
07292005, 05:51 PM

#1

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Thread Starter
Join Date: Jun 2005
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Posts: 11

HP vs MPH?
I read somewhere on here that to get 1 MPH increase it takes an increase of 5 or 10 HP. I was reading some of the literature on the boat I'm preparing to purchase (Four Winns 225) and I found some interesting performance graphs. The graphs listed both the Volvo Penta 5.7 (280 HP) engine and the 5.7 (320 HP) engine. They also listed performance parameters for those engines with both the monoprop and the duoprop drives  so four graphs total.
The difference between the 280 HP engine with the monoprop drive and the 320 HP engine with the duoprop drive was only 1 (one) mph on the top end.
Now, here are my questions:
 if 510 hp is supposed to increase mph by 1 mph, why is there only 1mph increase with an increase in HP by 40? For example, the top speed graphed is at WOT and 51 MPH on the 280 where the top WOT speed for the 320 is 52 mph( to push this further, on the Mercruiser 6.2L top WOT speed is 53 mph). What's up with that?
 The performance (as far as fuel flow and speed for a given RPM) is the same for a monoprop as a duoprop. I thought you were supposed to get better performance from a duoprop? The graphs don't show it.
Does anyone have a good reason for getting the top end engine & duoprop if you're only gaining one mph?



07292005, 06:11 PM

#2

Senior Member
Join Date: Jun 2003
Location: Wellington , Fl
Posts: 1,581

Re: HP vs MPH?
Remember its not all speed with the horsepower ,, that elusive Torque also is a factor,,Big Blocks today give us some impressive torque curves , and with a 4 stroke that helps hole shot ability.



07292005, 06:49 PM

#3

Senior Member
Join Date: Apr 2005
Location: james island SC
Posts: 511

Re: HP vs MPH?
I have never heard it put as x amount of horse power will give you Y amout of speed increase.
its more a percentage, like a 50% increase in HP will give you a 25% increase in speed. that may not be exact, but you get the idea. and keep in mind, the faster you go, the farther aprt the numbers get.



07292005, 07:33 PM

#4

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Join Date: Aug 2002
Location: Greensboro, NC USA
Posts: 1,005

Re: HP vs MPH?
here is no equation for HP vs Speed. If you bolt a 200 hp outboard to the queen Mary it will not move it. each individual boat loaded with X amount of weight etc etc has a unique relationship of HP to speed



07292005, 07:57 PM

#5

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Join Date: Jan 2002
Location: Foley, Alabama
Posts: 2,174

Re: HP vs MPH?
there is formula out there called "Crouches Planing Speed" formula:
It is not an exact mathematical equation, but it serves as a good approximation.
Speed (knots) = C / {sqrt(weight/hp)}
C is drag coefficient and is 170 for most planing boats
weight is TRUE weight including any fuel, water, people and gear on board.
hp is prop/shaft hp.
formula assumes a certain prop efficiency (55%?)!
Play around with it. I have found it to be fairly accurate. The only difference will be on boats that have a higher hp to weight ratio the C tends to go up towards 190, and on heavier wider beam boats with lower hp that the C will drop towards 150. remember that the speed is knots so to convert to mph multiply by 1.15.



07292005, 09:47 PM

#6

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Join Date: Feb 2003
Location: Hammond, LA
Posts: 17,387

Re: HP vs MPH?
Quote:
modunlavy  7/29/2005 6:57 PM
Speed (knots) = C / {sqrt(weight/hp)}
C is drag coefficient and is 170 for most planing boats

dang that's right on for my boat
3200 lbs, 140 hp, 41 mph...



07292005, 09:59 PM

#7

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Join Date: Apr 2005
Location: james island SC
Posts: 511

Re: HP vs MPH?
I think prop efficiency is more like 90+ percent
there use to be a link to a prop calculator, you could insert variables like pitch, rpms, or speed. with all but one of the variables, it would figure the one left off
anyone know what I am talking about
I have a 3 blade stainless prop 19 pitch, turning 5500 rpms, and I run GPS speed just under 55 MPH
its on a 21 foot 1990 vhull wellcraft cc



07292005, 10:39 PM

#8

Join Date: May 2003
Location: New Braunfels, Tx
Posts: 8,764

Re: HP vs MPH?
Modunlavy, using your formula I am actually doing better than expected..
My figures came out to about 35 MPH, but I actually have a max of about ~4143 which seems to me to be a significant difference.
So am I correct in assuming that my 4 blade prop is more efficient for my boat and/or my hull (tunnelVee) has less drag than a full deep vee?



07302005, 12:21 AM

#9

Senior Member
Join Date: Jul 2003
Location: Beaufort, SC
Posts: 212

Re: HP vs MPH?
Reelwork,
Check and make sure you have converted the KNOTS to MPH. I ran the estimated numbers for your Sea Fox and came up with 35.91 KTS or 41.35 MPH
Tried the formula for my boat and I got 36.8 kts or 42.32 mph and my boat does almost exactly that speed.
Thanks modunlavy, interesting formula.
Mark



07302005, 12:29 AM

#10

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Join Date: Nov 2004
Location: Galveston, Texas
Posts: 1,190

Re: HP vs MPH?



07302005, 08:42 AM

#11

Join Date: May 2003
Location: New Braunfels, Tx
Posts: 8,764

Re: HP vs MPH?
Thanks MArk  and yes, the knots to MPH conversion... I kind of skipped that step!
Based on that conversion, no lie  that is within 1 MPH of actual!



07302005, 11:24 AM

#12

Senior Member
Join Date: Apr 2005
Location: james island SC
Posts: 511

Re: HP vs MPH?
using the prop calculator, my slip is .036
does this sound reasonable?
the motor is a 2002 225 optimax
5500 rpm 1.75 GR 19 pitch prop speed is gps 54.5
that makes a prop slip of .036
that 3.6 percent right? is that possible?



07302005, 11:50 AM

#13

Senior Member
Join Date: Apr 2003
Location: Newport, RI, USA
Posts: 350

Re: HP vs MPH?
Chris V,
I run the same setup, motor, prop and ratio with a TS of 50 MPH and I get a slip of .115.
That 4.5 MPH makes a pretty big difference.
JK



07302005, 01:46 PM

#14

Join Date: May 2003
Location: New Braunfels, Tx
Posts: 8,764

Re: HP vs MPH?
Using the prop calculator rotor posted, I got a slip of .0755 which would translate to 7.5% right?
That's pretty good isn't it?



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