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Old 15 June 2006, 17:16   #1
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Prop torque.....

OK i recently fitted a 115 Merc to the back of my Osprey. When I get above about 4000 rpm the boat begins to "lean" to the left and feels quite flightly. Trimming out seems to cure the lean to a certain degree, although it still feels a bit uneasy, plus it causes the prop to cavitate.

What is the solution? Im currently running a 14" ali prop. Could changing the prop (maybe to a stainless one??) affect it at all. Failing that would taking the engine down a bit on the transom be of benifit. I am reluctant to do this for testing purposes as the engine is on the top holes at the moment, so the only way to move it any more is to drill new holes in the transom. I personally think this is going to be the way to go, as the cavitation plate is currently about an inch and a half above the bottom of the hull.....

What do people think?
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Old 15 June 2006, 17:25   #2
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Tim
I had the same problem when I first got my Humber, the problem was caused by the engine being fitted slightly wrong . It was not vertical on the transom and at speed the engine in effect tilt itself to vertical which meant the baot listed . I have now had a alloy plate made to cap the transom and the engine refitted using a jig to drill the holes . Result is perfect with the added bonus of a complete new feel to the boat and no ventilation in turns , this may be the stainless prop but she now rides true .this post refers Offset and cav plate height
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Old 15 June 2006, 17:27   #3
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I would have thought on your rib with that engine a slightly higher pitch might be appropriate.

Ref: The lean, sounds like the anode trim tab behind the prop might be out of place, or the engine may not be mounted dead vertical on the transom.

Is the anode in good nick, or is it a bit degraded?
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Old 15 June 2006, 17:30   #4
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Is the anode in good nick, or is it a bit degraded?
This model has a plastic trim tab not a zinc one. I have to say this was something that I had overlooked until now. I'll try adjusting it when I take the boat out on Monday.

Jimbo what size prop are you using. I know your boat is a bit heavier but it is a similar setup.....

Lurcher - I'll get a tape measure and check that the engine is on dead square tomorrow.
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Old 15 June 2006, 17:51   #5
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Check the offset as well... is the engine central or slightly to one side? I would guess that it may need to be 30-50mm to the right (starboard), but someone who's set up a similar sized engine on that type of hull would be able to advise you more accurately.
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Old 15 June 2006, 18:20   #6
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Thats alot of power on a small boat Tim, I would think the 14" is a little under proppedd, im running a 17" on the Tohatsu.
Likely to be the set up of the engine I would think, may be worth offsetting with all that power, a mate had a similar problem with a 75HP Mariner on a Valiant 5.2m, the boat became known as Eilean!!
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Old 15 June 2006, 18:21   #7
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the boat became known as Eilean!!
Surname Dover?
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Old 15 June 2006, 18:24   #8
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Surname Dover?
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Old 15 June 2006, 18:32   #9
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I don't know if the engine is off-set. If it is it's only by a small amount (it looks central). I just bolted the new engine onto the exsisting holes....

I don't know anything about propping boats. What sort of pitch would be more suitable? I suppose it would help if I said what the engine tops out at rev wise, but I can't confirm this until Monday.
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Old 15 June 2006, 18:33   #10
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What engine was fitted before? How did it perform with the previous motor?
Did you use existing mounting holes?
Does it lean to port while accellerating onto the plane, or only at high RPM?
Presumably, the boat is loaded evenly?
It does sound like insufficient offset, from what you have posted.
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