hp vs mph
03-04-2002, 12:19 PM
#12
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Clay, I think We're in vilolent agreement 
thrust ( or force) IS a function of velocity squared. Power, by definition IS force x Velocity. therefore HP (power) is a function of velocity cubed.
1 horse can lift 550 lbs at 1 ft/sec. that same horse can pull 1100 lbs at .5 ft/sec = both are one horsepower. the force is doubled in one case, while it's velocity is halved.
Torque x rpm is the equivelent to force x velocity. 600 ft-lbs of torque at 3000 rpm is 1/2 the power as the same 600 ft-lbs turning at 6000 rpm.
through our boating experience we have come up with "emperical calculations " that approximate these physical equations and in many cases are more accurate because they take into acount efficiencies and such that we choose to neglect in the physics calcs. things like prop effeciency chances, trim angle, aero lift in cats, etc.
ie 15 hp per mph. for an example. but this estimate applies only around say the 400--500 hp range, or 60 mph range. At 70-80mph it is a difference story because the drag is not linear.
(61mph/60mph) cubed = 1.05
1.05x 300 = 315 !!! there's the 15hp needed.
(81mph/80mph) cubed = 1.037
1.037 x 600 = 622.
22 hp more from 600 to get 1 mph
any of the calc mentioned in this string will give you an estimate to stat with. I'm an engineer so I like to try to take the physics approach to understanding this. hopefully saving my money by reducing or eliminating trial and error. One thing that is evident here . It takes a lot of HP to gain a little speed.
Where's Tomcat. He always has insight
thrust ( or force) IS a function of velocity squared. Power, by definition IS force x Velocity. therefore HP (power) is a function of velocity cubed.
1 horse can lift 550 lbs at 1 ft/sec. that same horse can pull 1100 lbs at .5 ft/sec = both are one horsepower. the force is doubled in one case, while it's velocity is halved.
Torque x rpm is the equivelent to force x velocity. 600 ft-lbs of torque at 3000 rpm is 1/2 the power as the same 600 ft-lbs turning at 6000 rpm.
through our boating experience we have come up with "emperical calculations " that approximate these physical equations and in many cases are more accurate because they take into acount efficiencies and such that we choose to neglect in the physics calcs. things like prop effeciency chances, trim angle, aero lift in cats, etc.
ie 15 hp per mph. for an example. but this estimate applies only around say the 400--500 hp range, or 60 mph range. At 70-80mph it is a difference story because the drag is not linear.
(61mph/60mph) cubed = 1.05
1.05x 300 = 315 !!! there's the 15hp needed.
(81mph/80mph) cubed = 1.037
1.037 x 600 = 622.
22 hp more from 600 to get 1 mph
any of the calc mentioned in this string will give you an estimate to stat with. I'm an engineer so I like to try to take the physics approach to understanding this. hopefully saving my money by reducing or eliminating trial and error. One thing that is evident here . It takes a lot of HP to gain a little speed.
Where's Tomcat. He always has insight
03-04-2002, 12:35 PM
#13
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Yes Rambunctious. We are in close agreement. However, my calculator does not have a cubic root, only a square root. So, I use squares!
Your math is correct. But, boats never understood math!
It is all a close guess. As you say, a starting point.
There is also another formula: It take $1000 for each 1 mph on the water. How fat is your wallet?
20 degress last night in Dallas, TX. Will winter ever be over?
Your math is correct. But, boats never understood math!
It is all a close guess. As you say, a starting point.
There is also another formula: It take $1000 for each 1 mph on the water. How fat is your wallet?
20 degress last night in Dallas, TX. Will winter ever be over?
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03-04-2002, 12:49 PM
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thanks to sharky
http://www.boatramp.com/prop_applet/...erAppletG.html
hey clay, you can always consult the base 10 log tables for roots!!!
hey clay, you can always consult the base 10 log tables for roots!!!
03-04-2002, 01:38 PM
#16
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Hey 26Sonic - Asking the speed question; and everybody's answers are right in one way or the other. There are so many variables that empirical equations based on many, many tests by Mercury and others are the easiest way to go. If you want a true calculation based on physics, you better save up and buy Hydrocomp software for thousands of dollars. No offense Rambunctious; I have to figure it out by first principles too, or I don't really understand what's going on.
