Scorched Props

Andy_Rs600 · 14 June 2009, 08:07

Hi all,

Heard a rather unusual comment yesterday. Someone (who should know what he's talking about) said that he had looked at the props on all the RIBs we use at my local sailing club and noticed they were "scorched". He said this is caused by superheated air "scorching" the prop when the engine is trimmed up too much and ventilates. He further said he was going to try to stop all the safety boat drivers trimming up, and rather to use more throttle to prevent this "scorching".

I've been a poweboat instructor for five years, and been around boats my whole life and i've never heard this before. I have seen caviatation damage to props, but not on outboards. Has anyone else heard of this idea?

Have fun.

Hightower · 14 June 2009, 08:40

I've never heard of Ventilation damage to a prop either. But sounds more like cavitation, where the water superheated right at the surface of the prop and creates billions of little bubbles that damage the prop surface.

willk · 14 June 2009, 09:09

Yeah, that's a bright lad you have there

, burn more gas every day to save a prop change every five or six years! In any case, you'll damage more props launching/recovering than you will from pitting. What's next on his list?

Reduce trimming of sails, to save wear on sheets?
Keep Yachts beached to reduce erosion to antifoul?

He reminds me of certain ribbers (no names!) who buy a £30,000 rib and obsess over whether they use 1 or 1.1L/Nm.

Andy_Rs600 · 14 June 2009, 09:15

Quote:

Originally Posted by Hightower

sounds more like cavitation, where the water superheated right at the surface of the prop and creates billions of little bubbles that damage the prop surface.

That's what i thought, he might just have the words confused. Have you ever seen significant cav damage on an outboard prop? I've never had to change a RIB prop because of that kind of damage.

Hightower · 14 June 2009, 10:15

I've only ever seen props where the paint is damaged. Obviously on Alloy props. But to be honest if you did have problems with serious cavitation, would you seek a soloution, so not that many damaged props around to look at.

BTW, I was told all props suffer with Cavitation given the right situation. That's why billions of £'s has been spent researching props for submarines.

Nos4r2 · 14 June 2009, 10:43

Quote:

Originally Posted by Andy_Rs600

That's what i thought, he might just have the words confused. Have you ever seen significant cav damage on an outboard prop? I've never had to change a RIB prop because of that kind of damage.

I've seen it, but it's never been bad enough to be a problem.

nugent · 14 June 2009, 11:14

chuffin ell,
i was showing Ian sharlot my prop only yesteday, and asking his opinion why the prop has lost the paint on the tips and only on one side,

looks as if i have run it in the sand, but i know i have not done so, + its one side only and not the tip edge,
my conclusion engine was set to high on transom i have lowerd it an inch now + up a pitch on new prop see how it goes but yes was prone to rev its nads of if trimed any thing above half way, i thought the prop bush was naff,
but have seen 52 on the gps

Rigaud · 14 June 2009, 12:19

Quote:

Originally Posted by Andy_Rs600

Hi all,

Heard a rather unusual comment yesterday. Someone (who should know what he's talking about) said that he had looked at the props on all the RIBs we use at my local sailing club and noticed they were "scorched". He said this is caused by superheated air "scorching" the prop when the engine is trimmed up too much and ventilates. He further said he was going to try to stop all the safety boat drivers trimming up, and rather to use more throttle to prevent this "scorching".

I've been a poweboat instructor for five years, and been around boats my whole life and i've never heard this before. I have seen caviatation damage to props, but not on outboards. Has anyone else heard of this idea?

Have fun.

Wow, makes me laugh because that would mean all offshore racing or drag boats would have this problem. I run a 23" cleaver with my prop shaft 4" below pad and zero problems. It's still too flighty up front so setting back 6" plus lifting 2" more. My props look great and really this guy should stay away from boats since he's a little

BogMonster · 14 June 2009, 12:34

So far over the last three years I have found the cavitation caused when the prop starts digging gravel to be more of a problem than anything else

Chris · 14 June 2009, 18:05

I believe that this can also be caused by having a prop thats diameter is too small.

Andy Young2 · 15 June 2009, 02:49

Getting back on thread - Is this an example of cavitation erosion about 10mm in from the leading edge of all three blades on this prop. Both props were like this and looking decidedly flimsy on the leading edge. I was planning to replace the pair beforeI struck some concrete in an indicated 13 foot of water 150m off the beach. I have since been able to source one replacement and awaiting another. The second prop has this sort of pitting damage but the blades are okay - is it possible to have the better of teh two props rebuilt as a spare?

I also wondered whether this damage as linked to anodes being ineffective or the effect of missing earthing straps between leg and engine mounting bracket?

Any advice gratefully received

jyasaki · 15 June 2009, 07:55

I ran a too-small (pitch) prop for a bit with a missing spacer. Ended up with a line cut about a half-inch in from the tips, starting about a half inch from the leading edge, and about an inch and a half long. It ate through the plastic (epoxy?) paint, and into the aluminum. ended up removing perhaps .040" depth, about 1/4" wide.

If it's eating metal over most of the blade width, I'd suspect electrolysis. if that's the case, the surface should have a definite pitted appearance.

jky

Simon B · 15 June 2009, 08:33

Quote:

Originally Posted by Andy Young2

I also wondered whether this damage as linked to anodes being ineffective or the effect of missing earthing straps between leg and engine mounting bracket?

Any advice gratefully received

Paint damage on hub looks more like electrolitic corosion,

Maximus · 15 June 2009, 09:13

Quote:

Originally Posted by Chris

I believe that this can also be caused by having a prop thats diameter is too small.

This was certainly the case with me,while sorting out the right prop for the new e-tec tried a few different sizes,finaly settled on 19''[engine supplied with 17]sorted!...ALSO!
CONGRATS TO CHRIS AND FAMILY!!! BONNY NEW BABY BOY 'CHARLIE' ARRIVED AT THE WEEKEND, All the best from Karen and me!....
See the trouble being a 'Born fiddler' can get you into!

Chris · 15 June 2009, 09:54