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Old 14 June 2009, 04:07   #1
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Scorched Props

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?

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Old 14 June 2009, 04:40   #2
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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.
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Old 14 June 2009, 05:09   #3
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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.
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Old 14 June 2009, 05:15   #4
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Quote:
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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.
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Old 14 June 2009, 06:15   #5
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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.
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Old 14 June 2009, 06:43   #6
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Originally Posted by Andy_Rs600 View Post
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.
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Old 14 June 2009, 07:14   #7
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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
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Old 14 June 2009, 08:19   #8
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Quote:
Originally Posted by Andy_Rs600 View Post
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
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Old 14 June 2009, 08:34   #9
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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
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Old 14 June 2009, 14:05   #10
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I believe that this can also be caused by having a prop thats diameter is too small.
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