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Old 16 December 2002, 05:45   #21
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Mike G.

Mike, Golden rule no.1 - Never change two things at once.

Your proposed change is a lot. Take care.
JW.
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Old 16 December 2002, 06:06   #22
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JW - my golden rule is "Don't get caught!"

But I take your point. Thanks.
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Old 16 December 2002, 08:16   #23
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Mike G. sorry mate, as you know I have been a bit busy and missed most of this thread. I have a selection of S/S props that you should try before having one worked. If you are running a 18.3/4 then I would recommend a 21 as the first choice. It should take you from one end of the problem to the other and you will proberbly settle on a 19. Have you contacted your local Yanmar dealer and organised a selection of props on sale or return?
They will probably ask for 500 deposit but as long as you do not damage them this is refundable. Of course they expect you to urchase a prop at th eend of the test which is understandable.
Let me know if you want to borrow my 21 or like.

Alan P
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Old 16 December 2002, 09:08   #24
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Hi Mike,

2 inches sounds a lot strait off,as you can put pitch on but not take it off.

The advice you were given about speaking to Yanmar is good advice,bye the sound of it you may have a rocket on ya hands,if it good with a 165 the its going to be great with 315hp.

The man who knows everthing about Props and drives at Barrus is called Joe Poulter. Im sending you his mobile number through the personal message system.

Good luck

Crazyhorse
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Old 16 December 2002, 14:37   #25
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One, Two, Three, Four, Five..........

Mike, maybe this guy has a spare prop or two.



Toby
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Old 17 December 2002, 02:32   #26
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Country: UK - England
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Thanks, Alan. Thanks, Crazyhorse. And thanks to Toby, too. I'll update this thread from time to time as I make progress. If I'm luck it will be two steps forward and one backwards. Still, I'm learnin'.....
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Old 24 March 2003, 02:53   #27
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Prop Sizing - Progress Report

Thought I'd revive this thread for a bit.

I recently made contact with Jo Poulter - the engine/prop supremo from Barrus - and he made the long trip down Cornwall to join Brian and me to experiment with Prop sizes on Magellan Alpha.

He connected various sensors and guages to my Yanmar 300 and we set off to check out whether the engine was delivering the right performance through the 1.65:1 ratio Bravo 2X leg and 17 3/4 x 21" aluminium prop. It was also to be a test run for the Floscan fuel meter I had recently fitted.

We went through various speeds from 5 kts, 10 kts, 15kts etc up to WOT which delivered just below 35 kts. Jo monitored his guages and at the end of the test pronounced that the engine was delivering exactly the right power across the range with that prop.

Brian was keeping an eye on the fuel consumption and at WOT we were burning 68 litres per hour - again, apparently exactly the right level.

So - all in all - it seems that that the ali prop/leg/engine combo is right for Magellan Alpha.

Jo did lend me a steel 17 3/4 x 23 inch prop to try as an alternative. I tried this a few days later. The boat was slightly slower onto the plane as you would expect but nothing significant. Top speed at WOT was about a knot faster.

However, the boat felt "lumpy" at low revs doing 5 to 10 knots and there was noticeably strong torque steer at WOT.

My conclusion?

Think I'll stick to the 17 3/4 x 21 inch ali prop and get an ali spare on the basis that...

1 - For me, it seems all round performance is about the same as steel.

2 - Ali props are more likely to break blades rather than gear boxes if they hit something at high speed.

3 - Ali props are cheaper than steel ones. A couple of hundred quid spare will pay for a lot of French booze.

We are now scratching our heads and trying to resolve Brian's leg/prop problems. But that's another story and he can tell the tale. If he has the mind so to do!.
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Old 24 March 2003, 03:51   #28
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Good. It's nice to hear the end of a story.

JW.
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Old 24 March 2003, 04:23   #29
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I am pleased for you Mike - although somewhat surprised at the results of your tests. I have always understood that an ali prop is 'softer' (obviously) - therefore it bends more and subsequently suffer from slippage.
Given your results I would do exactly what you have decided on.
Years ago when I had my Hysucat the rib suffered terribly from cavitation of the ali props. I ran some tests with a wide range of props and settled on s/s which actually transformed the boat. No slippage or cavitation and I picked up and extra 3/4 knots.
Props, I think, are no longer a science, rather an artform based on technical parameters.
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Old 24 March 2003, 04:56   #30
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Thats the theory isnt it. However on my last boat I found I got better performance with an ali prop than the stainless equivalent so I stuck with the Ali! Mike, sounds as though your engine/drive/prop set up is doing exactly what it says on the tin so stick with it! 35kts for Mag A sounds very reasonable to me and quite fast enough for an old chap with sciatica like you

Cheers,
Alan
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