Cavation or aeration problem?

Discussion in 'Boat Design' started by explorer3114, Jul 6, 2008.

  1. explorer3114
    Joined: Sep 2007
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    Location: Melbourne, FL

    explorer3114 Junior Member

    I have a 1958 14’ Orlando Clipper with a 25 hp motor on it. This boat is very nimble and will turn on a dime, top speed is 30 mph. This hull had a lot of corrosion on it and I found a “new” 1952 14’ Orlando Clipper with no corrosion on it to replace my current clipper. The new hull is the same three seat open hull design with one difference. On the new hull it tapers in 2” on each side towards the back (so the transom is 4” narrower). When I mounted the motor on the new boat I shimmed the motor up 1” to get the motor cavation plate close to the bottom of the hull (I had to do this on the old hull too). On the test run I found the new boat cavates when you accelerate up on a plane and cavates bad in turns. It would still get up to 30 mph like the old boat. I pulled the shim out dropping the motor down 1” and this helped some…but I still have the same problem. I also adjusted the angle of the motor and found with the pin in the hole closest to the transom created a little less cavation. I never would have thought the narrower hull would have handled so differently. My first thought is to put a larger cavation plate on the motor (like a Stingray or a Doel Fin); my other thought is to add a set of trip tabs. Any idea’s on this? Do I have a cavation or an aeration problem? For reference…I weigh 220 lbs and the fuel tank is in the center of the boat (just like my old boat).
    Bill

     

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  2. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Those skegs and keel extending all the way to the transom look like the culprit to me. Did the other hull have the external bottom ribs like this? Keels should be cut away forward of the transom to prevent ventilation in turns.
     
  3. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    (1) 25 hp is a lot of power for a boat of that style and size. Indeed, I doubt it would get a rating of more than 15-18 hp if built and registered under current regs. Granted, your old one worked fine with that engine, but maximum power ratings are a bit more conservative now.

    (2) Sounds more like ventilation than cavitation to me, and I agree with Tom that the skegs/strakes/keel/whatever hanging off the bottom are part of the problem. Trouble is, they look like they're more for structural stiffening than for hydrodynamic gains. You might run into some flexing problems with the bottom panels if they get cut back. But my guess is that's how the air is getting into your prop.
     
  4. explorer3114
    Joined: Sep 2007
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    Location: Melbourne, FL

    explorer3114 Junior Member

    The other boat had the same stringers on the bottom of the hull although I believe they were plugged at the ends (I'll plug them to see if that makes a difference). Orlando Clipper used that design to keep the floor inside smooth. They seem to help with tracking too as the other boat handled like a sports car going up little rivers and creeks. I did not post a photo of the inside but the transom has three brackets that attach the transom to the floor. It's pretty beefy, they rated the 14' for 30 hp.
    Bill
     
  5. Bullshipper
    Joined: May 2008
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    Location: Mexico

    Bullshipper Bullshipper

    There is very little V on your hull, so on turns the motor will lift the prop higher, Bt the cavitaion plate that would normally be placed 5/8" above the keel, also appears to be much lower, which contradicts the motor height theory IMO.

    But it also looks like there is less than 14-16 degrees of angle on the transom, so this may be your problem when you apply power. Bob's Machine shop sells wedges for $30, and this may help to correct the angle if that is the case after measuring.

    To me ventilation is when the prop is too close to the surface, where cavitation is caused by buubles mixing with the water going to your prop.

    Both scenarios are likely.
     
  6. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Your anti-ventilation plate (what ever one is calling a cavitation plate) appears to be located properly, which is about an inch below the bottom of the boat. Attach a Dolfin or other anti-ventilation plate addition. This will likely solve the problem, though like Tom, I don't like the rubs right at the transom either.

    On those early double wedge hulls, the transom angle is often not enough, particularly for reasonably modern outboards. Some wedges could help, but I suspect your sucking air from the turbulence off those rubs. The Dolfin will help keep the upper side of the prop pressurized.
     
  7. explorer3114
    Joined: Sep 2007
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    Location: Melbourne, FL

    explorer3114 Junior Member

    I checked a few dimensions; the motor cavation plate is currently 1 3/8" below the hull. Transom is 15 1/2" tall and 42" wide. The angle of the transom is 11 degrees. I'm not sure what you mean when you say to install a wedge. Can't I accomplish the same thing by adjusting the trim pin? I included a photo of the trim pin in the second hole outboard position (which makes it about parallel to the hull). I also included a second picture of the braces inside the transom (in case y'all were curious).
    Bill
     

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  8. Bullshipper
    Joined: May 2008
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    Location: Mexico

    Bullshipper Bullshipper

    motor height

    look at the cavitation plate when running at a good clip going straight and when you turn

    if the plate skims along the top you should be fine in terms of motor height.

    If it comes out of the water, then lower the engine accordingly. If its under water, you are dragging too much lower unit and slowing yourself down.

    all hulls are different in terms of bouyancy at speed.

    Motors tilt up a lot more than they tilt down, so the wedge increases the down angle at the starting point.
     

  9. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Some good suggestions here from all, and it looks like you now have a few avenues of testing to explore, Bill. Nobody can say for certain what it will do next, but by trying a few of these suggestions independently and in combination you should be able to find one that works.

    Just to make sure we're all using the same terminology here:

    Ventilation refers to air entering the propeller arc, usually from the surface. This is what the plate above the prop is for on your outboard, to keep the water flow in the vicinity of the prop separate from the air above it. Surface-piercing drives like the Arneson are designed to operate while ventilating; most normal drives are not.

    Aeration refers to small bubbles of air being mixed in with the flow. This can be accidental and detrimental, or it can be intentional. (Mercury has a few props that intentionally bypass a bit of exhaust out in front of the blades to aerate the flow, which can be beneficial in a few particular cases.)

    Cavitation can be caused by several things, but the distinguishing factor is that cavitation bubbles are bubbles of vacuum- not air. The pressure on the back of the blade drops so low that a vacuum bubble forms. With nothing inside to support it, the bubble is unstable and violently implodes as soon as the pressure begins to stabilize- unfortunately, this usually occurs just a bit farther down the blade, and repeated cavitation bubble implosions can pummel away at the blade until it begins to erode. Usually, cavitation is only a problem if the propeller speed is excessive or if the prop is overloaded; a cavitating propeller can still produce plenty of thrust but will wear down rather quickly. It is possible to design a propeller to operate well in a fully cavitating state, but this is tricky and is generally reserved for larger, faster craft than are being discussed here.
     
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