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Old 14 February 2008, 05:13   #11
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You have written that you hit 6100rpm "whith travel left in the throtle speed 34knts"
Then you said:"6100rpm is Wot"

So a bit contradictory but presuming that the current prop 15.25 X19 will over rev the engine I would try that 14.75 X 23.

Because your engine specifically produces its extra 25hp (over the DF150) from 5500 to 6100rpm IMO you should really endeavour to get the WOT to max out right around the 6000 to 6100rpm - If you only hit 5500rpm then you might as well have bought the DF150 !!
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Old 14 February 2008, 05:23   #12
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Hi, Aidan.
Supplier is recomending 21 pitch but my reason for thinking it might be 23 inch is because they initially recomended 17 or 19. Not sure if they are forgetting the 1:2.5 ration on the gear box.
That will make a difference, my opti is 1:1.87. So you will have a big slow turning prop. Go for the 23, you can always have it adjusted by an inch to fine tune it.

Pete
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Old 14 February 2008, 05:33   #13
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Many thanks for taking the time to post your reply Polwart.

The reason the engine was supplied with the 17 inch pitch i was told was because that was the only one available at the time of collecting the boat and to be fair to the supplier it arrived at the factory an hour after we did.

I think your calculation matchs my gut feeling, so thanks

Tndb - It does sond like a contradiction, i will try and explain a bit better :
WOT is 6100 (as prescribed in owners manual). When the boat reachs 6100 the throtle is at approx 3/4 of its travel. The engines 'brain' prevents any more rpm being produced, although the engine doesnt sond / feel like it is working hard to get to these revs. That and the rather pathetic top speed lead me to think i need the higher pitch. Does that help or did i confuse the matter further lol

I dug out a copy of Rib international and looked at a boat test they did back in Dec / Jan issue.
The boats were solent 6.5m ribs one had a verado 175 and the other had a suzuki 175

the suzuki recorded 47.95 knot

I know its not the same boat as mine but does suggest that there is a lot of speed lacing in mine.




So with all this in mind and all your help i am off to wrap the old prop for sending back to exchange for the 23. will hold off until tomorrow just incase someone comes along with more info.


Many thanks to you all for your help on a question that must have been asked a thousand times.
Kim
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Old 14 February 2008, 05:35   #14
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Thanks Pete 7 - good idea Anyone you would recomend to make the alteration if needed?
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Old 14 February 2008, 07:14   #15
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Thanks for you explanation re the 6100 and half throttle -I understand now . Basically looks like you are severely under propped - , my Df150 will push my 6.5m to 5900rpm with the 14.75X23 so your extra 25hp should certainly manage an extra 200rpm (to your max revs) one would think ?

You may even end up with a 14.5 X25, but the 14.75 X 23 is certainly worth trying since as I said before you really want to use the full 6100rpm rev band as that is what you have paid for (DF175 vs DF150)
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Old 14 February 2008, 07:53   #16
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Yep all makes sense TNDB and as Pete 7 suggested I could always have it altered to 24 pitch.
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Old 14 February 2008, 09:02   #17
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Thanks Pete 7 - good idea Anyone you would recomend to make the alteration if needed?
Yep, Geoff at Streamlined, over the years he has helped quite a few folk out on Ribnet so worth pointing business back in his direction.

http://www.streamlinedpropellers.co.uk/

Pete
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Old 02 March 2008, 15:22   #18
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Hi, was out this weekend with new prop (23 pitch)
managed to get 6000 / 6100 rpm and speed of approx 41knts
a much better set up with this propfuel figs later in the season
many thanks for everyone's help
K&S
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Old 06 March 2008, 04:51   #19
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Hi, was out this weekend with new prop (23 pitch)
managed to get 6000 / 6100 rpm and speed of approx 41knts
a much better set up with this propfuel figs later in the season
many thanks for everyone's help
K&S
Excellent - sounds spot on
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