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  1. #1
    casual poster Flatline's Avatar
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    Default Cavitation Plate Adj.

    Hi all,

    New to this forum. Have water skied all my life and have finally got my own boat. Flightcraft Banshee with 350 chev mid mount, Edelbrock inlet, Rolco exhaust/heat exchanges, Holley 600 set up as double pumper. Other than that standard. 12 x 14 prop. I have stripped it down to gelcoat and rebuilt/Re-paint it. My question is regarding the cavitation plate adjustment. Can someone advise whether it should be level with hull bottom or slight down thrust. PS how do you insert a picture for your profile and other views. would like to show what i have done.

  2. #2
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    IMO, cav plate should be level with the bottom of the hull. A centre mount sits on the plane very flat (compared to a rear mount), so you don’t want to bury the bow any further in the water by giving the cav plate any down angle. You could experiment with it though to see where it sits best.

    To upload pics, have a read of this link
    http://www.aussiev8.com.au/aussiev8-...-aussiev8.html

    To change avatar:
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  3. #3
    Senior Member cactus's Avatar
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    Hi Flatline..........great little hull you've got yourself there.......
    ..........if its just the standard size cav plate ,then adjusting it up / down won't have a dramatic effect on the peformance of the hull
    having said that.........I'd be inclined to wind it up as far as it'll go to try to get the nose of the boat up as high as possible , reducing the wetted surface area of the hull,to increase
    speed..........if this affects getting up on the plane/ cornering/steering too much, then wind it down till your happy with it.

    Cheers Cactus

  4. #4
    casual poster Flatline's Avatar
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    I have no trouble planing or pulling out of the water. sits pretty flat i think. did notice r/h side of plate was higher than left. As far as speed goes (only had it in the water twice since rebuild) 80kph at 3000rpm. Speed not an issue, as we are very keen skiers, tubers. How do i upload photos.

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    Senior Member Tinman's Avatar
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    If it is not level it could make the boat tilt to one side or even make it want to turn a bit with the rudder straight. Unless it has been done deliberatly to compensate. I would even them out and see what happens. Most mid mounts dont have anything more than enough cav plate to mount the steering rudder because the prop angle pushes the boat up out of the water.

    I think you need to have a minimum of 10 posts before you can upload photos.

    12 X 14" prop should be about 60k at 3000RPM you would need to have about 19 inches of pitch to do 80 @ 3000
    Cheers Tinman

    Jack of all trades master of none

  6. #6
    casual poster Flatline's Avatar
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    Default

    Thanks for the replys. I adjusted the plate stright and level. Steering has improved each way. Might try and lower a smidge to help with low oil pressure problem when towing slow. If it doesnt work i can always put it back. As far as 80kph @ 3000rpm, i'm not telling porky's. I have no speedo at the moment, and had 2 new jet ski's pace me to mark speed at rpm. 1500rpm @ 30kph, 1800 - 2000 @ 40kph, 2500rpm @60kph, 3000rpm @ 80kph. I have no real intrest at top speed, because we are serious skiers and boarders. Just thought i would let you know. Dont know if its boat weight or what. It is a brand new Austral Compuski 12 x 14 witha large cup.

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    Quote Originally Posted by Tinman View Post
    12 X 14" prop should be about 60k at 3000RPM you would need to have about 19 inches of pitch to do 80 @ 3000
    +1

    Not trying to get too far of topic here Flatline, but I find your rpm/speed figures very interesting.

    Using a 1:1 gear ratio and adding an extra 1” to the pitch for the cup, I punched your numbers into a Prop Slip Calculator……

    1500rpm @ 19kph = +10% slip
    2000 @ 25 = +11%
    2500 @ 37 = -3%
    3000 @ 50 = -16%

    It’s impossible to get (-)ve % prop slip because it means theoretical speed is slower than actual speed.

    By all means 80kph is impressive, I’m just thinking your tacho maybe inaccurate (or the Jet Ski speedo is inaccurate).

  8. #8
    Senior Member cactus's Avatar
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    Hi Flatline......your tacho is obviously inaccurate..........
    I'd be fixing the low oil pressure problem before anything else............. if you want to save your engine!!!!!!!!!!!!!!!!!!!!!!!

    What is the volume of the sump?How much oil is in the engine?What type of oil are you running?
    Sounds like there may be too little oil at the pick up when at the lower speeds/or maybe a windage prob

    ....good rule of thumb is 10 psi for 1000rpm

    .................ie.......2000rpm....>20psi
    3000rpm....>30psi
    4000rpm....>40psi

    Hope this helps

    Cheers,Cactus.

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    Senior Member cactus's Avatar
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    ................just read your other thread.............stick some more oil in it quick!!!!!!!!!!!!!
    ................
    I'd try at least 8 litres in it.....and go from there.

  10. #10
    casual poster Flatline's Avatar
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    I think i have sorted out the oil problem. not enough for the type of sump. Pressure is fine any other time. As far as speed V RPM i was hoping you guys would have the answer. i have hooked up a digital tacho at the same time and both read the same +/- a bit. I am 42 years old and i dont give a rats about my boat is faster. Would just like to know why? I have just painted it in two pack and it has a mirror finish under and on. 3 coats of wax. I have owned a couple of big yachts and done a lot of sailing. so know a little about shaft alignment, thrust and slippage. These boats are new to me and i am trying to get a grasp on it all. I just posted a thread in the general section in regards to engine numbers. you guys might be able to help. 307 or 350? CID 3914636 Model V0523CNR. I have uploaded some photos to my album if you want to look.

  11. #11
    Do you ever leave? Mickss355's Avatar
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    Getting back to the cavitation plates - for what its worth - years ago i went for a ride in a ski boat that had the cavitation plates hooked up to a link that went to a foot pedal under the dash to control the boat during hard cornering and also to stop porpoising when crossing another boats wake !

  12. #12
    casual poster Flatline's Avatar
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    Yeh Mick i have seen a few photos of boatsfor sale with the same setup.

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    Do you ever leave? Mickss355's Avatar
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    I don't profess to know a lot about ski boats but i can tell you this ! when my mate stuck the boot into it on a turn and pressed on the cav plate pedal it tracked that well it almost made me feel sick in the stomach with the Gs hahahaha !

  14. #14
    casual poster Flatline's Avatar
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    I have a Rolco dog clutch. Is it 1:1?

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    Yeah Mick, very standard for a rear mount, and does make a huge difference. I have the same set up in my rear mount just hand operated (looks exactly like a handbrake lever!). The cav plate only has about 10mm movement but really brings the bow down when in choppy water. And when fully up can get a lot of boat out of the water at high speed.

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