Ventilation Or Cavitation

[Bill kubiak]

What are the differences and how can I tell one from the other????

 

[walleyehed]

Re: Ventilation Or Cavitation

Ventilation is when air is induced/sucked into the path of the blade, causing "usually" a rise in RPM, and is not known for damaging the prop.
Ventilation is simply loosing bite, but there can be many reasons for this issue.
Cavitation is an extreme low pressure area that builds so quickly it boils and explodes on the surface of the blade causing "burn" marks-a porous, "lost-casting" look usually on the inner third of the blade. On aluminum it appears as a spot of paint coming off and something has rubbed the blade in the center of that area. Cavitation can be caused by a mis-matched prop turning more RPM then the particular design was intended for, or possibly a poorly made prop or even damaged.

 

[JUSTINTIME]

Re: Ventilation Or Cavitation

cavitation happens underwater
ventilation happens above water

 

[walleyehed]

Re: Ventilation Or Cavitation

Ventilation can occure at any position on the prop...perhaps a tab, gearcase skeg or anything in front of the prop or it's path can create a pocket of air that can induce Ventilation.....a poorly designed prop, then yes, the surface, or too close to, can be the reason for Venting.

 

[TilliamWe]

Re: Ventilation Or Cavitation

And what most people call cavitation, is really ventilation. walleye, would you agree that most people don't know they if they are experiencing cavitation? But whenever some one ventilates a prop, they should know it right away (the increase in rpms accompanied by a loss in speed). But they usually say they "cavitated", when they really "ventilated".

 

[steelespike]

Re: Ventilation Or Cavitation

Isn't it true all props experience a certain amount of cavitation?
It seems that I heard or read that minor evidence of cavitation can be
an indication of good prop efficientcy.

 

[Texasmark]

Re: Ventilation Or Cavitation

Most of us that were around in the "brown shoe" days (reference to the US Army Air Core before 1947 when it became a separate branch of the service...they wore "army brown" shoes which switched to black after separation; cliche commonly used usually in humor); were fed the "anti-cavitation plate" theory, as the name of the plate immediately above the prop on an outboard/outdrive.

Since joining this forum a couple of years ago, I have been taught the correct terminology which I agree is a more accurate definition.....but, after referring to it that way all my life, it is hard to get accustomed to the change.

I only had one prop that truly exhibited the results of cavitation. It was on a 1971 Chrysler 85 hp with a factory alum prop. The prop had a peculiar leading edge that, rather than flow smoothly with rake and all like today's SS props (in particular), it had a reverse curvature right at the root of the blade on the leading edge. It came off the hub and curved forward, then in an S curve swept back in the normal manner. It looked really great....but looks can be deceiving.

Cavitation erosion occurred right behind this S curve and ate the paint off and into the alum. Really bugged me as I hated to see a prop with some of the paint off it.

Just some trivia.

Mark

 

[oops!]

Re: Ventilation Or Cavitation

walleyehed
was just reading
have i got this right?
my props paint is coming off, ...or should i say ..all my props paint comes off.
looks like i been runn thru sand.
this is after a short period of time....10 hrs??
is that an indication of cavitation?
or did you mean by the inner third -"close to the hub"
thanks
oops

 

[walleyehed]

Re: Ventilation Or Cavitation

Usually, Cavitation starts as a spot as small as a dime...it can get bigger, it can even leave a trail that looks somewhat like a comet.
If the paint is coming off all of the blade-NOT the tips, (which is likely sand on the tips), it could be a referb prop or just bad paint, although rare for a factory prop at 10 hrs.

 

[steelespike]

Re: Ventilation Or Cavitation

Cavitation generally occurs on the back side close to the hub.I believe a little indication of cavitation is normal.

 

[QC]

Re: Ventilation Or Cavitation

Another little tip might be that when you first experience serious ventilation you might think you broke your drive or lower unit. It can rev as if you're in neutral. I have had it so bad in a tight turn that I almost had to stop to get the bite back, and then I have had lesser cases that I could control with throttle alone. Cavitation on the other hand you would only see evidence on a victimised prop as described above . . .