AVP Theory Vs Practice

[Sea Rider]

Wanted to start a talk with you fine boaters, regarding engine's anti ventilation plate height theory versus practice. Everybody states as cast on stone that AVP should be about even with hull, seen vids on which a long ruler is used to raise AVP to match same hull height. Well this is not so, if you take the time to read about every brand engine owners manual would see that most states 10 to 30 mm/0.4 to 1.2 inches and will depend on a given hull shape.

Providing the above measurements, have 2 simple questions to ask : How do you know engine has reached perfect AVP plate height, how do you determine that ? There's a visible aspect that 99.9% of posters here and out there are olympic skipping or not considering at all. Give your best own or others best theory, practice, word of mouth hearing.

Happy Boating

 

[bonz_d]

Re: ACP Theory Vs Practice

Re: ACP Theory Vs Practice

I guess that's why on Hi performance outboards you will see the engine mounted on an adjustable jack plate. Used to tune the engine to the optimal height.

 

[JimS123]

Re: ACP Theory Vs Practice

Re: ACP Theory Vs Practice

I'm most familiar with OMC engines that have an adjustable bracket, with a series of mounting holes 3/4" apart.

I mount the engine all the way down, and then move it up 1 notch at a time. At each position I do a sea trial and plot rpm vs. mph at about 8 different levels. From that data I roughly determine the best cruising speed and then check the fuel consumption at that speed over my standard local 26 mile course.

Obviously, this takes days to complete and I try to repeat each test under the same environmental conditions as best as possible.

Compare all the data and the right height will jump right out at ya....LOL. In my case, with the several boats i did it with the optimum was about 3/4" above the bottom.

As an aside, I always repeat the test every year now. An ideal way to know when your performance drops off so you know service is needed.

 

[gbrowne9]

Re: ACP Theory Vs Practice

Re: ACP Theory Vs Practice

every boat is different and it,s a matter of trial and error.on my apollo 660 i had the motor 2 hole above the transom and i was getting to much cavitation and over reving. dropped one hole and everything came into sync. it was a 1989 115 merc.

 

[ondarvr]

I don't think many people here recommend the AV plate to be even with the bottom of the hull, they do say it's a good starting point, but like already mentioned, every boat, motor and prop combination is different, plus when you toss in how the owner wants to use the boat it may change the results again. Now add a set back and it all starts over.

There is "no cast in stone" rule other than a great deal of testing needs to be done to achieve the best performance for how the owner wants to use the boat.

Now there are some general principals that are used to determine what changes may be need to be made to achieve a certain result, but it still needs testing to fine tune it. Most owners don't want to do that much testing, and many never do any testing, they just run it as is and don't know it could be better.

 

[Faztbullet]

This only a starting point to keep engine from being to deep on hull. It depends on hull design,horsepower,boat weight,jackplate setback, prop etc....For example my 17ft Bullet with a 200+hp motor with 14.5x28 pitch prop was set on jackplate with cav plate about 7"above bottom(pad) which left propshaft about 1-1/2 below bottom of pad.

 

[Frank Acampora]

First off, there is no theory! The anti-cavitation plate is a misnomer. This plate is actually an ANTI_VENTILATION plate and its primary purpose is to prevent the prop from entraining air when running. Theory and practice are the same: The av plate should be at the surface of the water with the hull on plane when using a regular prop. Surface running props and racing props are a different animal..

Since all hulls are different, how this is accomplished differs. Back to theory: All props generate a negative pressure on the forward face of the blades. Because of this, the blade nearest the surface will tend to suck air into itself. The av plate provides a barrier to air trying to enter the prop. Thus, setting the prop deeper into the water is a good thing EXCEPT it does increase drag and heavy steering due to increased torque reaction. So if we are looking for maximum speed and easier handling we try to set the prop as high as possible.

TYPICALLY, a given planing hull will ride at a slight angle to the water, displacing water and cutting a groove in the surface. Aft of the transom, the water runs back, restoring the level. THUS: Somewhere aft of the transom, the water level will be higher than the bottom of the boat. Additionally, with stepped hulls or set-back plates, the water level will be a good bit higher than the hull bottom.

With most recreational hulls, the outboard can be mounted on the transom so that the av plate is set about one inch higher than the hull bottom. With steps or set back the av plate can be set even higher, sometimes up to 6 inches.

Unless you have a known hull with data accumulated, trial and error is the only way to accurately set the av plate height. You start raising the engine in increments until the engine starts to ventilate on plane on the straight. Then you lower the engine until the prop only ventilates during a sharp turn. This can become a PIA unless you have a jack plate and most people are content to leave the engine setting on the top of the transom.

 

[Sea Rider]

Frank,

Correct, the term is wrong. Know perfectly well what's the AVP and how does it work. So not to miss the idea I want to explain and what's not mentioned on any engine Owners Manual or even by word of mouth, let's not mix different boat types in same basket.

If fine boaters want to follow llet's just leave this tech conversation limited to standard recreational boats with tiller driven, manual trim engines to max HP that uses clamps screws as opposed to ones that can be raised/lowered by moving holes on engine mount.

Going back to original question : How can you determine if engine has finally reached its sweet transom height resting position ?

-Do you play all day long raising, lowering engine on long sea trails.
-Do you consider water conditions for sea trails.
-Do you test by sound if prop is ventilating and adjust height accordingly.
-Do you do a visual check at back engine to see its water performance.

There's one simple test that can put your beloved engine near spot on achieving best sweet engine/transom height. Although this simple test can be applied to all standard recreational boats wanted to start with the mentioned low standard class tiller driven boats and then go from there up to upper ones.

If we can follow and be tech tuned same, do you find anything strange, unusual if any in this posted pic, give your best perfectionist boater answer if you presume being one as myself.

Sorry folks, system went mod cracy with triple post in a row...

Happy Boating

 

[Sea Rider]

Check posted pic

Happy Boating

 

[Sea Rider]

Check posted pic

Happy Boating

 

[Sea Rider]

Check combo

Happy Boating

 

[ondarvr]

There is no difference in what's done in testing whether it's remote or tiller steering, manual or electric tilt, or clamp vs bolt, the same principals are applied in every situation.

 

[Sea Rider]

There is no difference in what's done in testing whether it's remote or tiller steering, manual or electric tilt, or clamp vs bolt, the same principals are applied in every situation.

Everyone has his own testing methods to maximize their engine to optimize boat performance. The theory I like to apply could be much different than the one you like to apply. And every type of engine HP wether small, medium large has it's own particularity, can mix in same sac. To each his own for each type of boat..

Anyway ,state which principals would it be ? How about the posted pic, anything unsual, or probably used to ride or see someone ride that way ?

Happy Boating

 

[Bubbasboat]

I don't see the posted photo. Still getting used to new format. Where might I find it?

"Anyway ,state which principals would it be ? How about the posted pic, anything unsual, or probably used to ride or see someone ride that way ?"