HP and Torqe
HP and Torqe
05-27-2002, 08:46 PM
#2
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Torque is a measure of "twist force". Max torque on a motor is usually its most "efficient" rpm.
Getting best speed or selecting the correct prop for performance usually is achieved at the engine's most powerful rpm.
The reason for this is that POWER is a measure of Torque x RPM.
Torque by itself is irrelevent to speed. More TORQUE can exert more FORCE. More FORCE does not mean more speed.
POWER is force over time.
To transfer energy from a motor (heat) to water (thrust), you must do so via a screw (propellor/impellor). Turning a given screw of a given pitch at a certain speed is what does the trick.
At a motor's TORQUE peak, it is invariably at a given RPM, let's say 3000rpm. Taking this torque, and transferring it at 3000rpm into a gearset of 1.50:1 and into a prop of a given pitch, you get the resultant POWER transferred into THRUST.
At a motor's POWER (horsepower) peak, then a "smaller" measure of torque is transferred at a greater rpm through the same route to a prop.
Getting best speed or selecting the correct prop for performance usually is achieved at the engine's most powerful rpm.
The reason for this is that POWER is a measure of Torque x RPM.
Torque by itself is irrelevent to speed. More TORQUE can exert more FORCE. More FORCE does not mean more speed.
POWER is force over time.
To transfer energy from a motor (heat) to water (thrust), you must do so via a screw (propellor/impellor). Turning a given screw of a given pitch at a certain speed is what does the trick.
At a motor's TORQUE peak, it is invariably at a given RPM, let's say 3000rpm. Taking this torque, and transferring it at 3000rpm into a gearset of 1.50:1 and into a prop of a given pitch, you get the resultant POWER transferred into THRUST.
At a motor's POWER (horsepower) peak, then a "smaller" measure of torque is transferred at a greater rpm through the same route to a prop.
05-27-2002, 10:05 PM
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norway, The answer to your question is yes, the best way to maximize thrust is to run the motor at maximum torque. Power is a calculated number ((torque in ft-lbs x RPM) / 5252) and represents the rate at which an engine makes torque. The only real measure is torque.
Now, the question is what do you want to do, go fast or generate a lot of thrust ?
Torque provides the grunt, but remember power is the rate at which the motor makes torque. So if your a semi truck or a tug boat, you want the motor to generate max torque, cause you need the grunt. But to go fast, like in a car or (my personal favorite) performance boat, you want the motor producing torque at as high a rate as possible, so you run the motor where it produces the most power.
My explanation is very simplistic. You can get into discussions about gear ratios, prop sizes, power and torque curves, torque rise, etc. but the bottom line is when they are talking about "power" they are talking about torque, the rate at which the motor is producing it.
The easiest way to remember it is torque is for pulling and power is for speeding.
No mater how you slice or dice it, run it through this ratio or that, Torque Rules
Now, the question is what do you want to do, go fast or generate a lot of thrust ?
Torque provides the grunt, but remember power is the rate at which the motor makes torque. So if your a semi truck or a tug boat, you want the motor to generate max torque, cause you need the grunt. But to go fast, like in a car or (my personal favorite) performance boat, you want the motor producing torque at as high a rate as possible, so you run the motor where it produces the most power.
My explanation is very simplistic. You can get into discussions about gear ratios, prop sizes, power and torque curves, torque rise, etc. but the bottom line is when they are talking about "power" they are talking about torque, the rate at which the motor is producing it.
The easiest way to remember it is torque is for pulling and power is for speeding.
No mater how you slice or dice it, run it through this ratio or that, Torque Rules
Last edited by h2owarrior; 05-27-2002 at 10:31 PM.
05-29-2002, 12:15 AM
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The best way I can explain it is that torque is acceleration and force, Horsepower is speed. Everything boils down to torque. torque creates horsepower, not the other way around.
An engine can capable of big HP, but not have enough torque to overcome resistance after some given RPM point. The motor will not have the torque to accelerate past that RPM. It is loaded up and cannot go faster. There are basically 2 options:
1- A smaller prop is then needed. The smaller prop will require less torque to turn, letting the engine accelerate to a higher RPM. This higher RPM may partially offset the lower pitch prop, resulting in a net speed gain. The motor was over-propped
2- Another solution is to increase torque. The higher torque can turn the prop harder, which will again result in more RPM and more MPH. The prop was under-motored (haha)
If setup #2 turns the same RPM as #1, all other things equal, setup #2 will be faster due to the bigger prop.
Incidentally, 2 500HP engines can make peak HP at 2 different RPM depending upon camshafts, stroke, etc. One may produce 500 HP at 4000 RPM for about 656 ftlbs torque. The second might have 500 HP at 5252 RPM, producing 500 ftlbs of torque.
