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Old 20 July 2012, 17:06   #1
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Bad cavitation

Hi all hope someone can help I'm getting alot of cavitation from my 70 yamaha on my 6m rib, top speed of only 24mph today on flat but it's still cavitating and from stand still it really wants to go but you can hear the air around the prop, the cav plate the dead level with the bottom of the hull, cheers tim
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Old 20 July 2012, 17:21   #2
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It sounds as if you may need to have the rubber bushing on the prop replaced? Have you had "ground contact" with the prop recently?
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Old 20 July 2012, 17:24   #3
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Hi Willk I did think this, I just bought the boat and is reving really bad, is there a way to check if the bush is bad, many thanks tim
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Old 20 July 2012, 17:28   #4
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Dunno. I've been out on one with this trouble. It would increase the revs nicely until you gave it the beans and then it would "slip". Just as though someone hit the clutch, except it didn't have a clutch...

Another prop would be a definitive test

The repair is simple.
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Old 20 July 2012, 17:35   #5
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I think your right I will try another prop tomorrow and let you know , cheers tim ( fingers crossed!!!! )
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Old 20 July 2012, 20:06   #6
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One way to see if the bushing has let go is if it is slipping you should see a circle of set molten rubber at either the inner or outer diameter of the bush. If you need a photo example, let me know.
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Old 21 July 2012, 02:20   #7
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Quote:
Originally Posted by tim2658 View Post
Hi all hope someone can help I'm getting alot of cavitation from my 70 yamaha on my 6m rib, top speed of only 24mph today on flat but it's still cavitating and from stand still it really wants to go but you can hear the air around the prop, the cav plate the dead level with the bottom of the hull, cheers tim
Are you trimmed right down before take off?
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Old 21 July 2012, 05:51   #8
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Mark the prop nut and prop with marks that line up. If the prop's slipping, they won't line up after you use it.
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Old 21 July 2012, 11:26   #9
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Quote:
Originally Posted by tim2658 View Post
Hi all hope someone can help I'm getting alot of cavitation from my 70 yamaha on my 6m rib, top speed of only 24mph today on flat but it's still cavitating and from stand still it really wants to go but you can hear the air around the prop, the cav plate the dead level with the bottom of the hull, cheers tim
Cavitation and ventilation are actually different things. You can get cavitation without having air in the prop. If the prop bush is spun I wouldn't expect it to sound like air. If there is definitely air around the prop then engine trim etc - might be more relevant. If it is genuinely cavitating (which if you do it long enough will start to show up as little pock marks on the prop) then I think the most common cause is a damaged prop.

What are the engine revs doing during this? What size is the prop? If you've just acquired it do you know if it is being run with a different load (e.g. was it full of divers before) etc.
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