outboard motor - propeller blowing air - SailNet Community

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Old 10-31-2010
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outboard motor - propeller blowing air

Hey guys,

I have a 1991 yamaha t9.9 elhp outboard motor (9.9hp) and when I put it in forward gear and open the throttle sometimes the thrust isn't really there; when looking aft it appears that the motor is expelling a lot of air. Then, after anywhere between 2-10 seconds normal thrust resumes as the air disappears (or returns to the nornmal output it has).

Searches for the have been fruitless.. Anyone have any idea what this is? It happens occassionally and I feel the drive assembly may be going south.

Thanks,
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There are two things

(Ventilation) occurs when air or exhaust gasses are pulled into the propeller blades causing high RPMs and loss of speed.

(Propeller cavitation) is less obvious than ventilation,


Its pretty common on sailboat outboards as in the best case the prop is in just enough water and the motor is often not mounted level
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Incanation,

Tommays points are good. If the motor is not deep enough that
could be causing the problem. Also, if the prop is bent or has rough
edges, nicks, or gouges, or if it is loose, or covered in growth,
that also may contribute to the problem.

Dabnis
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thanks guys.. she does sit pretty shallow but it didn't look as though it was sucking water from the surface. is that what you mean?

so there is no internal mechanical cause for this? (good news)
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This does sound like you're running things too shallow.. do you have any further lowering adjustment on the bracket? As a quick check put more people in the stern and see if that improves things...
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If the exhaust is through the prop hub and if the motor is somewhat
shallow you may be blowing a big hole in the water when giving full throttle
when stopped. You might try applying the power a little at a time until
the boat starts making a little headway. Did this just start or has it always
happened?

Dabnis
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You know, the more I think about it the more I'm realizing it must be too shallow. I had 3 people on the bow (boat is a Hughes 22) and I had this problem in a big way and immediately moved people aft. Yesterday it happened when I was out myself in some less than smooth conditions and no, I didn't gradually engage.. But once the motor was "digging in" the bow was up more with the stern deeper and she was steady with no issues..

Thanks for putting my mind at ease; I will simply engage the throttle more gradually and be more mindful of keeping weight as far aft as possible under power..

Cheers,

Last edited by Incantation; 10-31-2010 at 06:37 PM.
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Depending on where your rudder is, ventillation also may be triggered by a rudder hard-over. At least it is on my boat - if I turn to port too quickly, I am guaranteed a few minutes of bubbling noises, vibration and low speed.

Just something to keep in mind.
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