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Old 30 June 2014, 18:18   #1
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Ventilation: Prop fix?

This is a 4.7m Zodiac Milpro SIB (WB465 - no speed tubes). Can this degree of ventilation be eliminated by finding a different prop? I currently have the OEM aluminum prop that comes with an ETEC30 (10x13).

4-blade, perhaps? Stainless steel? Anybody with personal experience with this?

AV plate is 1/2 to 3/4 inch below bottom of transom. Hoping I don't have to lower motor ...

(Please don't anybody tell me to make sure that she's - keel included - properly inflated.)
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Old 30 June 2014, 18:41   #2
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Is the prop the correct diameter?

Have you tried it with a 10 x 15 prop?

You may have a bit of cavitation at the start and getting up on the plane with the longer pitch.
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Old 30 June 2014, 18:49   #3
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Yes, prop is correct diameter. It is the OEM prop for this motor.

I have not tried any other props, yet.

Do you have experience with higher pitch prop reducing ventilation?
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Old 30 June 2014, 19:23   #4
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Quote:
Originally Posted by Projectile View Post
Yes, prop is correct diameter. It is the OEM prop for this motor.

I have not tried any other props, yet.

Do you have experience with higher pitch prop reducing ventilation?

Is this your problem?

IE The boat gets up on plane OK, then levels off, then when it gets to cruising speed, then it feels like the prop is losing its grip and the revs go up, and the boat slows down?

You could be suffering from 'hook or 'hollow'.



AFAICR ventilation /cavitation also occurs when
a) the prop diameter is too small i.e. the engine can turn it, without the prop being able to grip in the water
b) the prop pitch is far too small i.e. he engine can turn it, without the prop being able to grip in the water,
c) if the prop pitch is too large and the diameter too small i.e. the engine can turn it too fast, so the prop being able to grip in the water.
If the prop diameter is too large you will not get full revs from the engine.

If you have a longer pitch, (and not too heavy a load) and you slowly apply throttle you will eventually plane. Quick adjustments to the throttle will not help.

Does any of this apply to your experience?
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Old 30 June 2014, 19:29   #5
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The problem is not the wrong pitch, going up in pitch won't help. It'll just be over-propped.

I had the same problem in turns, but only when heavily loaded. Mine was so bad the boat would come off plane, stop and ventilate until I steered straight for a bit and got it to bite again.

I switched to a 4 blade ali and I don't get the slip/ventilation on turns like before. Its gone. You need to go down in diameter or pitch otherwise the extra blades will make you over propped. How much down? Trial and error. Its why I went with an Ali. At the time I didn't know how much it would influence the problem and how many trials I would need.

I have a 60hp Yam so my experience is only partially relevant to yours...
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Old 30 June 2014, 21:04   #6
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Doug_G,

The video I posted should answer your questions.

Please note that my questions are very clear and specific. I'm not interested in getting into a discussion about a hooked hull here. (Been there, done that: Cav plate height for Zodiac Mk4)

If my SIB hull is hooked, I'm screwed. I want to hear from people with experience dealing with ventilation and the simple solution of finding a different prop.

Thanks,
P
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Old 30 June 2014, 21:09   #7
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The problem is not the wrong pitch, going up in pitch won't help. It'll just be over-propped ... I switched to a 4 blade ali and I don't get the slip/ventilation on turns like before. Its gone. You need to go down in diameter or pitch otherwise the extra blades will make you over propped. How much down? Trial and error. Its why I went with an Ali. At the time I didn't know how much it would influence the problem and how many trials I would need.
Thanks captnjack.

Notice that I get ventilation even when travelling in a straight line, or at least that's what I'm trying to do in this video. Coincident with the ventilation, the boat does change direction but I don't think that that's the cause of the ventilation...

I have read that 4 blade props grip the water better and therefore reduce ventilation (or was it cavitation?), so it's good to hear that backed up by your personal experience.

My TinyTach is in the mail and when I get it, I will start experimenting with different props. In the meantime, I'm keen to hear about the experience of others in the hopes that it will help me pick the first prop to test.

SibRider, who frequently has good comments 'round here, has suggested that a higher pitch prop might help, but I haven't come across that particular advice regarding ventilation before or since ...
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Old 01 July 2014, 13:28   #8
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Does it feel like the boat is arching under the floor boards, then releasing right as it cavitates?
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Old 01 July 2014, 14:22   #9
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I know I have a diffrent setup but I had exactly the same problem with a 3.2m v airfloor and 9.8 twatsu, I had to cut 3/4in out of the transome, yes the floor and tubes were inflated to the correct pressure and yes it really does p..s you off when you think you have a good setup, I have to shim the engine back up when I have a freind with me.
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Old 01 July 2014, 15:23   #10
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Does it feel like the boat is arching under the floor boards, then releasing right as it cavitates?
I'd have to say 'no' to that, but it all happens very quickly.

What were you getting at?
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