree A O A.
    All heavy people were seated at the rear seat. Next heavest was in the passenger seat. Lightest person was alone in the bow close to the walkthru opening. I increased power until the boat was running in the deepest " squat " possible. That is how a short boat can have a bow so high in the air. Seeing where I am going is safely accomplished by slowly swing the bow from side to side. That trim condition is ONLY done when a large boating picnic is over & you are about 1/2 way back to your loading dock. Normal wind & light rain was predicted for the day. I found myself in a squall with 30 to 50 mph gusts in less than 5 minutes. Some of the waves went to 4' in a heartbeat.

    Best thing was that my course home was directly into the wind. Turning & running with the waves seemed suicidal with a boat full of seniors in lifejackets in 60 degree water, with those waves.

    The stubby boat was outstanding when I was in way over my head. The boat has a 200 hp V8. I am quite sure that contributed to being able to swing the bow side to side without the wind causing the bow to go into a broaching position.



    Edit.

    The above experience is absolute proof of my saying about life.

    Better to be lucky than very experienced.

    Luck always wins.
     
  13. cyclops2
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    cyclops2 Senior Member

    I would like to thank those who have stated that the 200 hp Chaparell has a weight distribution problem. It most likely does. The boat is designed for a 4 cylinder engine. V6 is the next weight option. I have the heaviest V8 option.
    The 5.0L mercruiser.

    Ballest of flat lead to be placed at the lowest accesable points on centerline ?? With a reduction in the # of people to be placed in the boat ?

    It would be so nice to approach a dock slip in bad winds with a boat that does not steer me around.
     

  14. cyclops2
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    cyclops2 Senior Member

    This is what I already have pre cut the BS 1088. All frames from the transom to 14' foward are 27 degrees. Keel & engine stringers are also cut. :rolleyes:

    Before I glue up this pile of future fire wood. :)

    Or should I just do a static loading of the hull dockside with 40# bags of weights in locations that represent all weights when complete?

    Laughter is welcomed. :)
     

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