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Old 24 May 2008, 00:23   #1
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Effect of s/s prop?

Thinking of upgrading the prop on my 90 Yam 2-stroke (on 6m Bombard RIB) to s/s (currently ali and cant remember the exact size/pitch off-hand). With a like for like replacement, what effect should I expect on: WOT rpm? max. speed? time to plane? Is it likely to be worth doing (prop cost ~200)

All comments/advice welcome.
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Old 24 May 2008, 01:58   #2
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I know, I know - search old threads before you post.

Have done now and I get the impression that s/s probably won't offer much benefit if my existing prop is well tuned to the engine/boat. Would that be generally correct?

Think I will experiment a bit with the selection of ali props I have at the moment before I decide to switch to s/s.

However, any specific advice still welcome.
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Old 24 May 2008, 03:53   #3
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I borrowed several SS props and did some extensive tests for my last RIB which had a 80 yam and found no real improvements over my genuine Yammi prop. Top speed was always the same if not slower (Guess I'd maxed out the hull performance), I did see gains in mid range speed at a given RPM (30knotts as opposed to 28 knotts with alloys one at 4000). However I came to the conclusion that it wasn't worth spending any money as the gains wheren't really there, plus if you ding a SS prop you can say goodbye to the gearbox, where as an Alloy prop will probably discentigrate before gearbox damage is done.
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Old 24 May 2008, 16:28   #4
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When I had my Humber Assault I had a yam 60 with a 17" prop on it. Given my nature as a tinkerer I bought a four bladed SS prop to try..........
The boat went much slower.
I then tried three other SS three bladed versions of various pitches and even though I could reach WOT with them...........
The boat went much slower.
After the last try I gave up and took the props back and stuck with alloy ever since. The difference in speed was about two knots slower with SS which on a boat that only did 27 knots empty was a lot.
I never did find out why the effect on my hull trying the four different SS props was negative instead of positive as it was contrary to expectations.
If I ever get a chance I would try another four bladed SS but I doubt I would buy one, would need to be a loan to try and I know no-one locally with one so unlikely to happen.
Will see what happens when the boat is run in, prop is a "best guess" at the moment.
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Old 24 May 2008, 17:29   #5
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I have never had a stainless prop but I did think about it at one stage. Since then I have pranged two alloy ones, one quite badly. I have to launch on rocky beaches and sometimes the wash can pick up the odd rock if you give it too much breakfast, on this occasion I got the boat stuck on the trailer, gave it a burst of throttle and there were some loud expensive clangs as several large rocks went through it, and then some swearing )

I now have no intention of making the weakest link more expensive, 100 a go is enough for a prop never mind 300 for a prop and for a new gearbox as well

I guess they are fine if you launch on a concrete slip and never go in shallow water but they wouldn't do me here.
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Old 25 May 2008, 03:30   #6
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It does not just come down to the prop. If you are fitting a stainless prop you should get more grip so the engine will not cavitate as easily. This then means you can raise the engine on the transom an inch or two or three. Raising the engine means less gearbox in the water so should provide you with a speed increase.

I have done a lot of prop testing on Danny Boy and found that a good stainless prop coupled with some engine height changes gave me another couple of knots over any alloy prop.

Chris
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Old 25 May 2008, 03:51   #7
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Quote:
Originally Posted by Chris View Post
It does not just come down to the prop. If you are fitting a stainless prop you should get more grip so the engine will not cavitate as easily. This then means you can raise the engine on the transom an inch or two or three. Raising the engine means less gearbag in the water so should provide you with a speed increase.

I have done a lot of prop testing on Danny Boy and found that a good stainless prop coupled with some engine height changes gave me another couple of knots over any alloy prop.

Chris
Yes but I think that the gains for a stainless prop only realy start to kick in with more powerful engines and larger boats. For the sake of a couple of knots, is it really worth it?

I have never run an alloy prop on Solent Viper, but would like to give it a try and see what happens.

People are always looking for the "Holy Grail" with regards top speed, however how many times do we travel that fast?

Much more important to get in the WOT opperating range for your engine so that you know it's not labouring by swinging a too largerer prop and also getting acceptable "holeshot" and cruising speed.
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Old 25 May 2008, 07:09   #8
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Quote:
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People are always looking for the "Holy Grail" with regards top speed, however how many times do we travel that fast?
Erm... as often as possible ??
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Old 26 May 2008, 01:57   #9
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Erm... as often as possible ??
Yeh, but realy. How often do you actually travel at WOT
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Old 28 May 2008, 11:19   #10
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Like for like doesn't work with a switch of materials. Assuming your aluminum prop is perfect for the boat and load, and you find the identical prop in stainless, the net result will be that you will be down 100 to 250 rpm at WOT due to the stiffer blades. That *should* result in a loss of top end speed.

You have to select the stainless prop for proper performance, just like you did with the aluminum (well, maybe not you, per se, but someone did.)

jky

Actually, the additional weight of stainless would have an effect on rpm as well. Same result, though.
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