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20 July 2012, 22:06
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#1
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Member
Country: UK - England
Town: portsmouth
Boat name: patch
Make: avon sr5.4/4m
Length: 5m +
Engine: mercury 90/honda 30
Join Date: Oct 2008
Posts: 291
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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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20 July 2012, 22:21
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#2
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RIBnet admin team
Country: Ireland
Length: 4m +
Join Date: Feb 2008
Posts: 14,910
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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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20 July 2012, 22:24
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#3
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Member
Country: UK - England
Town: portsmouth
Boat name: patch
Make: avon sr5.4/4m
Length: 5m +
Engine: mercury 90/honda 30
Join Date: Oct 2008
Posts: 291
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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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20 July 2012, 22:28
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#4
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RIBnet admin team
Country: Ireland
Length: 4m +
Join Date: Feb 2008
Posts: 14,910
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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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20 July 2012, 22:35
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#5
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Member
Country: UK - England
Town: portsmouth
Boat name: patch
Make: avon sr5.4/4m
Length: 5m +
Engine: mercury 90/honda 30
Join Date: Oct 2008
Posts: 291
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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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21 July 2012, 01:06
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#6
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Member
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,671
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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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21 July 2012, 07:20
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#7
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Member
Country: UK - England
Town: South Yorks
Boat name: Black Pig
Make: Ribcraft
Length: 5m +
Engine: DF140a
MMSI: 235111389
Join Date: Feb 2008
Posts: 12,178
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Quote:
Originally Posted by tim2658
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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Are you trimmed right down before take off?
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21 July 2012, 10:51
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#8
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RIBnet admin team
Country: UK - England
Town: The wilds of Wiltshire
Boat name: Dominator
Make: SR5.4
Length: 7m +
Engine: Yam 85
MMSI: 235055163
Join Date: Jul 2005
Posts: 13,069
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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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21 July 2012, 16:26
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#9
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RIBnet admin team
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: Tohatsu 30HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 11,632
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Quote:
Originally Posted by tim2658
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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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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