hp vs mph
#1
Platinum Member
Platinum Member
Thread Starter
Join Date: Feb 2001
Location: poughkeepsie, ny, usa
Posts: 1,198
Likes: 0
Received 0 Likes
on
0 Posts
hp vs mph
is there any rough way to calculate the amount of hp it takes to get 10mph more ? does this change when speeds will be over 70mph ? i know there are different variables that come into play . am thinking of pro charging my boat with 502 mpi,(415hp) am running 70mph gps right now and hope to see 80 mph to 80+ mph ,is it possible?
#2
Registered User
Join Date: Feb 2002
Location: Toms River NJ
Posts: 1,422
Likes: 0
Received 0 Likes
on
0 Posts
SuperCharge
The only way to get to those numbers are either build your motor more H/P or Super Charge. You will need at least 600 or better horse. To get that hull over the 80 mph mark,
#3
Registered
physics says that power is a function of Velocity cubed (i.e. to the third power) for viscous fluids (air and water)
so the theoretical answer to your question is.
if 415 hp to go 70mph, then to get 80 (14.3% more speed)
will require (1.143 cubed) or 1.49 x power, or 619.38 HP.
This assumes same prop slip/efficiency. so being realistic, you will probably need a little more that the 619. but 619 is the minimum requirement.
you can use this function in reverse too. if you build an engine up from 415 to 500Hp for example you should expect a speed of approximately ((500/415) cube root) x 70 =
74.48mph
so the theoretical answer to your question is.
if 415 hp to go 70mph, then to get 80 (14.3% more speed)
will require (1.143 cubed) or 1.49 x power, or 619.38 HP.
This assumes same prop slip/efficiency. so being realistic, you will probably need a little more that the 619. but 619 is the minimum requirement.
you can use this function in reverse too. if you build an engine up from 415 to 500Hp for example you should expect a speed of approximately ((500/415) cube root) x 70 =
74.48mph
#4
Registered
Where does torque fit into this equation? Supercharging has dramatic effects on boats because they add a lot of torque, which is needed to push through the water.
You will definately gain more than 4.5 mph!
You will definately gain more than 4.5 mph!
#6
Rough Seas Lie Ahead
Gold Member
Join Date: Feb 2002
Location: Virginia Beach, VA
Posts: 2,465
Likes: 0
Received 0 Likes
on
0 Posts
Screw the math. Old rule of thumb, not considering hull variables, is 15 ponies per mph gained....Anyone ever hear anything different?.....
Rick
Rick
#7
Registered
Join Date: Dec 2000
Location: Austin, Texas
Posts: 6,986
Likes: 0
Received 0 Likes
on
0 Posts
Here are two formulas that are suprisingly accurate.
New Speed = Old Speed X [the squareroot of (New Horsepower / Old Horsepower) ]
or
New Horsepower = Old Horsepower X [ (New Speed / Old Speed) squared]
It takes alot more horsepower for each mph.
Note - These formulas are simular to the ones that Rambunctious stated. However, he uses cubes and I use squares.
Good Luck!
New Speed = Old Speed X [the squareroot of (New Horsepower / Old Horsepower) ]
or
New Horsepower = Old Horsepower X [ (New Speed / Old Speed) squared]
It takes alot more horsepower for each mph.
Note - These formulas are simular to the ones that Rambunctious stated. However, he uses cubes and I use squares.
Good Luck!
Last edited by Clay Washington; 03-04-2002 at 11:27 AM.
#8
Platinum Member
Platinum Member
Clay,
I'll have to agree with your method above. I've also used that formula before and like you said is surprisingly accurate. I think the hyperbolic(square) function is more accurate:
Horsepower is what we're all accustomed to using as a determinant in our speeds, however, in essence what you're really interested in is how many pounds of thrust you can translate that horsepower into.
Maximum speed will be achieved at the precise point when the thrust produced by your engine equals drag(hydrodynamic=hull, outdrive & aerodynamic=hull).
And drag(hydro, aero) is a function of velocity squared.
I'll have to agree with your method above. I've also used that formula before and like you said is surprisingly accurate. I think the hyperbolic(square) function is more accurate:
Horsepower is what we're all accustomed to using as a determinant in our speeds, however, in essence what you're really interested in is how many pounds of thrust you can translate that horsepower into.
Maximum speed will be achieved at the precise point when the thrust produced by your engine equals drag(hydrodynamic=hull, outdrive & aerodynamic=hull).
And drag(hydro, aero) is a function of velocity squared.
#9
Rough Seas Lie Ahead
Gold Member
Join Date: Feb 2002
Location: Virginia Beach, VA
Posts: 2,465
Likes: 0
Received 0 Likes
on
0 Posts
There you go. That's the first explanation I've heard that sort-of takes into account hull variations...I'm jottin' that down dude...
Rick
Rick