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Old 23-02-2008, 10:23 PM   #1
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could anyone help me please

im trying to work out how to turn 57rpm in mph on what scale is it done i got a yam 130 on a phantom 19 in which im starting ocr this season hopfully
thanks wayne
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Old 23-02-2008, 10:38 PM   #2
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Hi Wayne, nice to see you will be joining us in the fleet. Who are you racing with? I will get Mark to answer your question. See you soon mate.
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Old 24-02-2008, 02:53 PM   #3
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Quote:
Originally Posted by badbouy View Post
im trying to work out how to turn 57rpm in mph on what scale is it done i got a yam 130 on a phantom 19 in which im starting ocr this season hopfully
thanks wayne
57mph near as damn it !! and a speedo wont help !!
and its 5700 rpm . i can email you the prop calc spreadsheet if you want.

Bob
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Old 24-02-2008, 07:27 PM   #4
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Type "prop calc" into Google and take your pick of web based programs - Bob will tell you what your gearbox ratio is if you don't know.
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Old 24-02-2008, 10:15 PM   #5
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Testing

Quote:
Originally Posted by Cookee View Post
Type "prop calc" into Google and take your pick of web based programs - Bob will tell you what your gearbox ratio is if you don't know.
Its 2:1 and with the yam 24 overhub you are running,
revs and speed are almost equal as bob says, A speedo is helpful to help you get used to where the boats fastest trim is for acceleration and top end as over trimming can make it seem faster when it actually makes it slower.

You need a trim gauge aswell

Also the revs/speed equation is not an exact science, Different props,ride height and conditions all cause variables.

Make a log of what you do so you can look back to see what actually improves as well as disproves!!

Most of all TEST and lots of it, I have racked up over 10 hours in the new cat in 5 weeks and still learning hugely
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Old 24-02-2008, 11:29 PM   #6
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Out of interest I just used this site;
http://www.go-fast.com/Prop_Slip_Calculator.htm

Using an Argo Cat with a 90 hp yam, running a 29" pitch prop we managed 83 mph (one way) on Windermere (timed not GPS).

The site gave me 77 mph with zero slip, so I guess not an exact science.
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Old 24-02-2008, 11:52 PM   #7
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Out of interest I just used this site;
http://www.go-fast.com/Prop_Slip_Calculator.htm

Using an Argo Cat with a 90 hp yam, running a 29" pitch prop we managed 83 mph (one way) on Windermere (timed not GPS).

The site gave me 77 mph with zero slip, so I guess not an exact science.
Don't forget that props have a variable pitch, so it's that part that is not exact - especially if it's been worked or had its diameter reduced.
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Old 25-02-2008, 12:50 AM   #8
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Don't forget that props have a variable pitch, so it's that part that is not exact - especially if it's been worked or had its diameter reduced.

I speak from experience with a P19 and yam 24 they are pretty in line..Strange thing is today i ran 7600RPM @76MPH must be a strange coincidence. mind you that's with a Standard 23P old style pre laserII Quicksilver out of the box, cast with dinosours around...

Every boat/prop/set up varies

Trying bunny eared cleaver tues dawn!....excited
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Old 25-02-2008, 03:07 AM   #9
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I speak from experience with a P19 and yam 24 they are pretty in line..Strange thing is today i ran 7600RPM @76MPH must be a strange coincidence. mind you that's with a Standard 23P old style pre laserII Quicksilver out of the box, cast with dinosours around...

Every boat/prop/set up varies

Trying bunny eared cleaver tues dawn!....excited
Surely at 7600RPM you are well out of the "powerband" ?
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Old 25-02-2008, 03:18 AM   #10
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We managed in my 21' Campbell Cat in 1988 at Windermere with one of the first XR2's - 75.8 mph (I know that because the certificate is hanging up on my wall in the office here!!)

That was running running a very worked on QSS 26"
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Old 25-02-2008, 03:26 AM   #11
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As BluFin on this forum has said many times average race speeds have not dramatically increased over the last 25 years, because Hotboats and Argos go quick now, is no different than Robert Cook doing over the ton in 1980 in a 3D boat.
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Old 25-02-2008, 09:31 AM   #12
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Strange thing is today i ran 7600RPM @76MPH must be a strange coincidence.

Trying bunny eared cleaver tues dawn!....excited
I wouldn't spend to long at those RPMs with an XR2 - get a bigger prop!
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Old 25-02-2008, 10:07 AM   #13
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Originally Posted by ciao! View Post
Out of interest I just used this site;
http://www.go-fast.com/Prop_Slip_Calculator.htm

Using an Argo Cat with a 90 hp yam, running a 29" pitch prop we managed 83 mph (one way) on Windermere (timed not GPS).

The site gave me 77 mph with zero slip, so I guess not an exact science.
I guess that your Tach showed wrong !!! You must have turned more than 5600 rpm more like 6400rpm and 6% slip (usually the the slip on a Argo flat out) to reach 83mph with a 29 !
I guess that Harri F was pleased with that speed !
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Old 25-02-2008, 10:09 AM   #14
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Don't forget that props have a variable pitch, so it's that part that is not exact - especially if it's been worked or had its diameter reduced.
What has the diameter to do with his actual speed ?
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Old 25-02-2008, 11:12 AM   #15
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Don't worry cookee i realize it was overrevving!

Was just one quick squirt, That was with a standard 23. The next test is with the 26 Bunny. Also the last test was the first outing having gone from 1:87 to 2:1 dramatically increasing the revs

Thanks for concern anyways
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Old 25-02-2008, 07:28 PM   #16
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What has the diameter to do with his actual speed ?
Reducing the diameter will increase the RPM therefore changing the ratio of RPM/Speed/slip
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Old 26-02-2008, 01:06 AM   #17
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How do you calculate slip?
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Old 26-02-2008, 10:16 AM   #18
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Quote:
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How do you calculate slip?
It's a bit of a chicken and egg thing really - if you put pitch, gear ratio, and speed into a prop calc programme then it will give you slip - the trouble is that even factory pitch measurements are only an average of pitch over the whole blade, and once you reduce the diameter a bit or tweak it slightly it will be some other pitch entirely, and at higher speeds it could make quite a difference to the slip.

http://www.go-fast.com/Prop_Slip_Calculator.htm
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