[Question] Formula
+ Reply to Thread
Results 1 to 10 of 10

Thread:
Formula

  1. #1
    Member
    Join Date
    Jan 2010
    Posts
    41

    Default Formula

    Can anyone provide me with the formula for calculating MPH using RPM, gear ratio, and prop pitch? I use to know this but have forgotten it over the years.

    Thanks
    Scott

  2. Remove Advertisements
    PerformanceBoats.com
    Advertisements
     

  3. #2
    Senior Member ol guy's Avatar
    Join Date
    Mar 2008
    Location
    Lake Elsinore Ca
    Posts
    3,837

    Default

    Quote Originally Posted by 712Catman View Post
    Can anyone provide me with the formula for calculating MPH using RPM, gear ratio, and prop pitch? I use to know this but have forgotten it over the years.

    Thanks
    Scott
    I am in the same boat! Still want the formula.Also regarding slip of the prop. I always went with a 10% slip. M

  4. #3
    Senior Member Sangerboy's Avatar
    Join Date
    Dec 2007
    Location
    Moorpark and Loma Linda CA
    Posts
    662

    Default

    I do not remember the quicker formula but this will get you there

    rpm X gear ratio (if 15% overdriven use 1.15 if 15% underdriven use 0.85) to get prop revolutions/minute

    Multiply that number by 60 to get prop revolutions/hour

    Multiply that number by the prop pitch to get theoretical inches/hour

    divide that number by 63,360 to get theoretical miles/hour

    multiply by 0.8-0.85 to account for a more realistic slip ratio on a flatbottom

    so the numbers on a boat with 15's at 5000 rpm and a 16" pitch prop would look like this

    5000 X 1.15= 5750

    5750 X 60= 345,000

    345,000 X 16=5,520,000

    5,520,000/63,360=87.12

    87.12 X 0.85(15% slip)=74.05 mph which is probably right in the ballpark for a true flat with those specs.

    Runner bottoms would likely be a little more efficient and hydros even a little more with less slip. V-bottoms would be a bit less efficient and have more slip.
    http://www.performanceboats.com//signaturepics/sigpic415_1.gif

  5. Remove Advertisements
    PerformanceBoats.com
    Advertisements
     

  6. #4
    Distinguished Member David 519's Avatar
    Join Date
    Jan 2008
    Location
    NorCal
    Posts
    5,766

    Default

    Try:
    RPM x Pitch x OD x 60 / 5280= MPH with no slip.

    Pitch is in feet (16" pitch is 1.33333')
    OD is overdrive in box; 25% over is 1.25
    60 converts minutes to hours,
    divided by feet in a mile...

    My Canyon:
    16 pitch prop, 29% gears, 7500 RPM

    7500 RPM x 1.3333 x 1.29 x 60 / 5280= 146.6 MPH

    That combo typically nets me about 128 MPH or about 13% slip. My Canyon is pretty efficient, meaning it cleans up pretty well. Rides on the center pad and the chines dry. I've played with the numbers of some pretty fast flats and they're in that ball park. I'd guess realistic slip numbers for a hot lake boat (flat) in the 15%-20% range but YMMV.. Also, I've never messed with numbers for hydros, wackers or I/Os but the math is pretty much the same...
    A couple other factors. Drag increases at the square of the speed. Power to overcome that drag goes up at roughly the cube of the speed (for the engineers out there, yes there's some simplification here, but it's close enough for the girls we go with ) Again and as always, YMMV....
    Quote Originally Posted by gn7 View Post
    ....... David 519 is 100% correct........

    Quote Originally Posted by fuelinmyveins82 View Post
    .....I think people forget that racing is supposed to fun. Losing shouldn't be discouraging it should motivate you work on your pile to make it faster.....

  7. #5
    Distinguished Member David 519's Avatar
    Join Date
    Jan 2008
    Location
    NorCal
    Posts
    5,766

    Default

    Sangerboy tree'ed me
    Quote Originally Posted by gn7 View Post
    ....... David 519 is 100% correct........

    Quote Originally Posted by fuelinmyveins82 View Post
    .....I think people forget that racing is supposed to fun. Losing shouldn't be discouraging it should motivate you work on your pile to make it faster.....

  8. #6
    Senior Member Sangerboy's Avatar
    Join Date
    Dec 2007
    Location
    Moorpark and Loma Linda CA
    Posts
    662

    Default

    Quote Originally Posted by David 519 View Post
    Sangerboy tree'ed me


    I just ran the numbers for my Canyon and I was at 127 through the lights at 8700 rpm with 12's and a 16" pitch prop. Very interesting that that calculates with 13% slip also just like yours
    http://www.performanceboats.com//signaturepics/sigpic415_1.gif

  9. #7
    Member
    Join Date
    Jan 2010
    Posts
    41

    Default

    Thanks Guys.........great info. That's not exactly the numbers I remember using, but I'm sure they're very close. I'm running a 21' Tunnel, I have a Gaffrig GPS speedometer in my boat so I can do some comparison this summer during gear and prop testing. Once again, thanks for your input.

    Scott

  10. #8
    Senior Member Sangerboy's Avatar
    Join Date
    Dec 2007
    Location
    Moorpark and Loma Linda CA
    Posts
    662

    Default

    With a tunnel when your at top speed and packing air, 10% slip is possible if the set up is good
    http://www.performanceboats.com//signaturepics/sigpic415_1.gif

  11. #9
    Village Idiot fc-Pilot's Avatar
    Join Date
    Jan 2008
    Location
    Peoria, AZ
    Posts
    3,349

    Default

    And every prop and combo are different. We have one prop that has more slip than the rest, and yet it is the fastest. As far as the formula goes, these guys NAILED IT! But yes, the slip is a guess until you have a time slip or GPS numbers to verify it. At least it can get you in the ball park.

    Paul

  12. #10
    Junior Member
    Join Date
    May 2017
    Posts
    1

    Default Slippage Conversion Chart

    Quote Originally Posted by 712Catman View Post
    Can anyone provide me with the formula for calculating MPH using RPM, gear ratio, and prop pitch? I use to know this but have forgotten it over the years.

    Thanks
    Scott
    How about the easy way since they all like it the hard way, https://bblades.com/propeller-slip-calculator/, I can also give you weight variances and Deep V/Cat conversions as well, let me know what you need, https://www.facebook.com/drscottazink/?modal=admin_todo_tour.

    Scott

+ Reply to Thread

Quick Reply Quick Reply

Register Now

Please enter the name by which you would like to log-in and be known on this site.

Please enter a password for your user account. Note that passwords are case-sensitive.

Please enter a valid email address for yourself.

Please select your insurance company (Optional)

Log-in

Tags for this Thread

Digg This Thread

Posting Permissions

  • You may post new threads
  • You may post replies
  • You may not post attachments
  • You may not edit your posts
  •  
 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95