cavitation burns bum me out

[ziggy]

i've never ask any prop questions since the first prop i tried seems to fill the bill. but after last season. my props got some cavitation burns in it.

engine and drive in sig. prop, best i know is a mercruiser flo-torqueII aluminum 19P prop. part # 48 832830A45 19P

viewing cavitation burns on 2 blades

the major burn.

the minor burn

what i think are the offending defects in the prop. these defects appear on 2 blades. these chips are on the leading edge.

my wot spec is 3900-4300. i regularly achieve between 4200-4400 depending on the quality of the day and load. 4400 is only achieved at pretty high trim (no trim limit switch, it's broke and i get the feeling i'm pushin the limit when i'm doing a speed run. top speed is 41-42mph maybe 43, gps) on a cool day. mostly i get 42-4300 though.

so any ideas what's causing this? what do i need to do to repair my prop if anything?

 

[Dunaruna]

Re: cavitation burns bum me out

Cavitation will cause chips or pit marks.

All I an see in the photos are paint scratches. Nothing to worry about.

 

[ziggy]

Re: cavitation burns bum me out

while i do have a low resolution camera, put it in paint and blow it up. it's cavitation burn. pits.

 

[solar7647]

Re: cavitation burns bum me out

prop shop will get that fixed in no time. I would recommend you not do anything your self you could make it worse. You could also up grade to a stainless prop or a composite.... they will not chip as easy.

 

[ziggy]

Re: cavitation burns bum me out

prop shop will get that fixed in no time.

yep, i believe that.

You could also up grade to a stainless prop or a composite

i got the o ring groove so ain't upgrading to ss for fear of a prop stike and shearing off my lower drive shaft instead of a prop blade or piece of. composite, maybe. but i'm poor. i'm also happy with the performance this prop gives me.
last time i had a prop sent in. it was $75. that was a few years ago, reckon the price went up. they do come back nice though.... however, the question to me is why did it happen. i can fix the prop, but if it just reoccurs then it's a waste.
this is a used prop i got at my local dealer. a c note used. other than the burns. it looks about like the day i got it. the burns appeared in the third year of use. first two, no issue...

 

[Mi duckdown]

Re: cavitation burns bum me out

Those are not burns. run it. Most burns are on or near the hub That you would only see if you had a stainless steel prop at very high perfomance. That is aluminum reman prop that lost paint.
.02
Burns would have removed that paint

 

[hwsiii]

Re: cavitation burns bum me out

Ziggy, here is an excellent picture of prop cavitation presented by Ken of Prop Gods. It shows that cavitation forms when the pressure becomes low enough on the blades that the water starts to boil from the speed of the prop, and cavitation vapor forms in that region, at lower pressures water boils at lower temperatures (like boiling water in the mountains). cavitation is normally created by a combination of water temperature and blade speed, but it can be exacerbated by any protrusions under the boat, close to the propeller, creating air bubbles that can be sucked into the prop and helping the prop turn the water to vapor.

The water boiling and creating water vapor (steam) instead of solid water is not the problem, when the vapor moves down the blade into areas of higher pressure and collapses back into water and returns back to a liquid the implosion is what causes the erosion of the paint and the blades.

The speed of the water over different parts of the prop varies directly with the distance it is from the center of the propeller. The highest speeds occur very near the tip of the blade. A 16" diameter propeller that is turning 3,000 Gear Ratio RPM has a prop tip speed of 143 MPH, this will give you an idea of why a propeller can create cavitation. The pressure drop and then the subsequent boiling of the water and the vapor collapsing is called cavitation.


Prop Cavitation

Exaerbating reasons for cavitation can be nicks in the blades on the leading edge, a blade being bent and out of alignment a bent prop shaft or a prop that is very small in blade surface area for the weight of the boat and the power supplied by the motor, especially at take off, and bubbles from items that have been installed at or near the prop itself from protrusions under or at the transom of the boat .

I hope this gives you a better understanding of cavitation and its causes.


H

 

[ENSIGN]

Re: cavitation burns bum me out

You're going to worry yourself to death,get a can of phantom black paint!