HP is used for advertising and by itself can be very misleading.
Bulldog aka Ronnie
An engine can capable of big HP, but not have enough torque to overcome resistance after some given RPM point. The motor will not have the torque to accelerate past that RPM. It is loaded up and cannot go faster. There are basically 2 options:
1- A smaller prop is then needed. The smaller prop will require less torque to turn, letting the engine accelerate to a higher RPM. This higher RPM may partially offset the lower pitch prop, resulting in a net speed gain. The motor was over-propped
2- Another solution is to increase torque. The higher torque can turn the prop harder, which will again result in more RPM and more MPH. The prop was under-motored (haha)
If setup #2 turns the same RPM as #1, all other things equal, setup #2 will be faster due to the bigger prop.
Incidentally, 2 500HP engines can make peak HP at 2 different RPM depending upon camshafts, stroke, etc. One may produce 500 HP at 4000 RPM for about 656 ftlbs torque. The second might have 500 HP at 5252 RPM, producing 500 ftlbs of torque.
HP is used for advertising and by itself can be very misleading.
Bulldog aka Ronnie
05-29-2002, 02:17 PM
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Ok, I'm at bat.. Tomcat is probably on deck.....
anyway. I have to differ with some of the comments, and now you are really gonna be confused. Our Opinions are like......... bellybuttons... we all have them and here is mine, take it for what it is worth. I always reserve the right to get smarter...... but
Here's the physics fundametals.
work = force times a distance (ft - lbs) (ie. drag a rock 3 feet with a force of 100 lbs on the rope = 300 ft-lbs of energy exerted)
power IS BY DEFINITION the rate of energy (work)per time.
therefore power =( ft - lbs)/ second
or lbs x (ft/second)
or force x velocity
(or in rotating terms... torque times rpm )I'll get to that in a minute
a horse can lift (through a pulley) 550 lbs at 1 ft/second speed
550 ft-lbs/sec = 1 horsepower by definition.
that SAME HORSE can lift 1100 lbs at a rate of .5 feet/sec through a two pulley system. SAME POWER OUTPUT FROM THE HORSE.
( simlar to changing gears on a bicycle, less torque on the pedals x higher pedal crank rpm
same thing applies (by physics) for a boat motor.
an engine producing 400 ft-lbs of torque at 5000 rpm is THE SAME power output as an engine producing 500 ft-lbs of torque at 4000 rpm. (2,000,000ft-lbs-rpm/5252) = 280.8 Hp
(5252 is the constant to turn rpm into radians/sec and get back to Fundamental units)
peak power output will result in the fastest boat through the most power converted from the engine into thrusting the boat.
500 ft-lbs of torque at 4000rpm will not move a boat faster than 450 ft-lbs of torque at 5100 rpm
I do agree that when discussing comparisons at ONE COMMON RPM, more torque = more power, but its through the physics' multiplication of a higher toque x the same rpm that gets that higher power.
so...... prop the boat to get the most speed when the engine is producing the most power ( not necessarily the peak torque)
this is typically around 4600-4800 rpm for a stock engine and about 5100 for a HP Mercury engine
if the prop you choose lets the engine over rev past 5100 or so, or it only lets the engine get to 4200, then a pitch change is needed. you are not at the top of the power curve of the engine. actually running an engine at too low of an rpm is hard on the drive train because the torque may be higher putting undue strain on the driveline while not producing the top speed.
Tomcat has a program that overlays the theoretical power requirement to move your hull through water at various speed vs the power output curve of an engine. the program selects the prop pitch that will overlay these curves at the optimum speed.
Thanks for listening
Rambunctious
B.S. Physics
M.S. Mech Engineering
anyway. I have to differ with some of the comments, and now you are really gonna be confused. Our Opinions are like......... bellybuttons... we all have them and here is mine, take it for what it is worth. I always reserve the right to get smarter...... but
Here's the physics fundametals.
work = force times a distance (ft - lbs) (ie. drag a rock 3 feet with a force of 100 lbs on the rope = 300 ft-lbs of energy exerted)
power IS BY DEFINITION the rate of energy (work)per time.
therefore power =( ft - lbs)/ second
or lbs x (ft/second)
or force x velocity
(or in rotating terms... torque times rpm )I'll get to that in a minute
a horse can lift (through a pulley) 550 lbs at 1 ft/second speed
550 ft-lbs/sec = 1 horsepower by definition.
that SAME HORSE can lift 1100 lbs at a rate of .5 feet/sec through a two pulley system. SAME POWER OUTPUT FROM THE HORSE.
