Prop problem definitions

[AGCB]

I read a post here some time ago that explained all of the various problems like cavitation, ventilation, slippage and several others and now I have searched and cannot locate that post. It listed all of them in a paragraph I think.
Would someone point me to something like that and this time I'll save it.

Thanks, Aaron

 

[steelespike]

In todays terms cavitation is what causes scaring on the back of the blades.
There is a low pressure area that makes the water boil and it eventually errors the paint and the metal.
I have a Merc 500 prop from 1965 with many years of operation The back side of the blades are about 1/2 scarred.
I have 3 Merc props all scarred the same .You don't feel cavitation.
Ventilation is basically the prop gets fouled with air and loses its grip.Could be a number of reasons,poor application,
motor set up too high inferior prop, a distortion in the bottom of the boat.Anything that might disturb the flow of warter over the prop.
"SLIP" is the measure of how efficient a prop moves through the water For instance a 12" pitch theoretically moves 12" forward in one revolution.
But the prop is working in a liquid so that movement is actually less.Measured in percent.Typically a runabout would be at about 10 or 12%.
Boats that are hard to push would tend to have higher slip numbers.As prop speed(revolutions) goes down slip usually goes up.

 

[AGCB]

At higher speeds when I make a tight turn the motor speeds up and boat slows down. I assume this is because the water is coming across the bottom of the boat sideways and disrupting the flow. Is there any fix besides slower speed?

 

[Frank Acampora]

No, It is because at higher speeds in a tight turn the boat leans and the prop ventilates. If you have power trim, tucking the engine in will help. If you do not have power trim and if the tilt pin is set for best performance in a straight line, and assuming the engine is set to correct height on the transom,then there is no cure.

 

[Texasmark]

Cupping will help in addition to what Frank said. Cupping is the bending of the trailing edge of the prop slightly toward the hub......like taking a flat palm of your hand and bending the tips of your fingers in. Just as those bent finger tips would grab water better than a flat palm so does cupping. Turning Point Hustler is the only alum prop that is cupped as far as I know. Most SS props are as it is part of the high performance package that usually is part of the SS prop design.

 

[inthedirtagain]

At higher speeds when I make a tight turn the motor speeds up and boat slows down. I assume this is because the water is coming across the bottom of the boat sideways and disrupting the flow. Is there any fix besides slower speed?

My boat does this as well, but only in left-hand turns. I don't have t-n-t, have tried adding stabilizer fins, and have tried multiple props.....all with the same results. The only conclusion I can draw is that my fish finder sonar is mounted just left of the engine. So, when I bank into a left-hand turn at speed, the water is disturbed by the sonar cutting just before the prop. That's my best guess, anyway. Does your boat exhibit something similar, or when turning in either direction?

 

[AGCB]

I can't remember if I've noticed it in a right turn

 

[Texasmark]

My boat does this as well, but only in left-hand turns. I don't have t-n-t, have tried adding stabilizer fins, and have tried multiple props.....all with the same results. The only conclusion I can draw is that my fish finder sonar is mounted just left of the engine. So, when I bank into a left-hand turn at speed, the water is disturbed by the sonar cutting just before the prop. That's my best guess, anyway. Does your boat exhibit something similar, or when turning in either direction?

You'd be surprised how well your transducer works when mounted inside the boat!

 

[Sea Rider]

If prop aerates at both tight close turns, could be due to engine not being opt height transom seated for that set-up with respect to water flow passing by when boat is at plane. Will need a transom height optimization to be inside the 0.2% of boaters that goes that way.

Happy Boating

 

[AGCB]

If prop aerates at both tight close turns, could be due to engine not being opt height transom seated for that set-up with respect to water flow passing by when boat is at plane. Will need a transom height optimization to be inside the 0.2% of boaters that goes that way.

Happy Boating

Would you please say that again?

 

[Sea Rider]

There's a theory conflict theme going on here for some time about AV plate versus Water Deflector Plate as everybody swears by AV and near none gives a dime about WDP, theme which have been investigating in theory and practice for long time throughout several boats/engines set-ups, but that's me. Do you have a current engine/boat problem or just wanting to know boating general theory ?

Happy Boating

 

[AGCB]

Do you have a current engine/boat problem or just wanting to know boating general theory ?

Happy Boating

See post #3

 

[Sea Rider]

For that will need to know boat model, engine size, if it's the max transom rated for and several parameters which works near hand in hand. When boating, were do you have the trim adjusted to and if it's a hydraulic or fixed manual trim ? how tight close are your turns, would you say at medium open wheel or bit close.

