What does an outboard cavitation plate ACTUALLY do?

Discussion in 'Boat Design' started by jakeeeef, May 22, 2023.

  1. jakeeeef
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    jakeeeef Senior Member

    Hi,
    As part of a small electric boat build, I am electrifying a 4 hp 2 stroke outboard with a 6kw ebike motor. It's an easy way to upcycle an otherwise scrap motor, and this motor is certainly scrap! see pic!, Plus having a gearbox means I don't have to worry about changing motor direction for reverse. Plus it's all marinized etc. so I don't have to concern myself too much with anything below the motor tray.

    I was initially thinking of doing an inboard with conventional shaft, but going the outboard route means I can use the motor on multiple boats. The car boot full of junk even includes both a longshaft and short leg, so lots of options for powering future boat builds going this way.

    I'm getting rid of any excess weight and drag from the leg(s). Powerhead delete (obviously), exhaust tube delete, impeller delete etc. Carbs, pumps, recoil starters, coils, powerheads (there were (almost) 2 engines in my box of Suzuki DT4 junk), all on eBay already. For what the carbs alone are worth, the total project cost will be down below zero.

    I'd ideally like to experiment later with larger, slower turning, higher aspect two bladed props, as the standard aluminium bunny ears prop will be an efficiency limitation. Clearly the 'cavitation plate' as outboard manufacturers insist on calling it (surely it's to do with ventilation, not cavitation?) is somewhat in the way.

    So, I'm wondering how much of what the 'cavitation plate' does is to do with internal combustion, exhaust gases, quietening the exhaust etc. and therefore whether it can be removed, and how much of what it does is actually hydrodynamics?

    Note, if these plates are put on for planing only reasons, while this first use of the engine is for a displacement boat, future uses could be for planing hulls, and in the case of the long shaft, foiling perhaps.
     

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  2. gonzo
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    gonzo Senior Member

    The plate prevents ventilation of the propeller. In short, when the propeller gets close to the surface, the low pressure on the forward face will suck in air and the propeller will lose thrust. Also, having the exhaust at the forward face of the propeller will ventilate it. If you want to run a ventilated propeller, commonly known as a surface drive, then it is a different operating mode and the plate would be out of the water anyway.
     
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  3. fallguy
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    fallguy Senior Member

    I wouldn't remove it if you ever want to plane the boat.
     
  4. ondarvr
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    ondarvr Senior Member

    The AV plate uses tend to overlap a little, but the most important factor is as mentioned, stopping air from being sucked in.

    This is most helpful at low too moderate speeds under hard acceleration, at high speeds they don't help in the same way.

    They also get used to control bow rise and other things, but again, this is at lower speeds. They typically work best when they are out of the water at high speeds.

    I would wait to cut it off, if you ever do, it's a quick and easy job to remove later. I'd experiment with props first. And the weight savings would be minimal.
     
  5. portacruise
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    portacruise Senior Member

    From Jake,

    "I'd ideally like to experiment later with larger, slower turning, higher aspect two bladed props, as the standard aluminium bunny ears prop will be an efficiency limitation. Clearly the 'cavitation plate' as outboard manufacturers insist on calling it (surely it's to do with ventilation, not cavitation?) is somewhat in the way."

    The ventilation plate will block off when using no clearance, large diameter props, unless an extension propeller Drive shaft is extended beyond the length of that plate, and that plate plus crudely faired down leg may wind up introducing significant blocking disturbance/ turbulence intake or other issues with large diameter props. I would look for the longest down leg possible to keep any experimental larger props well submerged below the surface, so that they don't suck in air, and cut off the plate if necessary. Another down leg is probably easy to source should you ever need something with the cavitation plate, it can be done again...

    A much simpler / efficient /lightweight/ adjustable leg depth solution would appear to be the Asian trailing shaft design, at least that has worked the best for me, as a test bed for different propeller designs..
     
    Last edited: May 23, 2023
  6. jakeeeef
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    jakeeeef Senior Member

    Yes, trailing shaft very efficient and self balancing. I'm trying to build something more like an outboard though, that fits in a drive well on various boats etc. So I'm kind of stuck with the basic format of an outboard.
     
  7. portacruise
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    portacruise Senior Member

    Finding the most efficient prop doesn't necessarily have to depend on using a particular outboard setup. Sometimes, the outboard setup itself might have to be altered in a minor or if practical, in a major way to take advantage of a significantly more efficient prop. A simple "Asian motor" test bed for finding the most efficient prop can be made from an inexpensive motor rated at the particular design torque and RPM needed, Plus a long shaft, Plus Quick Change adapter at the end of the long shaft. As an example set up for low power applications, I have used a cordless drill, 4' threaded rod, and a handful of pre-screened RC props tested with different boat hulls ( hull design also makes a big difference to overall efficiency) tested on the water, tweaking each aspect, for the best efficiency..

    I've found that gear case losses can be quite significant and can be lowered significantly when running at lower RPMs associated with large propellers, at least for the simple tests that I have done.
     
  8. alan craig
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    alan craig Senior Member

    I've done a brushless conversion on an Evinrude 3hp leg, three boats driven by cordless drills - one was an airboat, the other two had long shafts, one also had a conventional stern tube. Also made a prop shaft extension for the Evinrude, to use larger props behind the anti-ventilation plate. R/C model plane props work great until they find some weed. Use props for IC engines; props for electric 'planes are not stiff enough.
    Re. what Portacruise said about gear case losses - I agree; I found that the gear case can actually feel warm after an hour at less than 1hp. It's been suggested to me to put much thinner oil in the gear case but I haven't tried it yet.
    If you search my content you should be able to find some of my pics.
     
  9. C. Dog
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    C. Dog Senior Member

    OMC used to recommend around a 30 weight oil for the smaller engines as the 90 weight sucks power.
     
  10. C. Dog
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    C. Dog Senior Member

    Cav plate/anti ventilation plate is generally surplus at displacement speeds, but during planing the tip of the blades is pretty much at the surface, and the prop will pull air and ventilate without that plate to prevent it.
     

  11. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member


    The AV plate doesn't do much at speed, it's primary function is to allow the prop to be closer to the surface at rest or low speeds and not ventilate. At speed the prop can even peirce the surface and not ventilate in the same way.

    For best handling and speed the AV plate typically needs to be out of the water when on plane.
     
    Last edited: May 28, 2023
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