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Old 05 October 2007, 18:26   #11
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You've been down the pub, haven't you Captn?
why yes I have never been any good at maths either...
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Old 05 October 2007, 18:32   #12
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Steady guys its a 21..

now any chance of refocusing on the question you pirates
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Old 05 October 2007, 18:39   #13
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Steady guys its a 21..

now any chance of refocusing on the question you pirates
Well now we have the full story............

With a slip of only 0.005 either you need to recheck your figures or you have the most efficient boat in the world!!!

You can think of slip as like wheelspin in a way - the more wheelspin the less speed but sometimes you need a bit of wheelspin to keep the engine on the boil.
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Old 05 October 2007, 18:52   #14
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Well now we have the full story............

With a slip of only 0.005 either you need to recheck your figures or you have the most efficient boat in the world!!!

You can think of slip as like wheelspin in a way - the more wheelspin the less speed but sometimes you need a bit of wheelspin to keep the engine on the boil.
Well thats surprising codders ... its a 4 blader .. and I have a narrow Osprey derived hull, perhaps the speed is a factor ? but why is slip an issue and what does it mean?
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Old 05 October 2007, 19:07   #15
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The maths would be:

5800 engine rpm / 1.85 = 3135 prop rpm
3135 prop rpm x 21 inch pitch = 65838 inches per minute forward travel speed
As there are 63360 inches in a mile (according to my grandfather!)
65838/63360 = 1.039 miles per minute
1.039 x 60 = 62.35 theoretical mph with your set up at 5800 RPM, assuming nil prop slip.

If you are getting true 62 mph, then your prop slip is 0.35/62.35 = 0.56% : very good

Just as a comparison, the other day I was on a boat running at 5300 rpm on a 1.87:1 gearbox, 21" pitch prop and getting 46kt (roughly 50mph) Theoretical speed on those figures would be 56.4 mph, so prop slip would be 11.35%

Do I need to go back to the maths class or was there perhaps a 10kt current running with you?
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Old 05 October 2007, 19:16   #16
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Old 05 October 2007, 19:24   #17
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Quote:
Originally Posted by Downhilldai View Post
The maths would be:

5800 engine rpm / 1.85 = 3135 prop rpm
3135 prop rpm x 21 inch pitch = 65838 inches per minute forward travel speed
As there are 63360 inches in a mile (according to my grandfather!)
65838/63360 = 1.039 miles per minute
1.039 x 60 = 62.35 theoretical mph with your set up at 5800 RPM, assuming nil prop slip.

If you are getting true 62 mph, then your prop slip is 0.35/62.35 = 0.56% : very good

Just as a comparison, the other day I was on a boat running at 5300 rpm on a 1.87:1 gearbox, 21" pitch prop and getting 46kt (roughly 50mph) Theoretical speed on those figures would be 56.4 mph, so prop slip would be 11.35%

Do I need to go back to the maths class or was there perhaps a 10kt current running with you?
Wow ... I appear to be maxed out ... and no these are on various tides and runs N1 guys ta
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Old 05 October 2007, 19:26   #18
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or confused?
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Old 05 October 2007, 21:51   #19
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Well thats surprising codders ... its a 4 blader .. and I have a narrow Osprey derived hull, perhaps the speed is a factor ? but why is slip an issue and what does it mean?
I tried to explain it with the wheelspin analogy - wheels slipping - you are wasting power!!!
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Old 06 October 2007, 04:29   #20
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Erk!

I just worked mine out and I'm getting about 24% slip. I think I need to invest in a decent prop.
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