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hp vs prop

josh_2562

Well-known member
Maybe some one can help me with this. Let just say I have 1000hp engine and I'm spin what ever prop that I would use with that much hp at 3000rpms. will a boat with say a 100hp spinning the same prop as the 1000hp engine perform the same. I was just think about this. sorry about the dumb questions
 
I think Burt answered it the best way considering the vagueness of the question. I'm assuming you say both engines are capable of turning the same RPM with the same prop. If that were the case, that would mean that the prop was designed for the 100HP engine and the only reason the 1000hp engine stayed at 3000 was because you did nothing more than let it idle. Of course if you mean a prop that can harness the 1000HP down to 3000RPM then the 100HP engine propably wouldn't be able to start without the use of a clutch.

But I feel you're asking more along the lines of gear reduction/HP vs/TQ theories....But the answer will be the same with mild if not moot points to make about "what ifs".

JMO
Felber
 
Another angle, IF everything is the same, same rig, weight, etc. If the 100 hp motor turns the prop at peak performance for the prop, adding more horse power will do nothing so they would work the same. The larger hp motor would push the prop past its ability and stop pushing. hm this means they do not preform the same, I may be confused also.
 
"If that were the case, that would mean that the prop was designed for the 100HP engine and the only reason the 1000hp engine stayed at 3000 was because you did nothing more than let it idle"

That's what I was reffering to.
 
For simplicity sake let's take the prop out of the equation for right now and replace it with a constant load - like a dyno. If 1,000 hp will spin the rollers 3,000 rpm at a given resistance, then 100 hp will spin the rollers at 300 rpm with the same resistance.

It's going to be similar with a prop instead of a dyno but probably not exactly the same. My gut tells me you might spin it faster than 300 because the prop isn't going to move much air at that speed so the load/rpm won't be proportional.

Bear in mind when you say "the prop" this assumes you mean the same exact prop - same hub, same blade, same # of blades, and same pitch setting.

And this setup will be a real dog. There isn't an airboat prop made that's going to push at 300 rpm.
 
That's like having Lance Armstrong on a bike and say well Lance just use a hundredth of your power and run in 1st gear! So we can use first gear to it's peak performance and efficiency! Then some big 300 pound fat guy like me would whip him with a stock ten speed while caring a beer! There is no perfect prop or combo. You need to run the prop that performs the best in the area you want your boat to perform it's best! You can have a prop that is more efficient at an RPM the motor makes no torque and it will not perform as good as a prop that is less efficient but produces more thrust at where the motor makes peak torque! This thread could go on to the new year.
 
the 3000rpms was the prop speed. from what I have read most porps are made to spin at 3000rpms so if you can use a engine with less horse power to spin the same prop at 3000rpms will you get the same perfomance. or does haveing the extra hp help get the prop spining faster give it more snap to run dry and take off better. thanks for the help
 
A 100hp motor won't spin the same prop as a 1000hp motor so what does that ? Have to do with anything?
 
An engine with a hundred HP will not turn a decent size prop 3,000RPM's. And as always the 3,000RPM is a B.S. number. Every length prop has a different RPM at which the tip's become super sonic. A 72'' prop can turn 3,000 and will have a much lower tip speed then a 80'' prop! You can't turn an 82'' prop 3,000 because the tips will break the speed of sound! I guess the best way to put it the more power a motor develops the more prop it can turn so the more thrust it creates. Each prop can only create certain amount of thrust and each motor can only generate so much power so you need to run the right prop with the right motor! There are dozens of factors involved and I have yet to meet anybody who understands them all! I feel I know the best prop and the best gearbox to run on a specific type of boat however I tell you I have no clue what an 0540 likes! All the formulas and all the data means nothing until your but is in the seat on the water. I have seen two people run the same exact boat and one will say man that thing has more snap and response then any boat I have ever ran. Then the other driver will say man that prop has no top end and sucks in deep water! IT's ALL preference!
 
If I remember correctly , the hp required to turn a fix pitch prop varies the RPM to the 3rd power. Hp/(RPM X RPM X RPM)= Hp/(rpm x rpm x rpm)
If you doubled the rpm the hp goes up by a factor of 8. If max Rpm is 3000 & you cut you rpm back to 2500 you hp to drive the prom is only 57.8% of max out put.
 
Moritz":9aespzcc said:
If I remember correctly , the hp required to turn a fix pitch prop varies the RPM to the 3rd power. Hp/(RPM X RPM X RPM)= Hp/(rpm x rpm x rpm)
If you doubled the rpm the hp goes up by a factor of 8. If max Rpm is 3000 & you cut you rpm back to 2500 you hp to drive the prom is only 57.8% of max out put.

That's true kinda. That works for a prop who's efficiency is the same throughout the rpm range you're looking at; which is exactly what you're not doing on an airboat. That's why I used the dyno example in my earlier post.

What you're referring to are the pump laws which state (roughly) to double the mass flowrate of a fluid (air is a fluid) through a system you must have four times the delta pressure across the pump (or fan). In the math world, it takes eight times (2 raised to the third power) to create the required delta p.
 
Why don't car motors have 5 inch pistons with a 4 inch stroke? It looks like a 4 cylinder with that bore and stroke could put out a lot of torque at low RPM. Car motors don't seem to have the big bore or long stroke. Is it because they would weigh even more than they do now? Just curious and I'm not trying to start anything. I know nothing about car motors but would use one if it could do the job without all the extras and weight.
 
Des Moines Boater":2czhbgld said:
Why don't car motors have 5 inch pistons with a 4 inch stroke? It looks like a 4 cylinder with that bore and stroke could put out a lot of torque at low RPM. Car motors don't seem to have the big bore or long stroke. Is it because they would weigh even more than they do now? Just curious and I'm not trying to start anything. I know nothing about car motors but would use one if it could do the job without all the extras and weight.

Well DAB, I think you answered your own question. Why would an auto mfgr build a dd airboat engine?
 
I think what's being asked is will the same prop produce the same thrust if it's being turned 3,000rpm if it's done with a 100hp engine or a 1,000hp engine.
Obviously it will, but to some the obvious isn't. 8)

From the stand point of fuel economy and longevity, having more hp available than what's really needed to turn the prop might be a good thing, problem is if we've got the power to turn a bigger prop, you can damn well bet that we will! :lol:
 
Des Moines Boater":20z2l379 said:
Why don't car motors have 5 inch pistons with a 4 inch stroke? It looks like a 4 cylinder with that bore and stroke could put out a lot of torque at low RPM. Car motors don't seem to have the big bore or long stroke. Is it because they would weigh even more than they do now? Just curious and I'm not trying to start anything. I know nothing about car motors but would use one if it could do the job without all the extras and weight.
Automotive mfgrs. seem to be all about high rpm's with small bores and square to just over square strokes....the irony is most mid to full sized vehicles cruise at 2,000~2,500 rpms, then again they use the higher rpm's for speed/acceleration.
The few cars that "Tucker" made used helicopter engines (Franklin I think) what a novel idea! Can you imagine a nice little car like an old VW bug with a O-360 Lycoming in it. :shock:
What kind of advantage does a V12 displacing 4 liters have over a I-6 with the same capacity? :?
 
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