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I saw a prop pitch,gear%,rpm. formula to figure out mph.on here recently,but cant find It.does anybody have it handy so I can figure mph. on my cruiser!
14pitch
12%v-drive gear
5100 rpms
thanks
 

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20% slip, 60 mph
 

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since you have a v drive I"m sure you spend a 100 dollars everytime you look at the thing. ( I know I do) :). Just spend another 100 on a gps and know for sure how fast you're going.

You'll be suprised how off those calculations really are.
 

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since you have a v drive I"m sure you spend a 100 dollars everytime you look at the thing. ( I know I do) :). Just spend another 100 on a gps and know for sure how fast you're going.

You'll be suprised how off those calculations really are.
Which calculations??
 

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Speed formula

Here's another formula to calculate the mph (with a description of each step);

7100 rpm x 1.18 (v-drive ratio) = 8378 rpm

8378 rpm x 1.25 (prop pitch) = 10,472.5 feet per hour
(A 15 pitch prop screws itself forward 1.25 ft. per minute each revolution)

10,472.5 x 60 (60 minutes in an hour) = 628,350 feet per hour

628,350 divided by 5,280 (feet in a mile) = 119.005 mph

119 mph x .85 *(15% slip factor) = 101.15 mph

* The slip factor obviously varies with every boat, however in my experience with Sanger flats very similar to yours, this slip factor on my Sanger (flat, not a runner) was confirmed by a timeslip at Bakersfield.

Jocko
Last edited by jockorace; 07-23-2009 at 11:03 PM.




Above was my original post regarding the old school 'long version' speed formula.
Applied to your info provided;

5,100 rpm x 1.12 = 5,712 x 1.166 (14 pitch) = 6,660 x60 = 399,600 div. by 5,280 = 75.68 mph

75.68 x.80 (20% slip at this speed) = 60.54 mph

Jocko
 

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I just use RPM X GEAR X PITCH / 1056

5100 X 1.12 X 14 / 1056 = 75.73mph no slip

20% slip would be 60.5mph
 

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these ones
Agreed. Especially when you are entering number you don't actually know.

The excel spreadsheet I use is just the simple calculation Jocko was talking about.

But it does have the variable of the slip. You need to back it up with a gps, you can then figure out your boats "Slip" you can then see how changes effect the efficiency of the boat.
 

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Get a GPS if ya wanna know. PERIOD! Don't waste your time guessing.

No matter how close you dial in your calculations with the slip and everything, its still gonna be off.

My cents is all
 

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Speed formula(s)

Originally Posted by dragboat4ever
since you have a v drive I"m sure you spend a 100 dollars everytime you look at the thing. ( I know I do) :). Just spend another 100 on a gps and know for sure how fast you're going.

You'll be suprised how off those calculations really are.
The calculations / formulas are absolute! The only variable is the slip factor used. As Morg said, 20% is typical at a speed of approx. 60 mph, and the slip factor becomes less the faster you go. However it is not proportional to the speed increase. In other words....if you go 120 mph, that does not mean your slip is only 10%. Every boat/prop is slightly different. I will agree that a drag race time-slip or GPS (also not always 100% accurate!) is best to confirm your actual slip factor. Jocko
 

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I just go by guys I've run in the past, The last time I won a race the guy told Me that he beat a guy that was running 78mph and he beat him pretty good so he figured that he must of been running at least 81-82.... And I beat him pretty good :)so I figure I must be running at least 85-86....Which I was really disappointed because I always thought that all Boats on the River run 100 mph.... :)sphss:D:D
 

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just a ski boat with bark
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Ok. Here's a test:

11x16 prop
22 gears
7100 rpm.

What's the speed?
Well I can't say for sure since you didn't say what type of boat but it should run some where in the range below depending on the actual slip. If this is your TR-2 I'd say 108mph :p

RPM Gear Pitch MPH(T) MPH(P) Slip
7100 1.22 16 131.1 104.9 20.0%
7100 1.22 16 131.1 106.2 19.0%
7100 1.22 16 131.1 107.5 18.0%
7100 1.22 16 131.1 108.8 17.0%
7100 1.22 16 131.1 110.1 16.0%
7100 1.22 16 131.1 111.4 15.0%
 

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Mr. Potato Head
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How about
Cole TR-2
7500 rpm
11 by 16 RB prop
37 gears
How fast do you think, I did the calc. But can`t imagine I`m going that fast. So I want to see what you guys think.THS BOAT
 

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Ran my 78 Tr2 the on friday. Marked off 1/4 mile(used GPS) + 125ft for rope to clock. Turned 7450rpm/ 29% gear/ 11 x 16 menkens. 123 mph(also by GPS) Is there a way to estimate E.T. with this info? Have race in two weeks..KDBA. Will be able to confirm results. If not at least you have another rpm/gear deal to go by.:D
 

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just a ski boat with bark
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How about
Cole TR-2
7500 rpm
11 by 16 RB prop
37 gears
How fast do you think, I did the calc. But imagine going that fast. So I want to see what you guys think.THS BOAT
Depending if your still accelerating or leved off I'd guess about 127-128mph depending on slip (I guessed 18%).
 

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just a ski boat with bark
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Ran my 78 Tr2 the on friday. Marked off 1/4 mile(used GPS) + 125ft for rope to clock. Turned 7450rpm/ 29% gear/ 11 x 16 menkens. 123 mph(also by GPS) Is there a way to estimate E.T. with this info? Have race in two weeks..KDBA. Will be able to confirm results. If not at least you have another rpm/gear deal to go by.:D
I have some formulas I use to claculate with but there's so much varience in set ups, driving styles, props and gearing it doesn't work well unless I have a baseline number for the particular boat and driver. My calculations show [email protected] but aren't you running NA? seems like the gear is a little high and the rpm is a little low for an NA piece to be really quick. I would think it would be plenty quick enough for TE though. Once you get some numbers I could rerun the calculations with your numbers.

BTW Good Luck, beautiful boat you built there! :)devil:)devil
 
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