Questions about rocker

Discussion in 'Boat Design' started by Bullshipper, Jun 12, 2021.

  1. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    I have a 28' symmetrical power cat that has no rocker.
    She likes to run around 40 mph in calm or rough conditions.
    I would like add about 5-6" of height to the bottom off her sponsons to increase here buoyancy and tunnel height over the water as I glassed this prototype too heavy.

    The plan is to lift here on 4' tall stands, apply foam to the bottom to shape by sanding prior to glassing to perform another dynamic test

    Would you add rocker?
    Would you make the lowest point of the dip around 10% forward of her Center of Gravity?
    My chord length is 240" and I have drawn a dip with only of 1% or 2.4" lower than a flat line. It looks about right to me, but its strikes me as way to low. Any comments on this?

    Thanks
     
  2. kapnD
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    kapnD Senior Member

    Pictures?
     
  3. messabout
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    messabout Senior Member

    If you are planning to run 40 MPH then no rocker need be added. There are a lot of variables here but if you have a planing boat it will most likely be slower if you add rocker to the bottom.

    Why do you need more buoyancy? Is is because the gross weight of the boat has increased because you have added weighty objects. If so the speed may be compromised regardless of added displaced volume.

    Cats are weight sensitive. Even a ham sandwich adds overall weight. So also will any additional construction components such as additions to the bottom.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    So what is the problem that demands a solution ?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Rocker is the last thing to add to a planing boat, particularly a cat, it will turn it into a porpoising nightmare, I assume you think the boat is too low in the water and slapping the tunnel ?
     
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  6. Bullshipper
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    Bullshipper Bullshipper

    As mentioned glass is thick and heavy. 6000 lb hull weighs 8100.Sponsons are 34" wide at bottom while others measure 42". Running in rough conditions is only ok when going fast to create lift.
     

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  7. Bullshipper
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    Bullshipper Bullshipper

    More fotos
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Situation normal for good rough water cats, they don't like going slow, and they can run hard through some pretty terrible water, if well- designed, do you have a picture of the stern end ?
     
  9. Bullshipper
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    Bullshipper Bullshipper

    Hull acts like it is overloaded when loaded light. Adding 6" adds 4000 lbs in bouyancy. Top of tunnel is only 3.5" above water at rest at 2 degree attack angle
     

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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    You really can't design such a boat to have high clearance off the water in the tunnel, when the boat is planing, the boat rises, the idea is to stay on plane, the wave slap will not go away at slow speeds by raising a few inches
     
  11. Bullshipper
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    Bullshipper Bullshipper

    Thank you all for confirming that rocker is not needed. Hull turns and leans into turns very well at all speeds.
    I have been running power planning cats (4 brands) for 24 years but appreciate you trying to also help me with the other basics.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I would say your 3.5" is within the range of normality, if you wanted to increase it at slow speeds, a belt of buoyancy on the outside would be much easier to do. But I don't think I would bother
     
  13. Bullshipper
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    Bullshipper Bullshipper

    This is loaded light. 625 lbs more will drop her another inch and it will be easy to add another 2000 lbs in fuel, passengers, and ice and fish
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Every planing cat I have owned or been aboard, the clearance was minimal, if it isn't a problem at speed, what are your concerns ? They all slap to some degree, under some circumstances
     

  15. Bullshipper
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    Bullshipper Bullshipper

    Glass thickness exceeded spec
     

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