As many of you know I wrote a program based on empirical equations, that not only predicts speed, but optimizes the prop choice to go with that speed and the actual dyno curve for the engine. When you calibrate with your own test data, the program will create the hull coefficent and slip for your current hull/drive/prop and makes pretty good predictions. The only thing I don't like is that the empirical equations are based on tests where only estimates of prop HP were known, not the actual thrust of the prop/drag of the hull. That means the program does not incorporate prop efficiency curves or drive heights and becomes less accurate when you deliberately put the wrong pitch into the program. I'm working on it. Check out my propeller efficiency thread and the Bravo vs. Hydromotive thread for more info on that.
I'm also working on getting this program into a form that can run on a website. It looks like a java calculator with interactive graph is the best way to go, just costs more money than I was planning to spend on a software "toy".
Anyway, down to business. Based on 26Sonic's test data and a few assumptions, I ran his boat with a 502 MPI and then supercharged it. The results are shown in the attached graph. With a true 700 HP kit, he will reach about 87 mph and spin a 30" pitch. I don't know what prop you are using now, but if it's a three blade and blows out a bit now, you will need four blades to handle the power. If you can give me data on weight, prop, drive ratio, max RPM and expected HP, I can finetune this number a bit.
As many of you know I wrote a program based on empirical equations, that not only predicts speed, but optimizes the prop choice to go with that speed and the actual dyno curve for the engine. When you calibrate with your own test data, the program will create the hull coefficent and slip for your current hull/drive/prop and makes pretty good predictions. The only thing I don't like is that the empirical equations are based on tests where only estimates of prop HP were known, not the actual thrust of the prop/drag of the hull. That means the program does not incorporate prop efficiency curves or drive heights and becomes less accurate when you deliberately put the wrong pitch into the program. I'm working on it. Check out my propeller efficiency thread and the Bravo vs. Hydromotive thread for more info on that.
I'm also working on getting this program into a form that can run on a website. It looks like a java calculator with interactive graph is the best way to go, just costs more money than I was planning to spend on a software "toy".
Anyway, down to business. Based on 26Sonic's test data and a few assumptions, I ran his boat with a 502 MPI and then supercharged it. The results are shown in the attached graph. With a true 700 HP kit, he will reach about 87 mph and spin a 30" pitch. I don't know what prop you are using now, but if it's a three blade and blows out a bit now, you will need four blades to handle the power. If you can give me data on weight, prop, drive ratio, max RPM and expected HP, I can finetune this number a bit.
Last edited by tomcat; 03-04-2002 at 01:43 PM.
03-04-2002, 02:30 PM
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I have a nice, little program that will give you an idea of the prop pitch that you will need for any given set of data...
WinProp
You can either run it in place, or save it to your hard drive.
The biggest "unknown" is prop slip. However, if you plug in your current numbers, you can calculate your current prop slip. Now that you know your approximate prop slip, you can determine the proper prop pitch (and gear ratio) for many rpm and speed combinations.
Of course, this all assumes that you have the horsepower to turn the prop at the rpms that you specify. For that, please refer to my previous post!
WinProp
You can either run it in place, or save it to your hard drive.
The biggest "unknown" is prop slip. However, if you plug in your current numbers, you can calculate your current prop slip. Now that you know your approximate prop slip, you can determine the proper prop pitch (and gear ratio) for many rpm and speed combinations.
Of course, this all assumes that you have the horsepower to turn the prop at the rpms that you specify. For that, please refer to my previous post!
03-04-2002, 03:06 PM
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03-04-2002, 04:16 PM
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Wow Marty! That sounds awesome!
So, how's it gonna work...Will it be computational hp or measured. I guess you only really need torque readings to derive hp. Is this going to be a strain guage set-up on the shafts to measure torsional displacement?
So, how's it gonna work...Will it be computational hp or measured. I guess you only really need torque readings to derive hp. Is this going to be a strain guage set-up on the shafts to measure torsional displacement?