Aeration is usually due to improper tail height, at close turns you have less water bed for prop to keep bitting properly as when going on straight water courses. Latter will post some info about engine height performance issues and go from there.

Happy Boating

 

[Sea Rider]

This is a condensed version of lower leg issues, 2 themes are involved : (1) Splash Issues (2) Lower Leg Performance.

-If having water splashes out ot over transom, raise engine accordingly till water splashes disappears completely.




-Depending on lower leg height cond could achieve 3 possible performance conditions provided that engine and prop are healthy, engine trimmed at 90? for AV p'late to ride parallel to water level and deck weight evenly distributed while combo is on plane.

Lower Leg Water Performances :
-1 Upper : Best performance under flat calm and choppy water cond, perfect tight close turns. Best prop grip & thrust.

-2 Middle : Good performance on flat calm water at straight water courses, some aeration on choppy water cond and at close tight turns. Good, not opt prop grip & thrust.

-3 Lower : Crappy performance on flat calm water at straight water courses, worst aeration on choppy water cond and at tight close turns. Worst prop grip & thrust, less water bed for prop to fully grip as in (1)

Matching best engine height and top performance will depend entirely on boat model, hull shape, transom height and HP engine brand. Under any dry installation, that's sitting any given engine on any given boat's transom all boats will achieve propulsion, but probably not top one specially if having a under powered engine. This is a exercise for engine height optimization, can go from there for a prop maximization to suit your boating needs.

Happy Boating

 

[AGCB]

I wonder how to measure depth while underway. There is only about 2" between def plate and AV plate.

Motor is late 90s mercury 25 HP and boat is Alumicraft V14. Trim is manually set w/ pin and I don't notice splashing over transom except a little from transducer. No easy way to raise or lower motor. May be stuck w/ it as it is.

Thanks.
Aaron

 

[Sea Rider]

In older engines that's usually the average height between Def Plate and AV plate, newer ones can have as much as 5", are more friendly to play with different engine heights. Is that a tiller steered combo or has a central console.



If tiller, pull you head over transom and have a look while running on a flat calm straight water course, place a mate up front to help better balance combo when pulling head out.. This is the sweet engine/transom height for any given engine. check how flat and even is middle wake.

https://www.youtube.com/watch?v=oBeZjaA76VM

To check your current boat/engine installation, need to "determine" at which height is water flow passing by at plane. Post results, just for the record.


Happy Boating

 

[AGCB]

this is a remote steered boat. I'll try to have a friend help.

 

[Sea Rider]

The center console is located near bow, around middle deck or aft ?

Happy Boating

 

[Texasmark]

I wonder how to measure depth while underway. There is only about 2" between def plate and AV plate.

Motor is late 90s mercury 25 HP and boat is Alumicraft V14. Trim is manually set w/ pin and I don't notice splashing over transom except a little from transducer. No easy way to raise or lower motor. May be stuck w/ it as it is.

Thanks.
Aaron

Don't measure, just walk back to the transom and look. Yes you will/could be running a little deeper due to your weight back there (could send someone to the bow to counter this effect). Once the water clears the transom it rises somewhat when it crosses the prop and then more in the prop wash and then back to the dormant level. Again as Rider said, amount depends on speed, hull shape, trim position, water conditions..........

Rider, on the name calling, for all of my life, till 2005 when I joined this forum, we called the plate just above the prop the "anti-cavitation plate" with no regard for what we were actually saying. Somebody way back when in my area misnamed it and so it was....everybody just took it for granted with no further thought.

Few times does the subject of the WDP located on the leading edge of the LU come into play as you probably well know.

 

[Sea Rider]

Texasmark,

You won't find in any forum worldwide not even on the Internet a word about WDP and tons of info about AV plate because nobody really cares, everybody except few boaters gives for granted that you only need to sit any engine on your transom and voil?. Wrong, all transoms being small, medium, extra long are not made same, could have intermediate lenghts among them, same with engines, slight short, slight long among brands and this makes a day and night boating difference. Going for the sweet engine/transom height is tuning your combo, get the max out of it specially if under powered. It's not for everybody. Gee so much work involved, got tired just thinking about it, will pass....

A simple boating question : assuming you have AV plate even or slight lower with respect to lower keel, what do you expect to accomplish when combo is placed on water and you go for a wot spin ?

Happy Boating