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Thread: Cavitating tinnie.

  1. #1

    Cavitating tinnie.

    Hi.
    The local cadet unit have this centre console tinnie. Think it's about 16ft. I got it running as it had been neglected and just needed a bit of TLC.
    I took it for a testrun and it absolutely flew. Its got a 60hp yammy 4 stroke from memory. As its so light literally less then 1 inch of the boat is in the water. The transducer throws up a rooster tail as its not in the water.
    I turned the boat and lost all drive. The engine revved out, dropped off the plane, that bad that i cut the engine and checked that i hadnt lost the prop.
    In a straight line it seems fine, but terrible to turn.
    Its got this big prop guard that is getting replaced withan even more restrictive one apparently.
    Now. I was thinking the motor might be mounted a bit high. Would dropping the motor a bit help cavitating? Or is it all to do with the prop guard and unavoidable?
    I took a couple of photos, hard to see though..... impossible to get the angle right.
    Now it's my understanding that the cav plate should be level with the lowest part of the hull. This one looks like it maybe an inch or so above that level.......
    Anyway photos attached, see what you guys think.20180309_211418.jpg20180309_211343.jpg20180309_211339.jpg20180309_211350.jpg

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  2. #2
    Ausfish Addict Chimo's Avatar
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    Jun 2006
    Location
    Gold Coast

    Re: Cavitating tinnie.

    Motor may well be a little high but with a load of cadets in the boat it would sit lower so the issue would be lessened.

    Why would you put a more restrictive guard on.?

    The prop guy in Southport could probably offer suggestions re the impact of the guard but I cannot, sorry.
    http://www.propellerrepairsaustralia.com.au/

    Cheers
    Chimo
    What could go wrong.......................

  3. #3

    Re: Cavitating tinnie.

    To me it looks like it is too high above where it should be. I don't think the height should be dependent on the amount of weight in the boat. If you had a full complement of people for that size of boat the hull would sit at a certain depth, however once it started planing the cav., plate would be set at the right height and the motor should perform as expected without problems.

  4. #4

    Re: Cavitating tinnie.

    Quote Originally Posted by Chimo View Post
    Motor may well be a little high but with a load of cadets in the boat it would sit lower so the issue would be lessened.

    Why would you put a more restrictive guard on.?

    The prop guy in Southport could probably offer suggestions re the impact of the guard but I cannot, sorry.
    http://www.propellerrepairsaustralia.com.au/

    Cheers
    Chimo
    Im not sure why theyre putting on a more restrictive guard. I gather that this one is not up to spec for the cadets. I can only imagine it would make the issue worse. The cavitation was insane. I literally cut the engine and checked i still had a prop the first time it happened. Zero drive.

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  5. #5

    Re: Cavitating tinnie.

    I can only assume the gaps in the guard are too large ie- you can get a hand or foot in there and thus it doesn't meet cadet specs. I have always found with pressed tinnies that motors need to be a bit deeper due to the keel extrusion taking water away from the prop during a turn. What you are seeing is pretty much an exact duplicate of my own results when messing around with prop heights on my old rig. I could get around it by heavily trimming down prior to turning but that introduced other handling issues. End result was to drop the engine to a comfortable medium between outright performance and breaking free on a turn when towing tubes etc.The only way to know for sure is to do the work and experiment - preferably with the guard both on and off . That way you will know for sure what is having which effect.

  6. #6

    Re: Cavitating tinnie.

    Looks too high and the prop guard will add to the problem of cavitation.
    If the boat gets on the plane fully loaded the water level will be in line with the bottom of the hull at WOT.

    You may also be arse heavy with an empty boat as well, this alone can cause cavitation on high speed turns.
    Jack.

  7. #7
    Ausfish Gold Member ThePinkPanther's Avatar
    Join Date
    Jul 2004
    Location
    On Moreton Bay

    Re: Cavitating tinnie.

    Had a mate with the SLSC at Moroochydore and they had the identical issue with some of their rubber duckies that also had huge prop guards like yours!

    They never beat it in spite of lowering the donk as much as possible and getting more compact guards, seems to go with the territory!

  8. #8

    Re: Cavitating tinnie.

    Thanks for the help. Might drop it an inch or so and take it for a run. As you say, the guard is likely the issue but any improvement at all would help.

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  9. #9

    Re: Cavitating tinnie.

    And if i can remove the guard ill have a crack with it off, thatll be definitive.....

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  10. #10

    Re: Cavitating tinnie.

    Prop guards really hurt performance, those yamaha props are very easy to add cup to and respond very well to it. I would send it to a shop and ask for heavy cupping to be added. Will cost about $50. Engine height wont matter after that, as it will work submerged or surfacing.

  11. #11

    Re: Cavitating tinnie.

    Let's see we have excellant performance in a straight line but severe cavitation on a turn. So the prop guard is not hindering performance but it could possibly doing so on turns, this would possibly indicate that the guard support blades are creating heavy aeration as they are coming out above the water flow. Dropping the motor should reduce this. The photos look like the motor is high but hard to tell, get a staight edge, a piece of timber will do and hold it up under the hull and see where the cav plates sit, a much more accurate way than by eye I've been surprised when comparing where I thought it was and actually was. You should also trim the motor out to normal running position when you do this.

  12. #12

    Re: Cavitating tinnie.

    Thanks for the replies. When i get a chance i might knock it down an inch. And if i can, ill take the guard off.
    Reckon a hydrofoil would help?

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  13. #13
    Ausfish Addict Chimo's Avatar
    Join Date
    Jun 2006
    Location
    Gold Coast

    Re: Cavitating tinnie.

    What do you want the hydofoil to do?

    So you dont need the prop guard any more?

    What about the cadets and liability ? OHS issues?

    Suggest whatever you do you limit changes to one at a time so you have some hope in deciding what helps and what does not.

    Cheers
    Chimo
    What could go wrong.......................

  14. #14

    Re: Cavitating tinnie.

    Sorry. Meant id take the guard off to see if it improves. Itll need to go back on, but just to see how much of the problem comes from that.....
    Not sure the theory behind the hydrofoil. Maybe holding the leg down a bit.

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  15. #15

    Re: Cavitating tinnie.

    Hydrofoil does the opposite. As Chimo says, one thing at a time, start with the straight edge to confirm the height, then take it from there. If there is some change when the guard is removed all it is proving at that stage I'd that it is a contributing factor.

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