( simlar to changing gears on a bicycle, less torque on the pedals x higher pedal crank rpm
same thing applies (by physics) for a boat motor.
an engine producing 400 ft-lbs of torque at 5000 rpm is THE SAME power output as an engine producing 500 ft-lbs of torque at 4000 rpm. (2,000,000ft-lbs-rpm/5252) = 280.8 Hp
(5252 is the constant to turn rpm into radians/sec and get back to Fundamental units)
peak power output will result in the fastest boat through the most power converted from the engine into thrusting the boat.
500 ft-lbs of torque at 4000rpm will not move a boat faster than 450 ft-lbs of torque at 5100 rpm
I do agree that when discussing comparisons at ONE COMMON RPM, more torque = more power, but its through the physics' multiplication of a higher toque x the same rpm that gets that higher power.
so...... prop the boat to get the most speed when the engine is producing the most power ( not necessarily the peak torque)
this is typically around 4600-4800 rpm for a stock engine and about 5100 for a HP Mercury engine
if the prop you choose lets the engine over rev past 5100 or so, or it only lets the engine get to 4200, then a pitch change is needed. you are not at the top of the power curve of the engine. actually running an engine at too low of an rpm is hard on the drive train because the torque may be higher putting undue strain on the driveline while not producing the top speed.
Tomcat has a program that overlays the theoretical power requirement to move your hull through water at various speed vs the power output curve of an engine. the program selects the prop pitch that will overlay these curves at the optimum speed.
Thanks for listening
Rambunctious
B.S. Physics
M.S. Mech Engineering
05-30-2002, 01:20 AM
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Rambunctous-
Horsepower is defined by that equation, but I find it easier to explain horsepower vs torque this way to a person not familiar with mathematics.
Given the SAME horsepower, boat and props, the higher torque engine will accelerate faster.
Boats DO get overpropped when the torque of the load exceeds the available torque of the engine.
In a rotational system, can HP exist without torque? Can torque exist without horsepower?
BS in engineering too
Bulldog aka Ronnie
Horsepower is defined by that equation, but I find it easier to explain horsepower vs torque this way to a person not familiar with mathematics.
Given the SAME horsepower, boat and props, the higher torque engine will accelerate faster.
Boats DO get overpropped when the torque of the load exceeds the available torque of the engine.
In a rotational system, can HP exist without torque? Can torque exist without horsepower?
BS in engineering too
Bulldog aka Ronnie
05-30-2002, 01:29 AM
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TurboDog
Bradperry-
Yes- I think the guys at the brewery saw me and Daisy (Black Dachsund) riding around in my triple black Porsche 930 Turbo. The original brewery is now the Brewpub and is within 5 minutes walking distance from my house. there is also an Abita Springs bottled water Co. here. I wash my cars, water my lawn, etc with water people pay $1 a gallon for. Whoda' thunk that a few years ago.....
PS- Ever try Dixie blackened voodoo? It was outlawed from Texas for a while. Or so I've been told.
Yes- I think the guys at the brewery saw me and Daisy (Black Dachsund) riding around in my triple black Porsche 930 Turbo. The original brewery is now the Brewpub and is within 5 minutes walking distance from my house. there is also an Abita Springs bottled water Co. here. I wash my cars, water my lawn, etc with water people pay $1 a gallon for. Whoda' thunk that a few years ago.....
PS- Ever try Dixie blackened voodoo? It was outlawed from Texas for a while. Or so I've been told.
05-30-2002, 10:26 PM
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Mr. Perry
I'm an electrical engineer at the NASA engine test facility in Southwestern Mississippi. We test shuttle engines for design mods and for Flight readiness where the engine recieves a very thorough test for flight certification.
We also test experimental engines such as the linear aerospike which may one day replace the shuttle. There is also a test area for peroxide based engines (bad stuff). The peroxide you buy at the drug store is about 3%. Rocket grade peroxide is greater than 90% and is HIGHLY reactive with all but a few metals and some plastics.
One small peroxide engine would make those turbine guys think they had weedeaters on the transom...
Bulldog aka Ronnie
I'm an electrical engineer at the NASA engine test facility in Southwestern Mississippi. We test shuttle engines for design mods and for Flight readiness where the engine recieves a very thorough test for flight certification.
We also test experimental engines such as the linear aerospike which may one day replace the shuttle. There is also a test area for peroxide based engines (bad stuff). The peroxide you buy at the drug store is about 3%. Rocket grade peroxide is greater than 90% and is HIGHLY reactive with all but a few metals and some plastics.
One small peroxide engine would make those turbine guys think they had weedeaters on the transom...
Bulldog aka Ronnie
