Can a motor produce the same HP at different RPM with different Loads

If the 27" prop is too much pitch for the boat, the engine will not reach top RPM and the boat will go slower. There isn't enough data for a significant answer.

 

The HP requirement for a given top speed is constant.

The constant slip of 10% and constant shaft RPM makes speed a linear relationship to pitch.

So I'd say you have a big problem ... theoretically.

 

Hough, would you please show me how you arrived at the statement that says,

I will also agree with your statement that

I do agree with your statement that it does make it a linear relationship, but if two props at 19" pitch and 23" pitch each have 10% prop slip, are you trying to say that they will not do 54 MPH at the pitch and RPM stated below, and there is no constant shaft RPM. Do I understand you to say that this is impossible, and I am wrong.

If what you are saying is true, please back this statement up with data, so I can find out where and why I am wrong and correct it.


Prop Slip




H

 

A higher pitch propeller will keep the engine from reaching top RPM, therefore not reaching top HP. The speed will be lower.

 

Well said. You would have to change gear ratios to address this issue.

 

Sure. Props are screws ... with zero slip they advance by their pitch each revolution. With 10% slip they advance 90% of their pitch each revolution. I took the question to be: "If all other things are equal, and ONLY the pitch is changed, what will the result be?" The obvious and simple answer is that the speed will change in direct relationship to pitch.

Of course at the same SPEED different pitched props will have different shaft RPM. I have no reason to question your data. I understood the question to require the shaft RPM be constant. Hence the question about HP at different engine/shaft RPM.

The idea is to match the prop with the engine and reduction ratio so that the full rated HP of the engine can be used.

I don't see a conflict between your data/formulas and what the engineer said.

Power required to reach Speed X is constant.
Speed X can be obtained at more than one Pitch and shaft RPM.
With all other things constant increased Pitch will give higher Speed at the same RPM ... IF there is enough power available to drive the boat faster.

What is the argument?

 

In real life propeller slip varies with RPM

 

"My question is can a 292 HP motor produce 280 HP at 5,000 RPM and also produce that same 280 HP at 4,130 RPM."

NO. it can't, well not very likely anyway.

Bar bet settled.

That was easy...

 

Yes, that is why I didn't understand the question. It was an odd set of conditions.

For X HP and 10% slip at V speed, the shaft RPM must change for each pitch.

For Y shaft RPM and 10% slip at V speed, there is only one value of pitch.

The HP value does not change for V speed. Only the RPM at which that HP is required changes with pitch.

I truly don't see what the argument was all about.

Randy

 

As I told you before, if the engine isnt almost max tuned you are able to change the effect curve. So you are able to make the engine settings to generate more power at lower rpm.
As I did read the thread start was if he was able to change the effect of the engine.

AND YES IT IS POSSIBLE! unless the engine isn´t almost max tuned!

This is what motor engineers wrestling with every day.

Often an original engine isn´t max tuned caused by the years of usage will be shortened.

For a streetrace car for an example there is two ways to meassure the power.
One way is to measure at the driving wheels.

Another way is to mount the engine in a tune bench.

To tune an engine so it will be able to gain the same power at a low rpm.
You usually increase the compression ratio. Or make the intake tubes bigger then the flow from the throttle hole. Or sometimes both.

The propeller does only transfer the power to the water and does not affect the engines output range. But in good kondition in ballance of engine power out, rpm, and effekt transfer the boat will go faster less fuel conumption and so on.

My opinion is!
YES OF COURSE YOU ARE ABLE TO CHANGE THE ENGINES EFFECT CURVATURE!
As people are doing this everyday.

 

Nukisen, your lack of knowledge in internal combustion engines is staggering.

While you can make small changes to the intake to tune the engine you will effectively only move the torque peak to a different speed. The torque peak does not change substantially. Peak torque is directly proportional to displacement, it only falls off at higher speed due to losses in the intake an cam timing that causes less cylinder filling. If you significantly reduce the speed you will only make less power because the torque peak is unchanged and the speed is lower. All internal combustion engines are effectively air pumps. The power you make is directly proportional to the amount of air you move.

If you want more power you need to move more air. That means for a lower rotational speed you need to have more displacement. Racers want more power so they find ways to increase the engine speed while retaining the same maximum torque by making intake and cam modifications. This is all pretty basic.

The changes to intake manifoilds and cam timing don't increase the torque peak, they simply move it to a higher speed. In this case, if the torque peak was moved up to 6000rpm (as opposed to 3200) then you will be making 400 hp at that higher speed. The torque is the same 370 ft lbs, but the speed is higher so there is more power. Tuning tricks are basically used to allow the engine to breathe at higher speeds, it simply doesn't work the way you are suggesting.

With regard to compression ratio. Once you reach the maximum cylinder pressure for a given configuration the engine will detonate and not last for even a few minutes. The reason that increasing compression ratio works on high speed engines is that the losses in the intake at higher speeds reduce the cylinder pressure. A typical low speed engine is generally tuned for the highest compression that matches the cam and the expected rotational speed of the engine and the fuel that is typically used. If you spin an engine to a higher speed, you can increse the compression ratio, but you can't raise the compression ratio at the same time you are reducing the speed, you will blow the pistions out of it.

The only way you can increase the power at lower speed is to supercharge the engine (push more air through it) so it acts like a bigger pump, but that is a whole different ball game.

 

Now it seems we do have an expert here!
And he is also very polite to!
Made it feel like the discusions used to be used at flashbackforum.
I am not an expert but I do know some.

I thought the question was about if you where able to move the effectcurve pitch down to a lower rpm. Or maybe increase the power so it is able to take out the 280hp at the lower rpm he asked for. Not make it to a racer. Still to supercharge an engine isn´t the only way. Also as I told before you are able to increase the compression ratio. This will make more vacuum in the intake. And also it will make the combustion faster.

YES MY OPINION STANDS!
IT IS POSSIBLE!
Feels like we should wait for an motor tuner!Who are able to show about this curvature.

 

"...but absolutely NO knowledge of motors."

Guys, lighten up a bit here, and nukisen, stop yelling!

The original poster has... "absolutely NO knowledge of motors".

He then explains it is an argument with his chums.

I believe the short answer, given the limited information and details is:

No.

 

Yes I do think this an correct analysis.
For me it does look like make sence.

I think this is possible if you are able to tune up the engine a step higher without increase the power pitch rpm. If you don´t scare the engine a bit by tuning and trim. Then it would not work.

 

He showed a HP curve. The question did not include retuning or hotrodding an engine. From your perspective we are comparing different engines.