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Old 06 August 2007, 15:01   #11
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Country: UK - N Ireland
Boat name: Muzungu
Make: Red Bay 650
Length: 6m +
Engine: Suzuki 200hp
Join Date: Jul 2007
Posts: 37
I downloaded the Prop Calculator, by roycruse, its very good. I still get the same answer that the slip is around 2% at 5400.

I have two GPS and the Lowrance is linked up to the engine giving all data, yes the tacho is a bit high but the Lowrance should be correct.

I think my boat is heavery wider and more dead rise than most. Sadly I have deleted the spread sheet I had of other boat spec.

Redbay 6.5 720kg - dead rise 25 - avg 200L fuel (max 315L) - it also has a widder beam
Humber 6.5 580kg
Ribcraft 6.5 460kg apox

What part in the rpm range should you look at the slip?

Thank you for all your help.
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Old 06 August 2007, 15:33   #12
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Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Oh yeh, it was roycruse, apologies for confusing him with Hugh Jardon.
Quote:
What part in the rpm range should you look at the slip?
The slip figure is not too important if the boat is performing well but it does give an indication of the load the prop is under. Given that your boat is fairly chunky and has a steep deadrise your 47mph isn't unreasonable as a top speed. Your 16"dia prop with help drive it well but you'll never reach anywhere near 2% slip. If all your figures are correct then the tacho has to be reading low so your 5400rpm will be inaccurate and your revs will be higher.

My previous boat was a 6.5mtr, 200hp and quite heavy. It did 50mph with a low fuel load and about 45ish when loaded so it is fairly comparable to yours. It had a slip of about 18% at cruising speed - around 40mph.
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Old 06 August 2007, 16:42   #13
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Country: UK - N Ireland
Boat name: Muzungu
Make: Red Bay 650
Length: 6m +
Engine: Suzuki 200hp
Join Date: Jul 2007
Posts: 37
I agree that 2% seems imposible but I am getting the rpm from Lowrance it is linked up to the engine giving all data, rpm, lph, temp, volt, hours, fuel used, and more. It comes from the engine processor so I would have thought it would be 100%
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Old 12 August 2007, 12:12   #14
1up
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Country: UK - England
Length: 5m +
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there is a good prop calculator here
http://www.mercuryracing.com/product...s/propcalc.php
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