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Old 12 September 2005, 13:23   #21
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HMMMM

I ran this hrough the prop calculator at http://www.go-fast.com/Prop_Slip_Calculator.htm and it gives a theoretical speed of 50mph which is only 43 knots and thats with no slip.

So maybe your 48 knots is not gps measured and just the result of a not very accurate paddle wheel speed sensor...
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Old 12 September 2005, 13:28   #22
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Quote:
Originally Posted by Alex Brown
You must have an incredibly low amount of slip on that prop to get 48knots at only 6000revs - I'd expect the engine to be nearer 6300revs to get that sort of speed.
No actually according to the prop slip calculator if you have a 21 inch prop with a 2.38 gear ratio and 5% slip you would need 7200 rpm to get to 48 knots

Nauti Buoy - best check your speedo or your rev counter one or both of them must be wrong...
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Old 13 September 2005, 03:50   #23
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Gps

I based the 48 knots from the fact that I used my GPS Street Pilot 3, admitably the one I also use in my car, however the speed did show on the GPS as being 55mph as max speed. I was also trying to see how accurate the speedo on the rib was which was about 4mph out as far as I could tell trying to watch both at once and see where I was going.

Like I said before I did have absolute ideal conditions, no wind a very very flat sea. What more can I say, should have taken the camera I guess.
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Old 13 September 2005, 04:56   #24
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Sorry I wasnt trying to say that you were tellin' fibs.

I was just questioning your instruments accuracy.

The calculation for speed is

(RPM X PITCH) / (RATIO X 1215) = THEORETICAL SPEED IN KNOTS

(6000 X 21) / (2.38 X 1215) = 43.6 KNOTS

So with that engine prop and rpm your theoretical max speed is 43.6 knots.

Actual speed is likely to be 5 - 10 % less han this due to slip.

If I am completely mad and there is a way for a boat to exceed this maximum speed then please can someone tell me how - short of driving downsream in a very fast current - this could increase speed over ground (which is what gps measures) to above this speed. Ive been in rivers with nearly 10 knots of flow before so this could give you the speed you are talking about.
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Old 13 September 2005, 10:15   #25
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df140

Wish I could get over 40 knots out of my DF140. 35/36 knots is all I can acheive with 21" ali prop at 5300 revs on a very good day. I accept my boat is bigger and heavier but a chum of mine has an identical boat and set up and he will get over 40 knots out of his. Engine in for a 190 hr service today so will see if that helps.
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Old 13 September 2005, 10:52   #26
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Quote:
Originally Posted by roycruse
HMMMM

I ran this hrough the prop calculator at http://www.go-fast.com/Prop_Slip_Calculator.htm and it gives a theoretical speed of 50mph which is only 43 knots and thats with no slip.

So maybe your 48 knots is not gps measured and just the result of a not very accurate paddle wheel speed sensor...
Hey Roy! The constant given in your URL example was 1056 whereas the constant in your own calc further down says 1215. Which is right?
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Old 13 September 2005, 10:59   #27
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Quote:
Originally Posted by hard1
Hey Roy! The constant given in your URL example was 1056 whereas the constant in your own calc further down says 1215. Which is right?
the prop calc on the website i gave a link to is 1056 which converts to imperial miles per hour and 1215 converts to nautical miles per hour (knots).

1056 being the number of inches in a mile devided by 60
1215 being the number of inches in a nautical mile divided by 60
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Old 13 September 2005, 11:04   #28
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'kin obvious innit?

Er, well not to mathematical Luddites like me.

Thanks.
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Old 13 September 2005, 11:15   #29
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you could also use

656 for kilometers per hour
2362 for meters per second
12 for feet per minute

ill stop now ...
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Old 14 September 2005, 05:14   #30
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Quote:
Originally Posted by Burdick
Wish I could get over 40 knots out of my DF140. 35/36 knots is all I can acheive with 21" ali prop at 5300 revs on a very good day. I accept my boat is bigger and heavier but a chum of mine has an identical boat and set up and he will get over 40 knots out of his. Engine in for a 190 hr service today so will see if that helps.
I agree Burdick, some of the guys on this site do appear to wring a lot more top speed out of their similar rigs than I manage. 41 is tops for me. Let's hope that speed, like size, isn't everything. Not saying that I'm small, Oh no!! Honest.
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