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Old 21 May 2013, 14:36   #1
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Power loss with Yamaha 40HP after rope around prop

I've got a 6.5m Chinook RIB with twin Yamaha 40HP outboard engines.

Last week when I took the boat out, a drifting piece of rope snagged around the starboard prop while planing at about 20 knots causing that engine to stall. I killed the other engine immediately, removed the rope, the shear pin hadn't sheared, and I was able to restart the engine. But once under load, when the revs reached around 4.5k - 5k, the engine that snagged the rope stopped giving thrust and the revs shot up really high. Running the engine under 4.5k and it sounds and works normally (no misfires, etc.) and gives expected thrust. Ticks over OK and all seems fine apart from this effect when the engine is under load above 4.5k at which point it seems akin to a clutch slipping on a car (I'm not an engine guy so don't know the internals).

When I took the boat out the water, one of the 3 prop blades is slightly split (about 2cm tear which I bent back with pliers). The same prop blade is also slightly bent out of shape - an area of about 5cm x 5cm bent by about 5mm.

After closing the tear (but not straightening the bend because I didnt want to metal fatigue to blade any more) I tried the boat again today and the same again - engine is fine up until 4-5k then loses thrusts and revs insanely high?

Any suggestions what it might be? Could the prop damage be causing cavitation? It seems like minor damage but then I guess it spins quick...
Or is it likely to be something internal to the engine? Help!
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Old 21 May 2013, 14:45   #2
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i reckon it's a prop issue, all your plates above the prop unharmed?
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Old 21 May 2013, 14:46   #3
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does it have shear pin? i'd have though it had a rubber bush and it sounds like its spun - works fine at low power but stops gripping at high power. put a line of tipex (or similar) on the prop crossing onto the rubber centre. Take engine for run until symptoms felt. Check it both halves of line still match. If so replace / get prop repaired and the problem should go.
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Old 21 May 2013, 14:51   #4
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Poly, apparently the shear pin hadn't sheared. 2nd sentence in OP.
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Old 21 May 2013, 14:54   #5
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Quote:
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Poly, apparently the shear pin hadn't sheared. 2nd sentence in OP.
Yes - that's why I am asking it actually has a shear pin as I thought (perhaps worngly) they were only found on small outboards and a 40HP would have a more conventional rubber prop bush.
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Old 21 May 2013, 14:55   #6
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swap the props over from each engine if the fault swaps you know whats wrong!
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Old 21 May 2013, 15:02   #7
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My 30 Mariner 2T has a shear pin and it's just a carb upgrade to a 40?
Maybe wrong but I remember that it's simply an induction upgrade on the 40 from the 30.
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Old 21 May 2013, 15:32   #8
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Thanks for all your responses guys.

Yep, they definitely have shear pins. But I'll try beamishken's idea and swap the props - surefire way to figure if it's the prop or the engine. Then I might come asking you again...
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Old 22 May 2013, 15:16   #9
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If the Prop's not 100% she's not going to run 100%
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Old 05 June 2013, 03:48   #10
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So I've swapped the props

Quote:
Originally Posted by dpa99c View Post
Thanks for all your responses guys.

Yep, they definitely have shear pins. But I'll try beamishken's idea and swap the props - surefire way to figure if it's the prop or the engine. Then I might come asking you again...
OK, I swapped the props and took the boat for a spin. Incidentally, I was wrong - the pins are split pins not shear pins - they just stop the nut that holds the prop on from spinning (see photo below). But the exposed centre isn't rubber, it's metal (as can be seen in photo)...???

Photo of prop

Anyway, swapping the props eliminated them as the cause of the problem. The same starboard engine is still showing the same symptoms, but worse now. By the time I turned the boat around to bring it back in after testing it, that engine is stuck in forwards gear (despite the position of the throttle lever which I can see is moving the gear shift on the outside of the engine casing via the cable) and the engine itself is not sounding 100% with a slightly metallic clicking sound. Hope I haven't knackered it by trying this

Any ideas again would be appreciated. I was hoping for a quick fix I could do myself as removing the engine or towing it along with the boat is gonna be a hassle and taking it to professionals is expensive....
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