Like Tree0Likes

hp vs mph

Reply
Old 03-04-2002, 04:30 AM
  #1
Platinum Member
Platinum Member
Thread Starter
 
26sonic's Avatar
 
Join Date: Feb 2001
Location: poughkeepsie, ny, usa
My Boats: boats, cars
Posts: 1,187
Default 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?
26sonic is offline  
Reply With Quote
Old 03-04-2002, 07:00 AM
  #2
Registered User
 
Shanghied Again's Avatar
 
Join Date: Feb 2002
Location: Toms River NJ
My Boats: Shanghied! 2004 Donzi 33 ZX
Posts: 1,422
Default 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,
Shanghied Again is offline  
Reply With Quote
Old 03-04-2002, 08:50 AM
  #3
Registered
 
Rambunctious's Avatar
 
Join Date: Jul 2001
Location: Holland, Mich
My Boats: 95 Formula 336 SR1 525SC's
Posts: 1,046
Default

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
Rambunctious is offline  
Reply With Quote
Old 03-04-2002, 09:38 AM
  #4
Registered
 
Pure Energy's Avatar
 
Join Date: Oct 2000
Location: N. NJ, Eastern LI
My Boats: 2001 Superboat Y2K
Posts: 3,027
Default

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!
Pure Energy is offline  
Reply With Quote
Old 03-04-2002, 09:57 AM
  #5
Registered
 
Join Date: Jul 2001
Location: oshawa ontario
My Boats: 31 Scarab Thunder
Posts: 4,699
Default

Where's Tomcat? he's the man to ask.
pullmytrigger is offline  
Reply With Quote
Old 03-04-2002, 10:11 AM
  #6
Rough Seas Lie Ahead
Gold Member
 
Reckless32's Avatar
 
Join Date: Feb 2002
Location: Virginia Beach, VA
Posts: 2,465
Default

Screw the math. Old rule of thumb, not considering hull variables, is 15 ponies per mph gained....Anyone ever hear anything different?.....

Rick
Reckless32 is offline  
Reply With Quote
Old 03-04-2002, 12:25 PM
  #7
Registered
 
Clay Washington's Avatar
 
Join Date: Dec 2000
Location: Austin, Texas
My Boats: 1990 Cigarette Bullet
Posts: 6,986
Cool

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!

Last edited by Clay Washington; 03-04-2002 at 12:27 PM.
Clay Washington is offline  
Reply With Quote
Old 03-04-2002, 12:46 PM
  #8
Platinum Member
Platinum Member
 
CigDaze's Avatar
 
Join Date: Jun 2001
Location: St. Petersburg, FL
My Boats: Cigarette 35 Cafe Racer
Posts: 21,346
Default

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.
CigDaze is offline  
Reply With Quote
Old 03-04-2002, 12:55 PM
  #9
Rough Seas Lie Ahead
Gold Member
 
Reckless32's Avatar
 
Join Date: Feb 2002
Location: Virginia Beach, VA
Posts: 2,465
Default

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
Reckless32 is offline  
Reply With Quote
Old 03-04-2002, 01:12 PM
  #10
Enjoy the show
VIP Member
 
Ron P's Avatar
 
Join Date: Feb 2001
Location: The Joisey Shore
Posts: 10,353
Default

Just stick a blower on it and see what happens. Remember you will need a bigger prop too. Go to 650 HP so you get over your 80 MPH. mark.
Ron P is offline  
Reply With Quote
Reply

Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
gto
General Q & A
3
09-12-2008 11:06 AM
sakoutis3
General Boating Discussion
14
02-26-2007 08:25 PM
aw61801
General Boating Discussion
21
05-22-2006 08:42 PM
jeffg
General Boating Discussion
2
05-16-2003 10:26 PM
BILL1134
General Q & A
9
06-11-2002 10:58 AM



Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 03:57 AM.


Copyright 2011 OffShoreOnly. All rights reserved.