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Old 01 September 2007, 14:24   #1
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Country: UK - England
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Over revving OB?

High guys, just returned from a first run in my new SIB and actually my first time ive ever taken a boat out!

It all went remarkably smoothly but one thing had me a little worried... when i opened her up a bit (a 40hp yamaha 2 stroke) it would occassionally maek a nosie of revving very high and we would lsoe momentum. If it was a car it would be like revving it out of gear, or with the clutch depressed or i guess a faulty gear box.

It sounded terrible as we sat there for a few seconds revving like mad and goign no where, it would suddenly engage, or i would kill all revs and build it up again.

Was *very* choppy out and i thought it might be because we were leaping out into the air and the propellor may of lsot resistance, but it seemed to be more dependant on speed and would do it whether we were jumping or not....

What could this be? Is it normal? thanks guys ive got to say everyones bee na big help to a new boat owner!
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Old 01 September 2007, 15:09   #2
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It is the prop ventilating - sucking in air. My SIB occasionally does this as well - othertime no sign of it so it can be hard to pin down. make sure the leg is deep enough in the water - it may be in the shallow water setting or timmed up too far. Also weight distribution in the boat can cause it - try moving people around. usually you want someone as far forward as possible to get on the plane - then move aft when up to speed.
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Old 01 September 2007, 15:25   #3
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thanks codprawn, can always rely on you to answer! As far as i know ive left the positioning the same as when i brought it. It was locked in its lwoest position and there is no shallow water lever i can find!

I'll try and track down the previous owner and see if it happend for him - hopefully it wont be so bad in calmer waters. Is it something i should worry about?
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Old 10 September 2007, 13:06   #4
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Shouldn't hurt the motor, assuming you're not overevving too often, but it can be aggravating.

I assume you do not have power tilt/trim? You might try changing the motor tilt position (usually accomplished by moving a pin that stops the motor as it's swung down), whilch will help prevent ventilation at the cost of increased drag and resulting in lower top end speed.

Another thing that may help is the addition of a hydrofoil; a wing shaped device that gets bolted to the anti-ventilation plate (often called a cavitation plate - the flat horizontal ledge that sits just above the prop.) Supposedly, they create a bit of lift as they move forwards, making it easier to get on plane. They also create a larger barrier to prevent air from getting sucked into the water around the prop, which may be of benefit to you.

Other than that, lowering the motor on the transom will help, also at the cost of increased drag, and a lot of labor.

Codprawns "shallow water setting" is usually a little lever that tilts the motor up a bit (typically, you lift the motor, and flip the lever, and the motor sits back down on a stop keeping it semi-tilted); though my Honda simply had a locking lever that would stop the motor wherever you wanted it (no shallow water stop, per se.)

Good luck;

jky
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Old 11 September 2007, 04:18   #5
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Country: UK - Scotland
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Andrew,

I would suggest leaving it alone until you can try it in calmer water. If it's a case of just going airbourne, then the simple answer might be to just not open it up so much on the uphill side of a wave..... But where's the fun in that?

I would think if it was the gearbox / dog clutch disengaging it wouldn't drop back in of it's own accord. I had that scenario on a prehistoric old Johnson, and they definetly don't drop back in!
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Old 11 September 2007, 10:37   #6
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Could be your propeller hub kit spinning. Many have torque control fitted into them (a bit like a clutch) they are supposed to protect your gearbox when you hit a rock...with your prop. But they can be faulty or worn and spin under high stress. Not sure how you would test it though
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Old 12 September 2007, 05:19   #7
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Back to my heap of a '72 Johnsorude.....

Older props had an anti vibration rubber bush 'tween the shaft splines and the prop casting proper, which i believe is injected in at the time of manufacture (i.e. not a replacable part). I suspect your engine might be newer than this technology, but although there is (should be) a shear pin, if the previous owner had repalced same with a sawn off 6" nail, then continually buried it in sand / mud , the knobbly bits on the rubber that stop the whole lot spinning on its own axis shear off, and so you end up relying on the surface friction between the rubber & metallic parts.

Way tou can tell is that round the hub at the joints, you'll see the paint flaked off or badly crazes, with hints of extruded rubber round the perimiter. It's basically the friction can't hold the torque, and it slips, generates heat, and extrudes the rubber. You probably won't notice it below about half throttle, and it would be a continuous effect of lots of revs combined with not a lot of action........

Having said that, it's a sib with a 40 - air time is an integral part of the trip!!! (I'd try a trip on flat water first before you start dismantling things)
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