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Old 31 October 2004, 16:47   #1
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Advice pls guys!

When cruising down Portsmouth harbour this afternoon, punching the tide, when I tried to go above 5 Knots the prop cavitated and lost drive. It's been OK all day blasting around the Solent at 30+ Knots. Turned to go with the tide and all was OK again! No gear crunching was heard, but there was a mass of microscopic bubbles in the water behind the transom when this appeared to happen. Sea was flat calm with very little wind. I've had something similar happen in a rough following Sea but not in the calm!

Has this happened to anyone else and what do you think were causing the bubbles in the water?

Andy
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Old 01 November 2004, 05:12   #2
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Prop slipping-nut not done up tightly???
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Old 01 November 2004, 05:32   #3
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Quote:
Originally Posted by Hightower
When cruising down Portsmouth harbour this afternoon, punching the tide, when I tried to go above 5 Knots the prop cavitated and lost drive. It's been OK all day blasting around the Solent at 30+ Knots. Turned to go with the tide and all was OK again! No gear crunching was heard, but there was a mass of microscopic bubbles in the water behind the transom when this appeared to happen. Sea was flat calm with very little wind. I've had something similar happen in a rough following Sea but not in the calm!

Has this happened to anyone else and what do you think were causing the bubbles in the water?

Andy
Andy,

I think that the with the tide / against the tide might be a bit of a red herring. However, microscopic bubbles sounds like either cavitation or ventilation. Could the propeller have been fouled by some weed or a plastic bag which has since come off?

Propeller bush starting to let go - lots of revs but little drive once you get above a certain RPM?

Have you since tried full power and if so does it all now work as it should?

I ought to make an effort to get up to the Churchillian - is it this week?

Regards
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Old 01 November 2004, 08:08   #4
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Could have been a plastic bag or weed I guess. Nut looks OK split pin is still in and I don't think the bush has gone as I was cruising at speeds in excess of 30 knots for a number of hours and this happened comming through the Harbour which is why I didn't stop and lift the leg to check for obstructions. On the return trip seemed OK but couldn't go too fast as MOD plod and Ribald were about!

I'll have the prop off tonight and check the seals.

Thanks and I'm afraid I'll miss the next Churchillian Pub night as will be away from home again.

Andy
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Old 01 November 2004, 09:00   #5
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Ahh so the cunning new device we've had installed in the harbour works then. Designed to curb the speed of errant boaters who think that the 10 knot limit does not apply to them . Funny thing is that it stops our boats as well .

Joking aside, it sounds very much like a bag/weed was picked up as has been previously mentioned. At least it wasn't a 20ft long railway sleeper of which there are a few still unaccounted for.
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Old 01 November 2004, 17:50   #6
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Hi

Used to get that some times on my old Flatacraft, but normally when mucking about going from Lock to lock doing doughnuts with water pouring in over the tubes. That was definatly cavitation. Always throttled back briefly to let the prop recover and it went away.

I've also experianced it at unexpected times when the combination of trim speed through the water etc. etc just conspired against me. But always throttled back briefly to cure it, feels like the props in a hole behind the boat.

I always suspected I should change the prop to cure it completly but the Flats gone to a new home up North and it was so much fun with the prop it had.

Nasher.
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Old 01 November 2004, 17:58   #7
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it's happenned to me at the harbour entrance. I found cavitation quite easy to get there........I wouldn't bother going there again theres plenty of better places!
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Old 01 November 2004, 20:31   #8
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Stuart, I'll come up Southampton next time and see what your cavitation is like . How are things? Bloody bow tube strakes still haven't been fixed properly yet, he's comming on Wed. so one last time and then name and shame time I think.Andy
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