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Old 22 May 2004, 17:59   #1
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Stainless prop performance

Fitted a new Titan ss prop today hoping to gain a little performance. Was disappointed
I fitted it for two reasons, alloys seem to take lots of minor dings from trailing about and a hope that I would see minor performance gains, mpg and another knot or so.
The boat is actually performing quite different, torque through the steering is higher and the boat is now doing a knot less!
The advice I was given was that revs would stay the same and very slight performance increases would be noticed, what actualy happened is that I lost 200revs.
Anybody have any idea what has happened, the prop size is identical to the alloy I replaced 13.75x15.
I will give SD a call monday for advice but would appreciate any views from out there.
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Old 22 May 2004, 18:55   #2
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S/S normally allows you to move the engine up a hole which reduces the drag and thats where the performance increase comes from.

Certainly worth a try. Put the engine leg on the deck and unbolt it. Use the trailer jockey wheel to move the boat transom up and down.

You will want about three lads to do it just to be sure nothing slips.

Pete
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Old 22 May 2004, 19:03   #3
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Yeah, but I am seeing a performance decrease
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Old 22 May 2004, 19:21   #4
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Rupert, the advice given to you doesn't make any sense to me. If the steel prop is better because it is less flexible than the aluminium one, then selecting the same pitch is actually increasing the pitch when the prop is under load.
If it is chosen for the increase in efficiency because the blades are a bit thinner, it is not great in normal applications and, of course, because the blades are thinner they are going to be able to bend a bit, aren't they? If they are stiff enough not to bend, then they are not that much thinner and you're back to point number one.
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Old 22 May 2004, 20:09   #5
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I know, I wasn't sure either, that is why I called SD and asked which size to get and what I should expect.
I am just disappointed as I have now spent £250 on a prop I didn't really need and am worse off
I was hoping for 0.4knt extra to push the boat speed over 30knts, now I am back at 28.6knts.
What I will say is that the accereration seems slightly better but if the pitch is now slightly higher this would make sense of the 200 rev/min drop as well.
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Old 23 May 2004, 05:46   #6
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As I said on another thread, they'll tell you anything. Listen to them but use your own brain for the final decision. It'll be every bit as good as theirs!
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Old 23 May 2004, 12:31   #7
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Rupert,

I'm sure Steel Developments will be happy to either alter the pitch or swap for a different propeller.

As for the advice; generally it is true that you swap an aluminium prop with the same pitch stainless steel. However there are so many factors that effect performance that is not always going to be true. Choosing the right propeller is more of and art than a science and most of the time it’s going to require some trial and error.

Increasing the efficiency of the propeller doesn't necessarily mean that the rotation speed of the propeller will be affected. You would take that into account during design, and alter the pitch/blade area/diameter/rake etc accordingly. You can make SS considerably thinner than aluminium and it still won't bend as much - I believe SS is around 5 times stronger than Al.
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Old 23 May 2004, 17:51   #8
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SS prop performance

Have just fitted a secondhand Laser II. It is a 20" pitch and replaced an aluminium 19" pitch.
Max Engine RPM about the same so I guess I might be going a bit faster.

The boat feels like it is riding higher in the water and the usual Searider chine walking is much reduced.

Very happy with it and at £110 I couldn't really have lost.

I hope SD can sort you out.

Duncan
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Old 24 May 2004, 06:29   #9
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Called SD this morning and they thought the cupping on the SS prop was probably the problem. I have been told to send it back and they will slightly alter the pitch to compensate FOC.
Hopefully this will sort the problem.
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Old 24 May 2004, 06:52   #10
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There was an excellent technical article on props and propping on RIB International's site. I've just had a look there but can't seem to find it.

Best described as a "black art" I think.
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