 

[Texasmark]

Re: cavitation burns bum me out

As usual Hwsii excellent post, information, and credits to the author (real professional).

Mark

 

[smokeonthewater]

Re: cavitation burns bum me out

looks like cavitation to me.... but heck..... run it and shop around for a 20 pitch your prop could have been bent just slightly or you mightbe blowing out a bit on holeshots and benifit from just a little more pitch..... also look for anything upstream that would disturb the water flow

looks like the most common causes for cavitation are imperfections in the leading edge and too much cup in the prop..... next would be too small of a diameter for the horsepower but not likely here..... I'd probably study the prop very carefully and look for another... maybe a 20 pitch to drop those rpm's just a little

 

[ziggy]

Re: cavitation burns bum me out

thanks for the feedback guys.

Those are not burns. run it. Most burns are on or near the hub That you would only see if you had a stainless steel prop at very high perfomance. That is aluminum reman prop that lost paint.

i still think it's a burn.

but i could be wrong.

Exaerbating reasons for cavitation can be nicks in the blades on the leading edge

that, i have. the question is are my nicks bad enough to cause this. to me i'm guess it is yes, otherwise i wouldn't have had the burn. + the 1st two seasons i didn't get these burns. perhaps over time i've hit enough debrit and dinged my leading edge enough that it is now starting to have ramifications. these dings are also mentioned in chapmans as a cause for burns too.

can i touch these up with a file and be a cheap azz or would it be better to leave well enough alone. guess the suggestion about taking it to the prop shop would be the right repair. may end up doing that if i can come up with the bread.

You're going to worry yourself to death,get a can of phantom black paint!

naw, but thanks for the concern.. only reason i posted up is cause i did note the burns at end of season and now it dead of winter and i'm bored and wanna talk about boats... might have to get a can of paint. need one in general. been using a black sharpe for minor touch up...

looks like cavitation to me.... but heck..... run it and shop around for a 20 pitch..... also look for anything that upstream that would disturb the water flow Today 10:09 AM

thanks for the agreement. i still don't think it's just paint chipping off so nice to have someone else think that may be the case too and that i'm no crazy. yep, being poor, i may run it. if i was changing props, i was thinking a 4 blade 18P (one forsale in my area right now too on craigs list, hummm) for a little lift at the stern though. i could probably use some tabs. my trim is pretty sensitive. when full down it's ok and i jump out of the water real fast to plane. but just touch the up trim and ya can tell right away the attitude changing. i got old style gauge and usual highest trim i run is up two bars. i have gone to three though, 5 and i believe it's in the red.

thanks again for the feedback.

 

[hwsiii]

Re: cavitation burns bum me out

Ziggy, that last picture absolutely looks like cavitation implosions, with all the very small pock marks in it, and thanks Mark.


H

 

[Yepblaze]

Re: cavitation burns bum me out

You're going to worry yourself to death,get a can of phantom black paint!

I agree

 

[Texasmark]

Re: cavitation burns bum me out

Only time I had cavitation burn on a prop was with a '71 Chrysler 85 hp. It had this unusual leading edge to the OEM alum prop....blade left the hub and then went forward slightly as it was formed then went back in conventional fashion to the rear....sorta like the props on the Solas advertisement here on the right side, only more exaggerated. The burn was right in the area of the leading edge of the blade where the S curve was formed. Burned off a spot about the size of a quarter and did it every time I went out.......come home and paint over it, next week go out and burn it off again. Very annoying.

Mark

 

[ziggy]

Re: cavitation burns bum me out

Very annoying

i'd think so. kinda why i wanna approach the issue i'm having. i'm assuming that what i got going will get worse with further use.
since i now have folks believing i got cavitation burns. best thought so far is my leading edge. ya can see some of my defects in my 1st pics. guess if i wanna get it back to biz as usual, i'm gonna have to splurge and send it in to the prop shop and quit doing the missouri river which is most likely where i strike stuff at... filthy dirty river... or figure on the prop shop every year or three if i want it perrrfect.. thinkin about filing it. but there's not really anything to file. kinda like little chunks missing from the end of the blades must be causing the disturbance of water flow... other than that, this seems like a proper prop for my rig and works well.