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Old 26 August 2014, 09:45   #1
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Cavitation issues----------------HELP

I have had a number of both wooden and aluminium floor SIBs but recently bought an Excel SD330 flat air floor which is outside my comfort zone. The engine is a 15hp Parsun with a standard shaft ( 18'' from transom to cavitation plate ).

I have so far only had one other person on board with them sitting forward and myself on the tiller. The boat and engine runs as sweet as can be at half throttle but when we open the throttle three quarters to full, cavitation is the result. If I drop the throttle back to half or there abouts, no problems. The venue is a river with very very little flow so almost still and no wave action.

The cavitation plate is approx 3'' below the transom with the upper plate about half an inch above the transom bottom. I have tried each trim position on the motor without improvement.

The boat pressures are .25 for the tubes, .40 for the keel and .70 bar for the floor. I use a Bravo BTP12 to pump the boat up and have found that a separate pressure gauge shows the floor at .60 and the hand pump which came with the boat shows just below .60 bar so perhaps the Bravo is not quite correct.

So far I have not tested the pressure of the floor, keel or tubes after being on the water for a short time which I will do next time I go out. I do wonder if the floor pressure is critical and if the pressure drops after being on the water for a while.

As I have already said, with this SIB/removable air floor, I have no experience so I am hoping that some words of wisdom will be forthcoming.

Phil
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Old 26 August 2014, 11:03   #2
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The floor and keel pressure are absolutely critical to the boats performance. Even if you have it perfectly inflated before you go in, the moment it hits the water the pressure will drop. Put it in the water for about 15mins then inflate to the correct pressure.

But even with the correct pressure, some sibs still have a habit of cavitating at speed with the biggest and heaviest engine options. Its because the floor isnt completely rigid and has a habit of drawing air in at the front and then dumping out of the back to cause cavitation. Even hard floor sibs can do this as well.
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Old 26 August 2014, 11:21   #3
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Have the passenger move back. It could be they are raising the stern too high.
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Old 26 August 2014, 12:07   #4
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Have the passenger move back. It could be they are raising the stern too high.
Hi all

Sounds like the engine needs to be raised up and up
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Old 26 August 2014, 12:35   #5
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When you say it cavitates can you clarify the symptoms? It is a commonly misused description for all sorts of things...
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Old 26 August 2014, 12:48   #6
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Sounds like the engine needs to be raised up and up
Can you explain why you think that?

If the OP is getting ventilation, barring any other changes, moving the engine up will worsen the problem, won't it?

jky
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Old 26 August 2014, 14:04   #7
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Thank you for your replies.

I must confess, I don't understand the idea of raising the motor, if anything, I would have thought lowering the engine by removing some of the transom would be better.

The effect that I describe as cavitation is the engine revs up as though it is not in water. Then winding the throttle right back and turning the throttle back to about half throttle seems to allow the engine to bite again. The process is repeated over and over as we worked our way down river.

Phil
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Old 26 August 2014, 14:15   #8
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The effect that I describe as cavitation is the engine revs up as though it is not in water. Then winding the throttle right back and turning the throttle back to about half throttle seems to allow the engine to bite again. The process is repeated over and over as we worked our way down river.
Did you look at the prop at the time (not that easy to do - and don't hang over the back!) but its usually obvious if there is a big air pocket behind the boat - compared to a nice "normal" wake.

It doesn't make sense that your engine sounds like it is 3" too low on the transom but has ventilation (not cavitation).

Certainly worth making sure that everything is inflated right. But if you are going to do another test run then I'd also make sure the prop isn't "spun" at the same time (see here: Mariner 25hp two stroke, loss of power.)
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Old 26 August 2014, 14:16   #9
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You're right it is confusing, but you say AV plate is presently 3" below keel, which would normally be to deep.
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Old 26 August 2014, 14:34   #10
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Hi all

You say your engine size is 18" and your transom is 15 I take it.Moveing weight to the back will make the engine deeper in the flow of water.Also is the prop straight if yes check for spun hub.Is it ventilation or cavitation anyway wheres Locozodiac would be interesting to have an explanation.checked my prop because of similar probs only no where near as bad turned out one of the blades is off pitch and my engine length is 17" still working through it though.

PS is your cav plate visible when on the plane.
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