Ventilation: Prop fix?

[Projectile]

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.)

[Doug_G]

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.

[Projectile]

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?

[Doug_G]

Originally Posted by Projectile

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?

[captnjack]

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...

[Projectile]

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: http://www.rib.net/forum/f50/cav-pla...-53656-11.html)

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

[Projectile]

Originally Posted by captnjack

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 ...

[Peter_C]

Does it feel like the boat is arching under the floor boards, then releasing right as it cavitates?

[slate1234]

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.

[Projectile]

Originally Posted by Peter_C

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?

[SIBRider]

Originally Posted by Projectile

I'd have to say 'no' to that, but it all happens very quickly.

What were you getting at?

Sounds/behaves pretty much like mine when under-propped. You need either larger diameter prop (probably ideal solution but you won't be able to find one off the shelf) or higher pitch and/or 4 blade. Tach will tell story. As you speed up with WOT, prop has to slip more and more to provide increasing force needed to propel boat forward. As you reach critical region prop caviatates. Resistance on prop shaft drops, engine has access of power, races to highest RPM somewhere 6200+. Fortunately in EFI system it's all about fuel to RPM mapping. Regular Etec don't have mappings to create correct air-fuel mixture at higher RPMs. ECU trims fuel supply, power drops, prop grips water again. Cycle starts over.

[Peter_C]

Originally Posted by Projectile

I'd have to say 'no' to that, but it all happens very quickly.

What were you getting at?

Think of it kinda like a caterpillar arching its back then continuing on. I have experienced that before on dead flat calm water, when loaded with three people and scuba gear. We tried moving things around and it never went away. It was quit evident though as you could feel the boat bow up and then release, at which point it would cavitate. I never have the problem when the water is even slightly bouncy as the boat releases the wave or whatever you want to call it.

[jyasaki]

Originally Posted by Projectile

If my SIB hull is hooked, I'm screwed.

I think all SIB hulls as a whole have some degree of hook. You can't tauten up a flexible piece of fabric enough to keep it flat when you're throwing 20mph water at it.

jky

[Locozodiac]

That's why you need to have tubes and keel top inflated to with gauge to achieve best water performance under flat and lightly choppy water conditions.

If your engine seats at 90° when sib is on plane, cavitates slightly when giving full throttle in straight line, much worst on tight coin turns, you neeed to lower that engine a bit untill cavitation disappears, only achievable under trial and error. If in need to chop transom down a bit as in Slate's example, it's bad musique and definitely you're not going for it.

Stay with that current delivered prop you need a engine/transom optimization not a prop maximization, you can do that afterwords if you like to, to take the max HP out of that engine.

Happy Boating

[Projectile]

Originally Posted by Locozodiac

Stay with that current delivered prop you need a engine/transom optimization not a prop maximization, you can do that afterwords if you like to, to take the max HP out of that engine.

That may be, Loco, but numerous people here have had ventilation problems improve or disappear with different props. Why do you completely disregard these anecdotes? After all, a new prop is a simple fix, while chopping a transom is both complex and irreversible?

[captnjack]

I didn't realize your ventilation issue was on straight runs (unlike mine which was on turns).

I had a similar issue in my old SIB (12.5ft Bombard with Ali floor). But it only occurred when lightly loaded and I attributed it to cupping of the floor fabric. The hull would lift off the water momentarily then fall back down. Because I had a Yam with a 17" shaft on a "short" 15" transom the engine wouldn't ventilate or race at these times. The prop stayed firmly in the water despite the fact that the hull was rising up several inches.

Is there any way to lower your engine a little and see if it changes the behavior at all? If not I would wait for the Tiny Tach and see just how much racing is going on.

[Projectile]

It seems that if I have an extra 250 lbs in my boat (in addition to me and a gas tank), she runs very well. No ventilation.

Does that indicate anything about the cause and solution?

[Searider]

What condition and age is the prop?

I suffered from ventilation/cavitation on my 5.4 Searider and cut down my (new) transom. Made no difference.

On closer inspection my ally prop had loads of fine cracks in it and must have been distorting under load. It was essentially worn out.

If there's ANY damage on your existing prop address that first.
Even if there's no damage apparent it might be worth having it inspected and balanced.

[C-NUMB]

Originally Posted by Projectile

It seems that if I have an extra 250 lbs in my boat (in addition to me and a gas tank), she runs very well. No ventilation.

Does that indicate anything about the cause and solution?

Increased weight makes the boat sailing deeper and little slower, both things have an impact on prop ventilation. Think SIB boats are kind of challenging for the props, as the hull flex, the water stream coming towards the propeller(under the transom)is not as stable as on a hard boat. Also the shape of the area just before transom tend to be different(not optimal) than on hard boats.

I would borrow and test a loads of different props, different brands. Can be huge different on two equal pitched props by different brands. On my SR5.4 a K series 19 pitch works fine but a black max 19 is no good, as the actual pitch is much higher than on the K series so it wont rev enough.

[Projectile]

Originally Posted by Searider

What condition and age is the prop?

Good point, but my prop is mint, hardly used, OEM aluminum. I bought the motor brand new, out of the crate, and these particular small engines come with a prop from the factory. She's probably got < 30 hours on her.