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Old 07 July 2014, 00:10   #21
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Originally Posted by C-NUMB View Post
I would borrow and test a loads of different props, different brands.
That is impossible.

I will have to buy whatever prop I try, so I will probably try, at most, 2 props.

I just have no idea where you get "loads of different props and brands" to try.
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Old 07 July 2014, 00:14   #22
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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.
I'm thinking that you might be right, Loco.

It seems to me that an extra 250 lbs in the boat will cause the boat to sit lower in the water. That would cause water to pass over the lower end higher up in relation to the AV plate when planing. That would reduce entrainment of surface air: ventilation.
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Old 07 July 2014, 01:06   #23
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I'm thinking that you might be right, Loco.

It seems to me that an extra 250 lbs in the boat will cause the boat to sit lower in the water. That would cause water to pass over the lower end higher up in relation to the AV plate when planing. That would reduce entrainment of surface air: ventilation.
On plane, moving forwards at, what, 20mph? The water has a foot to rise as it comes out from under the hull; doubt that's going to be whole lot different. Testing may show otherwise, but I'd suspect you've got more overall wetted area, so a lower speed, plus a more bow-up configuration, all of which changes the whole equation.

jky
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Old 07 July 2014, 01:34   #24
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On plane, moving forwards at, what, 20mph? The water has a foot to rise as it comes out from under the hull; doubt that's going to be whole lot different. Testing may show otherwise, but I'd suspect you've got more overall wetted area, so a lower speed, plus a more bow-up configuration, all of which changes the whole equation.
To be sure, there are plenty of variables, but raising or lowering the motor (or raising or lowering the height at which water flows by the lower leg) by 1/2 an inch can make all of the difference on the world to a problem like ventilation, can't it?

I'm not sure that your comments really harm the hypothesis. There are *always* plenty of variables and many sources for uncertainty ...

Now, if you are saying that you think that it's unlikely that the boat sits such that the water rises <1/2 inch higher relative to the motor under plane with an extra 250 lbs in the boat, then I retract the above.
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Old 07 July 2014, 14:16   #25
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Been reading lately many engine brands owner's manuals. All conclude that the best prop thrust and combo performance is only achieved with engine siting at 90, with boat evenly weight deck distributed and combo riding parallel to water surface when on plane at speed. No one specifies at which lower unit height must water flow pass. Seems too complicated to state as definitely will vary from hull to hull and boat type.

If you assume you have too much weight at rear, move it to bow, or even sit a mate up front to have a well ballanced boat. You must "visually" check water flow height passing through lower unit if having ventilation or over, out water splashes isuues which translates into unwanted LU excessive drag which will slow combo down.

Once knowing that, if going raising or chopping transom down that's entirely up to you. But you need to confirm that by simply looking at back transom what's happening there when on plane.

Happy Boating
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Old 07 July 2014, 15:08   #26
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On plane, moving forwards at, what, 20mph? The water has a foot to rise as it comes out from under the hull; doubt that's going to be whole lot different. Testing may show otherwise, but I'd suspect you've got more overall wetted area, so a lower speed, plus a more bow-up configuration, all of which changes the whole equation.

jky
Bow higher (up) does push the stern down into the water more. The fact that more weight reduces the effect, its most prevalent on straight runs, and not on turns is all telling me the engine is too high.

The OEM props typically are pretty close and useful although sometimes the pitch is a bit small for a lightweight sib. There is a small chance going up 2" or so in pitch will help but I doubt it.

Perhaps you can get a friend to film the engine and wake while in use and its doing its ventilation thing. From a separate boat since extra people in yours tends to reduce the effect.
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Old 10 July 2014, 14:00   #27
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5200-53300 rpm wide open, lightly loaded. That gets me 23mph.

When it ventilates, revs go up to 5600.

Evinrude recommends 5500-6000 at WOT

Lower pitch prop?
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Old 10 July 2014, 15:34   #28
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What trim position are you running at? Is boat porpoising when you ventilate? If you trimmed-in too much prop thrust will push stern up/bow down then possibly ventilate with light load. Heavier load will make it harder to lift stern naturally.

What is your prop exactly? In the other topic you mentioned 10.3x12 markings. 23 mph at 5200 rpm and such prop means reasonably good grip with 16% or so slip.
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Old 11 July 2014, 01:08   #29
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I was running in the first trim hole. When I do that, I get intermittent ventilation when travelling in a straight line while wide open (WOT) as seen in the video. I also get it on turns as the video shows as well. I don't get it when going slower than WOT. In the first trim hole, the RPM tend to be in the 5200 range wide open.

When I trim out to the second hole, I get ventilation as soon as I get onto plane (far slower than wide open). On one occasion, I was able to get up to wide open throttle in the second trim hole and the RPMs were in the 5300 range. Consistently about 100 rpm higher than in the first hole.

That last bit identifies another feature of the ventilation that might be an important clue about the cause: it's very finicky. While boating for 60-120 minutes, there might be 20 minutes that goes by without any ventilation, and anther 30 minutes where it's as frequent as seen in the video.

The ventilation seems to be very sensitive to weight distribution; it's seems much worse when the port/starboard weight distribution is uneven.

It also seems to be better if I'm travelling against the wind in a short chop. I get the sense that the bow is up a bit more at that time.
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Old 11 July 2014, 01:12   #30
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I'm inclined to think that my problem is that the AV plate is too high and not that I need a new prop (to fix the ventilation).

I need a shorter pitch prop to get more power out of my outboard, but I think that's an unrelated issue.

Of course, I could have a problem with the hull, but if that's the case, then I'm screwed. Zodiac won't hear it. They say that the boat is running properly.
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