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Old 01 January 2004, 12:10   #1
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Prop selection

Having now had a chance and the conditions to do a V-Max test i can confirm my maximum speed as:

46knts
5200rpm
21" prop
1.87:1 gearbox ratio

Triming out past 30% of the range gave no advantage...just instability.

Using this http://go-fast.com/Prop_Slip_Calculator.htm and the above figures it seams i have a 20% slip - this seams quite high.

Assuming the 20% slip is true, and will be constant with a prop change, it also seams droping to a 19", at 5750rpm will give me... 46knts, and a much bigger fuel bill.

Am i overlooking anything? Comments appreciated!

I am quite willing to accept that this is as fast as i can go, and i'm very happy with the set-up but if there's more in her 'capin...
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Old 01 January 2004, 13:49   #2
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Daniel if it's an ally prop try and borrow a stainless 20" - that will give you less slip and more speed, try a 19" if it's available for peace of mind as well and you'll be sure then

Happy New Year -
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 01 January 2004, 13:52   #3
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It's a Laser II stainless 21". I did not think you could get even pitch for my engine, or is that just in the Laser II range? I've arranged to borrow a 19" Laser II but am concerned the test will not be that realistic because it's not as new and shiney as my 21".
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Old 02 January 2004, 05:26   #4
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Try the 19 anyway - it will still give you an idea. If you can get hold of something like a Raker, they give more bow lift if the boat is a bit nose heavy or a chopper if you need some stern lift - sadly everything I have is a bit big for you!

If you get a chance Tempest are a good neutral prop and work well on a well balanced boat.

Hope the above helps a bit,
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 02 January 2004, 05:32   #5
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Quote:
Originally posted by Cookee
or a chopper if you need some stern lift -
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Old 02 January 2004, 06:08   #6
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That's what they seem to do to my boat Jon? Maybe my brain is a bit rusty from all of the celebrations - Which does your alchohol free brain say is the best stern lifting prop Jon?
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 02 January 2004, 06:47   #7
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Quote:
Originally posted by Cookee
or a chopper if you need some stern lift
Steady on Cookee, your starting to go all "Manos" on us.

I've always found the chopper to be an excellent prop, but it certainly don't give stern lift...

Mind you, I've never understood the obsession with propellor swapping, why not just accept that the rib does 46 knots (?) and be happy with it. As you should be aware through racing, it's almost impossible to find identical conditions to compare props, even with back to back testing the fuel load, tide, wind etc changes, and so you can never be 100% sure that a different prop is an improvement, unless of course your original choice was totally wrong.
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Old 02 January 2004, 07:57   #8
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Prop swapping, what's that?

TQ rules.
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Old 02 January 2004, 07:59   #9
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Quote:
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why not just accept that the rib does 46 knots (?)
Quote:
Originally posted by Daniel
I am quite willing to accept that this is as fast as i can go, and i'm very happy with the set-up
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Old 02 January 2004, 10:35   #10
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Mirage

Hi Daniel we had a Laser II picth 19 on a Viper 6m 150opti.
We changed to a Mirage 19 and that one made us go 1 1/2 knots faster, i don`t know about fuelconsumption tough.
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