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Old 13 March 2006, 23:18   #1
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Free Prop Calculator



There has been a fair amount of discussion about prop selection over the past day or so - many times people have gone off to do some calculations.

There are a few online tools that will do the job but I decided to write a little executable that will do the job for you on your own PC.

It has a couple of improvements over the online ones I've found - mainly in the interface.

You will need to have installed .NET Framework 2.0 before you can run it (this can be got as part of the normal windows update or downloaded from here (you should have this anyway or you cant run any apps writen in microsoft .NET)

Once you have the .NET Framework just download PropCalc.zip from here and unzip it anywhere you like and double click it to run it.

Once you have it running simply fill in the details that you know and click the = sign under the detail that you don't know - you can enter speed in knots, mph or kph. Easy as that.

It will calculate :-

1) SPEED from RPM, GEAR RATIO, PITCH and SLIP%
2) RPM from GEAR RATIO, PITCH, SLIP% and SPEED
3) GEAR RATIO from RPM, PITCH, SLIP% and SPEED
4) PITCH from RPM, GEAR RATIO, SLIP% and SPEED
5) SLIP% from RPM, GEAR RATIO, PITCH and SPEED

If anyone wants any more features adding or needs help using the app post here or drop me a PM.

JK if you want to add this to the downloads section - I'm more than happy for you to do so.
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Old 14 March 2006, 06:28   #2
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I have had one support question allready. So I may aswell publish thew answers here.

If you get an error message popup "Please Check Data" when you press the = button it is because one of the values is not a number or there has been a mathematical error for instance if you have a letter or a space in any of the values or if you set prop pitch to 0 when trying to calculate slip or you set speed to 0 when trying to calculate ratio. etc.

In a few days I will try and replace the meaningless single error message with more meaning full ones that tell you actually what piece of data is wrong.

For now - just use the example data that appears when you start the application as a guide as to valid values.
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Old 14 March 2006, 06:34   #3
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Cool. A suggestion for two wee improvements roycruse, if I may...check the entries in the data fields for validity before allowing the calculation and prop speed is interesting to know.
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Old 14 March 2006, 06:44   #4
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I was planning to do a more thourough check in the next version - it was 4am when I finished and a single "Please Check Data" message was all i was capable of at that hour

When you say prop speed - I presume you mean prop RPM ie engine RPM / Ratio.
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Old 14 March 2006, 06:46   #5
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Quote:
Originally Posted by roycruse
...- it was 4am when I finished and a single "Please Check Data" message was all i was capable of at that hour
I know that feeling.

Quote:
When you say prop speed - I presume you mean prop RPM ie engine RPM / Ratio.
Yup.
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Old 14 March 2006, 09:30   #6
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Version 1.1 is now available.

Re-download the zip file from the original link if you want it.

This new version now tells you what field has the incorrect data instead of the generic message if a calculation cannot be made.

I'm going to add the functionality to remember the values in version 1.2 so it doesn't keep reverting back to my Suzuki DF140 values each time you start it.
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Old 14 March 2006, 11:52   #7
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Version 1.2 now available for download.

PropCalc now remembers the values from your last calculation so when you re-start it after closing it down you don't have to re-enter the constants like max RPM and RATIO that are always the same on your engine.

JW - I started to add prop speed but then stopped as I cant see that it is actually useful for anything other than a step when doing the calculations by hand. No one is going to be able to measure there prop speed so its useless as an input field and i cant find a use for knowing how many RPMs the props doing other than to calculate the forward distance covered (which in this program is done for you). If you can come up with a good reason for needing to know the RPM of the prop ill add it in...
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Old 14 March 2006, 13:35   #8
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Hey Roy

great work

so if i put in my revs being 5750, gearing being 1.75, prop 23, slip at say 15% i get 53knots there about

so when i test on the water i then input the actual max revs achieved and speed produced and it will then tell me the slip? as above is theory at the moment.? that way a different pitch prop can be tried on the boat and slip reassessed and if too high then a grippier prop used

would it be fair to say slip is dependant on the hull shape and form, ie the drag produced by it, or in what way is the hull shape taken into account in the calculations
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Old 14 March 2006, 13:50   #9
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As far as my knowledge extends "SLIP" is the result of the force of drag acting to cause the prop to Spill some of the water its trying to force backwards.

If you take lots of readings at 500 or 1000 rpm increments and different trim angles you will hopefully find that when you get to flat out with the engine trimmed right out and a light load on board your slip will be down into single fingures. It is in that state there is least boat in the water and drag and therefore slip should be lowest.
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Old 14 March 2006, 16:50   #10
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**WANTED**

Smart arsed 9 year old to tell me why it doesn't work

"the application failed to initialise properly (0xc0000135). Click on OK to terminate the application."

Have set it going on an update. (-Windows XP)
Anyone got any other ideas?
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