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Old 15-08-2019, 15:17   #1
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Dinghy Cavitation

I have an Avon 10' RIB with a 2-stroke merc 5 hp short shaft. The engine ventilates/cavitates at full throttle up on plane, ever so slightly. The anti ventilation plate of the outboard is about 1" below the lowest point of the hull and the outboard is trimmed all the way down. There is no easy way to get the motor mounted lower without hacking at the transom. I have not found a "foil" that fits this motor and I'm not sure that would help. Any suggestions would be appreciated.
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Old 15-08-2019, 20:56   #2
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Re: Dinghy Cavitation

Try trimming out, move the leg away from the transom.
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Old 16-08-2019, 14:55   #3
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Re: Dinghy Cavitation

How about more weight? You could carry an anchor! Friends drifted all night once cause they had no anchor.
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Old 16-08-2019, 14:57   #4
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Re: Dinghy Cavitation

Do you mean directly aft-ward or tilt? Tilt definitely does not help.


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Try trimming out, move the leg away from the transom.
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Old 16-08-2019, 18:25   #5
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Re: Dinghy Cavitation

Locating the anti-ventilation plate 3/4 to 1" below the bottom surface of the hull is more or less standard practice for single engine installations in " normal" installations. Lowering the plate height (increasing X dimension) will mostly increase drag without lessening ventilation and raising the X dimension will increase speed somewhat at the risk of ventilation in hard turns. As you know, trimming the drive out will lift the bow of the boat and possibly increase speed, but also get the prop closer to the surface of the water and encourage ventilation and prop blowout during hard turns.

If you have prop cavitation and thrust breakdown (not ventilation) at high speed only, it might not be worth worrying about it, as little time is spent at WOT.
But if you are getting enough cavitation to cause prop burns, there are a few things you might check. These might be:
- dings on the leading edge of the prop blades....this can cause bubble streams flowing back across the blade and collapsing later on the blade....noise and blade damage
- disruptions on the bottom of the hull ahead of the prop, feeding turbulence and bubbles into the prop...causing cavitation, etc.
- oil drain plug on the lower unit protruding too far into the flow, or any other damage on the gear case, causing bubbles to flow into the prop, thus cavitation
- prop not true.....more pitch on one blade than the other(s)
- not enough blade area on the prop for the h.p. and speed....better design prop might help, or a prop tune up
- adding a little cup on the trailing edge of the blades might help retard cavitation

Hope this gives some ideas...
DougR
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Old 16-08-2019, 19:53   #6
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Re: Dinghy Cavitation

The reply by DougR is comprehensive.
Is that a thru-prop exhaust? Maybe the clearance between the box and the front of the prop boss is too big, (possibly a reversed thrust washer) Exhaust gasses being sucked out there?
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Old 17-08-2019, 09:47   #7
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Re: Dinghy Cavitation

DougR, thank you very much - I will begin digging. The engine was not behaving in this way on my last dinghy so I suspect it is something around the position or the hull - unless I messed something up in between dinghys on the outboard. And yes, thrust breakdown, sorry didn't really know the term - basically increase in RPM decrease in speed up on plane. Happens at 3/4" throttle + but the boat doesn't really plane below that... Should probably have an 8hp...

Quote:
Originally Posted by DougR View Post
Locating the anti-ventilation plate 3/4 to 1" below the bottom surface of the hull is more or less standard practice for single engine installations in " normal" installations. Lowering the plate height (increasing X dimension) will mostly increase drag without lessening ventilation and raising the X dimension will increase speed somewhat at the risk of ventilation in hard turns. As you know, trimming the drive out will lift the bow of the boat and possibly increase speed, but also get the prop closer to the surface of the water and encourage ventilation and prop blowout during hard turns.

If you have prop cavitation and thrust breakdown (not ventilation) at high speed only, it might not be worth worrying about it, as little time is spent at WOT.
But if you are getting enough cavitation to cause prop burns, there are a few things you might check. These might be:
- dings on the leading edge of the prop blades....this can cause bubble streams flowing back across the blade and collapsing later on the blade....noise and blade damage
- disruptions on the bottom of the hull ahead of the prop, feeding turbulence and bubbles into the prop...causing cavitation, etc.
- oil drain plug on the lower unit protruding too far into the flow, or any other damage on the gear case, causing bubbles to flow into the prop, thus cavitation
- prop not true.....more pitch on one blade than the other(s)
- not enough blade area on the prop for the h.p. and speed....better design prop might help, or a prop tune up
- adding a little cup on the trailing edge of the blades might help retard cavitation

Hope this gives some ideas...
DougR
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Old 17-08-2019, 14:38   #8
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Re: Dinghy Cavitation

Another possibility.....prop hub is spinning at high power load but holding at lower throttle settings.

Take the prop off and make a small punch mark on the inner hub and a corresponding adjacent punch mark on the outer hub right next to the first. Run the boat and check to see if the marks have moved relative to each other.

Or, borrow a test prop and see if it acts the same.

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