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Old 06 October 2007, 04:41   #21
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Country: UK - Scotland
Town: Ayrshire
Boat name: Raven
Make: Ribcraft
Length: 5m +
Engine: 150 suzuki
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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?
Dai,

based on your example

I get he following;

5,900 RPM
2.50 Ratio
25"
3.40 slip?
53.97 MPH

Have a go (if you don't mind) I've probably made an arse of it some where, if it's correct I'm quite pleased as I thought it was wasting a lot of power.
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Old 06 October 2007, 06:20   #22
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3.4% slip, not really feasible often something is amiss ie...the gps is not reading correct plus tidal movement, the prop pitch is not as stated or the gear ratio is different to what you believe...an ali prop on an ordinary hull set standard height, is not going to give outrageous results the answer lies elsewhere
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Old 06 October 2007, 06:52   #23
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Quote:
Originally Posted by ribit View Post
3.4% slip, not really feasible often something is amiss ie...the gps is not reading correct plus tidal movement, the prop pitch is not as stated or the gear ratio is different to what you believe...an ali prop on an ordinary hull set standard height, is not going to give outrageous results the answer lies elsewhere
Cheers,

The engine is one hole up,recently done, speed improved by 1 knot ,the prop is stainless,standard suz. The speed is an average over many runs, I don't see a problem,the performance is fine (ventalates a bit more now it's a bit higher)
if it is 3.4% (which I can't really see) I'm a happy chappy,if it's 34% I'm still happy.
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Old 06 October 2007, 09:26   #24
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Originally Posted by IBWET View Post
if it is 3.4% (which I can't really see) I'm a happy chappy,if it's 34% I'm still happy.
could not agree more if the boat does what you want goes as fast as you think it should then who cares about statistics anyway
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Old 12 October 2007, 21:08   #25
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Originally Posted by Bigmuz7 View Post
.... but I dont understand the importance of what it means ? and how it is relevant ?,
Ah well, it's not really slip at all or, at least, it shouldn't be, but it might be.... Your prop blades fly in the water just like a wing of a plane in air but instead of travelling in a straight line they are attached to the hub. For there to be useful lift*, the blade will need to be at an angle of attack to the water. For some reason it always appears easier to understand when considering the wing of a plane so I'll stick to that. Imagine a plane landing; to generate sufficient lift the flaps will be activated and the plane will be flying nose up to increase the angle of attack of the wing. This is kinda like your prop at slow speed. When a plane is flying fast, the angle of attack will be much less, about 3-4 would be good. So now imagine your prop; the blades are angled so when you start to move the angle (of attack) is steep because you're hardly moving though the water so you could describe this as high slip. When you are travelling fast, the water is approching the blades much more from the front so there is less slip. Therefore, the slip (angle of attack) varies with boat speed and load and it is plainly also going to differ prop to prop depending on the prop's blade section and to some extent on the blade profile.

* Some folk reckon there will be no lift without an angle of attack but I reckon there will because the blade (wing) section will generate lift by dint of it's shape. It might not be much lift but lift is lift.
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