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i got skiillllzzzz....
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884 Posts
Discussion Starter · #1 ·
was just wondering .say you have 18's in the v-drive with a stelling 2 blade prop 11 1/2- 15.,motor spins 7100 rpm.what mph should i be running.have not gps'ed my boat when i have ran w.o.t
any info would be cool.
 

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Short and stout
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210 Posts
Quote:
Originally Posted by Carnivalride
Not sure where your math is coming from but I get 159.6 with no slip using the formula I have.

eng rpm x gear ratio / (1056/prop pitch) = mph
6700 x 1.48 / (1056/17) =
9916/62.12 = 159.6


7600 x 1.48 / 1056 / 16

11100 / 66 = 168.18

168.18 x .15 (slip)

143 mph, just what we ran for a 7.85 @ 149. Math is fun

with this I came up with 74.47 mph. Don't know if it is right or not?
 

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i got skiillllzzzz....
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884 Posts
Discussion Starter · #3 ·
mph??

the gear that i was told that was in it might not be 18's.god i hope your math is off because if this boat only does 74 mph damn i should have kept the jet"yeah right" it ran 98mph.i know alot of people at cfw fathersday weekend would say it runs faster than 74mph.i guess i have to find out what gears are in it then i can find out.
 

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1,800 Posts
Motor RPM 7100
Gear Drive 18%
Prop. Degree 15.0
Prop. Slip. 16%



MPH 99.96

The above is pasted from an excell script I wrote. Send me a pm with you e-mail & I can forward it. After you have gps'd your deal you can figure out your prop. slip..
 

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Premium Member
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3,356 Posts
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
 

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i got skiillllzzzz....
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884 Posts
Discussion Starter · #14 ·
i think i need to use somebodys lifeline and take it out to gps it.i know somebody that has a water radar gun.i will see if i can get a speed number.no matter what it runs right now i am having fun with it.i can always ad more power when i get a job.:)Unsure
 

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Premium Member
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3,356 Posts
You are definitely correct about the Lifeline jacket! I made hundreds of 100+mph runs in front of Will-O-Point with either no jacket or just a ski jacket in my earlier "I'm indestructable" days. What a fool I was. Now I never run (at speed) without full race gear.
I look forward to meeting you next summer after I move back to California (Lakeport). Dave
 

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359 Posts
Motor RPM 7100
Gear Drive 18%
Prop. Degree 15.0
Prop. Slip. 16%



MPH 99.96

The above is pasted from an excell script I wrote. Send me a pm with you e-mail & I can forward it. After you have gps'd your deal you can figure out your prop. slip..
New to this also, whats prop slip?
 

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V-Drive
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283 Posts
it is the amount of slip that the prop losses trying to push the boat forward on each revolution . It varies between every boat and the prop combination.

I use a hand held GPS unit to get the exact slip ..

My flat is right at 17-18 % slip with the prop that I have on it now .
I would like for it to be about 15% for my boat .

I ran it on my spreadsheet
7100 rpm / 15 pitch / 18 gears/ 15 %slip : I came up with 101.13
 

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247 Posts
Speed

Fast 76 what is in the motor (heads, cam, carbs, make of T-Ram etc.).

My last Runner bottom Hondo with my first motor had a 468 11 1/2 - 1 comp. with Dart pro 1 heads a Dart tunnell ram 2 Braswell 750 Holleys. and a 700"lift solid roller cam. And @ Ming it ran mid 9"s @ 108 with a 15 gear and an 11-16 Menkens prop. The motor would turn 7800 -8000 rpm.

In all honesty it takes alot of motor to hit 100 mph in a true heavier flat in my opinion. I hope you dont take this the wrong way!!;)
 
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