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Old 14 August 2006, 08:30   #1
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Woe's since installing S/S prop

OK guys I know this isint on a RIB, but you might be able to help!

I have a Mercruiser 1.7 (120HP) Turbo-Diesel on my sports cuddy with an Alpha 1 Gen II leg

It came as standard with a 14 x 23 prop, but was over-revving, so I upgraded to a 14.25 x 24 stainless steel prop. I fitted it up nice and tight. Nothing is loose, there is no play on the spline. The prop rotates freely and doesnt come in contact with any part of the leg

When I put the engine into gear, there is a very loud 'thumping' noise until I hit 1200RPM. Once I hit this RPM it stops. This did not happen with the aluminium prop. I am worried that I might be doing damage to my gearbox. Has anyone had anything similar happen to them? The noise is definately coming from the leg, because the noise goes when I put the engine into neutral. Could it be the extra load thats making the noise?
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Old 14 August 2006, 08:52   #2
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Sounds like the blades are out of balance - or it could be the extra load on the leg but I would check prop balance first.
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Old 14 August 2006, 18:42   #3
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You may find a PM to Mike C useful.... he had some issues with prop on his RIB which has a similar engine and outdrive to yours... how much clearance is there betwen the prop and the anode?
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Old 14 August 2006, 19:30   #4
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All of the above, plus.

Could be a couple of things.

1. If you turn the prop by hand is the gear casing fouling any part of the prop, you could remove the prop and have a look for witness marks.

2. How good is the bush in the prop? Could this be turning relitive to the blades/shaft indicating a slipping bush.

3. How hard a rubber bush have you got? It would be possible that when in drive the bush gives slightly and causes the prop to "wobble". If the clearances are tight on any part of your casing/leg the prop might be hitting it! Again have a close inspection of the prop/casing/leg for witness marks/damage.

I don't confess to being an expert but do know that SS props are harder on transmissions, particularly when changing into/out of gear as the rubber bushes are harder and don't give....Also Solas props had a reputation for bush problems.

What make of prop is it?
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Old 14 August 2006, 23:17   #5
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You could also try taking the prop off and putting it back on in a different position - may be all that's needed to alter the balance.
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Old 15 August 2006, 18:14   #6
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Thanks for the replies guys, I think I have found the problem

I found an article online, which stated that the prop needed to be torqued down to 100ftlbs, instead of the recommended 55

As it turns out, the hub kit in it is designed for the Mercury outboard, and not ideal for the Alpha leg. A new hub kit is in order!
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