Ventilation: Prop fix?

[Projectile]

Originally Posted by C-NUMB

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.

[Projectile]

Originally Posted by Locozodiac

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.

[jyasaki]

Originally Posted by Projectile

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

[Projectile]

Originally Posted by jyasaki

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.

[Locozodiac]

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

[captnjack]

Originally Posted by jyasaki

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.

[Projectile]

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?

[SIBRider]

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.

[Projectile]

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.

[Projectile]

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.

[SIBRider]

Running on first hole should not be norm unless you have near vertical transom. Not much point trying to figure out what's wrong here.

Can you measure transom angle relative to floor boards?

Motor in second and/or third trim hole place level or ruler on AV plate. Where does it project relative to transom?

[Projectile]

In second or third hole, A/V plate is 3/4-1 inch below bottom of transom.

The most likely problem is that motor is too high. This would explain why ventilation gets better with more weight and worse as I trim out (up). Lowering engine will permit me to trim out to 2nd or 3rd hole.

RPM readings are not consistent with need for a higher pitch prop. Lower pitch, if anything, is more likely.

I plan to drop motor 1 inch and re-test.

[SIBRider]

Re-reading last year topic. Seems you raised motor about 2" with metal plate. Prior to that you had tested motor with 3/4 plank and that did not produce desired result. You said no improvement. Seems dropping 1" from current position you'd be back to square one or very close to that.

Makes me think you should focus on propulsion still. Can you confirm what you current prop is?

When you say you reached only 5300 RPMs on 2 hole WOT ventilating?
Was it babying, watching for ventilation or true WOT twist it all the way? It seems usually, ventilating prop experiences loss of resistance. Naturally leading to RPMs going up without restriction. Certainly my motor behaves this way. Going as far as hitting hitting rev-limiter.

[Projectile]

First, let me say a big thank-you for going back and re-reading the old thread and really trying so hard to help me out, SIBRider.

It turns out that back at that time, when I was having huge problems with bow down on acceleration and huge stress on the boat (but no ventilation unless trimmed out to hole 4 or 5), my boat was not correctly assembled. YOU actually brought this to my attention when you noticed a difference in the positioning of the floorboards relative to the elephant trunk hole on my transom. My dealer had not placed the rear floorboard over the groove on the transom, but rather, had it sitting on top of that groove. Accordingly, the floorboards would have been too short. The problems I was having then are perfectly described by a hook in the hull, and incorrect floorboard assembly would be expected to cause just that. Unfortunately, it went noticed, and all the attention and advice I received from the dealer that sold me the boat (Defender) and the dealer that assembled it and look after me locally (Automarine) was focused on the motor being too low.

When the transom was plated to raise the motor 2 inches, the boat was also reassembled properly.

Things got much better. The bow down attitude improved considerably. However, I had a new problem: ventilation as you see in the video in this thread. I have to operate the boat in the first trim hole, which, I think, also keeps the bow somewhat down (but that problem isn't near;y as bad as before).

I now believe that the original problems I had were due to incorrect floorboard assembly, and that the problems I am having now are due to the motor being too high. I suspect that plating my transom to raise it was never required. A 3/4 inch plank and proper assembly would probably have done the trick.

So at the end of the day, I suspect that the ideal position for my AV plate will be about 2 inches below transom bottom. I should be able to run in the second trim hole at that level, and then I will recheck the WOT RPM and see if I need a lower pitch prop.

My current prop is the Evinrude OEM prop I described earlier and that you mentioned here.

The engine reached 5200 RPM (and change) in trim hole 1 with intermittent ventilation raising RPM to 5600 when it was happening.

The engine reached 5300 RPM (and change) in trim hole 2 with more frequent ventilation also raising RPM to around 5600 when it was happening.

[SIBRider]

Lots of variables. Certainly worthy experiment.
at 5600 RPM do you let go throttle or it goes down by itself? 4 blade prop is something that can certainly address this problem or higher rake 3 blade.

Mine 3 blade has notable rake.

[Projectile]

RPM goes down by itself.

[Projectile]

I've been boating in the Okanagan, which is not at as high an altitude as Calgary. Here, I get 24 mph with my ETEC 30 at 5380 RPM in the second trim hole (1 mph extra and about 100 rpm higher than Calgary).

I still get intermittent ventilation but it's not that common anymore. It seems to be very sensitive to keel pressure. Interestingly, I get it if the keel is inflated to manufacturer's recommended pressure of 3.5 psi, but not if I pump it up to 5-6 psi.

It's also very sensitive to port-starboard weight distribution - something I never experienced in any of my other Zodiacs.

All in all, this 2 weeks vacation has been a blast and the boat has been performing mostly pretty well. I am inclined to try to get rid of the remaining intermittent ventilation with a new propeller rather than lowering the motor at this point

[Projectile]

3 questions:

1) What propeller to get? (Current prop is Evinrude OEM 3 blade 10.3 x 12)

2) A lower pitch prop will increase my RPM, which seems to be needed to get me into the sweetspot for this motor (5500-6000), but what will this do to my fuel economy and performance?

3) If I go to a 4 blade prop, what pitch should I aim for when going from 3 blade to 4 blade?

[captnjack]

Originally Posted by Projectile

I still get intermittent ventilation but it's not that common anymore. It seems to be very sensitive to keel pressure. Interestingly, I get it if the keel is inflated to manufacturer's recommended pressure of 3.5 psi, but not if I pump it up to 5-6 psi.

This is my experience as well. The lower pressure allows the keel to compress and the fabric of the hull to bow and that creates a hook.

Frustrating that the floorboard issue created the motor height issue and now the ventilation issue. Goes to show you how one design/assembly element can really mess with a lot of different performance problems.

[jyasaki]

Originally Posted by Projectile

3) If I go to a 4 blade prop, what pitch should I aim for when going from 3 blade to 4 blade?

Assuming all other variables (cupping, blade shape, all that) are the same, I've generally heard that you lose about the equivalent of 1" of pitch in rpm going 3 blade to 4 (more rotational drag, more weight.)

Unfortunately, all the other variables are never the same, so...

jky

Okay, that was clear as mud. #blade to 4, you'll need to drop an inch or so in pitch to get the same rpm. Take that with a grain of salt. The rules of thumb all depend on whose thumb you're measuring with.