Does cav plate create suction on the water surface?

Discussion in 'Hydrodynamics and Aerodynamics' started by Bluejinx202, Jan 9, 2015.

  1. Bluejinx202
    Joined: Apr 2012
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    Bluejinx202 Junior Member

    Could someone explain the reason why if I lift my cav plate up on my speed boat, it helps lift the front of the boat at speed? I have a problem where my hull is trying to "suck down" at speed (55mph) I have increased the size of the cav and lifted the angle to about 5mm above the the bottom of the boat and its helped and given the boat a better attitude, but only allowed me to get another 5mph before it drops the front again. Could someone explain what's happening here so I can keep trying to nut this out? 60mph feels a little hairy with the nose down.
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What is the boat powered by, outboard engine?
     
  3. Bluejinx202
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    Bluejinx202 Junior Member

    No, 6cyl midmount, direct shaft drive
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds like it is acting as a trim tab, might be some inboard ski boat people here who can advise you.
     
  5. powerabout
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    powerabout Senior Member

    What hull is that?
    Midmount at 55 is about it, at 60 its going to get hairy
     
  6. Bluejinx202
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    Bluejinx202 Junior Member

    It's a 1965 Seacraft 140 (14 foot, 6 inches) powered by a 202 holden inline six. I was out testing today, had the gps top out at 61.8mph while pulling to the right hand side. I'm thinking about changing the shape of the cav to try and get the boat back on its center point and get rid of this pulling down and sideways thing it's doing. I'm just looking for some info from some experienced boaties as there isn't much written down on doing what I am doing
     
  7. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    http://ssboc.com.au/
    The steering pulling is what happens when you get to 60 ish in mid mount.
    Thats good speed, impressive if its a stock 202.
    You need the weight back and smaller propshaft angle..
    Yes you need a vee drive to go faster comfortably.
     
  8. Alannanicole
    Joined: Jan 2015
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    Alannanicole New Member

    rise it up again, its becomming negitive wedge as the hull lifts basically the prop is lifting the stern and the running surface is causing water to lift against the plate have your prop worked to add bow lift and adjust the plate again the same distance you did the first time also since its shaft drive you may need to add a couple small lift strakes forward of the prop near the center if the hull doesnt have any
     
  9. Bluejinx202
    Joined: Apr 2012
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    Bluejinx202 Junior Member

    Could you explain lift strakes? I figure the cav plate is creating a net pressure at speed and sucking down and the angle is therefore lifting the front of the boat
     
  10. Bluejinx202
    Joined: Apr 2012
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    Bluejinx202 Junior Member

    http://youtu.be/bxNGgCjH0KE
    The above vid is of the boat at the ssboc 30th year regatta at echuca last year. The engine is far from stock;)
     
  11. whitepointer23

    whitepointer23 Previous Member

    i have to ask , what have you done to the motor. is it a blue , red or black motor. because that thing is flying for a holden 6. have you tried adjusting up the cav plate as far as you can and see what it does. is there a skeg, at that speed you probably need one a third of the way along the hull like a race boat. is the hull true. have a real good look along the bottom and make sure it is perfectly straight. because of the boats age it may be slightly distorted or warped somewhere. if this is the case the problem gets worse the faster you go.
     
  12. whitepointer23

    whitepointer23 Previous Member

    another problem i can see is in 1965 it would have been designed for a stock motor and speeds into the forties , i think you are pushing the hull well beyond its designed max speed and that may be the trouble. the term is hull racking i think.
     

  13. whitepointer23

    whitepointer23 Previous Member

    Wonder if he found the problem.
     
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