# How Torque, HP, and max RPM relate to speed/cruise? How do Gas/Diesel compare?

Discussion in 'Powerboats' started by leaky, Oct 20, 2015.

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### DMacPhersonSenior Member

This thread reminds me of the Star Trek episode with the white-black bi-colored race that hates their mirror-image black-white brethren. Power and torque are two sides of the same coin, and it is not at all useful to think that one is better than the other for calculations, or that one is even detached from the other.

Here are my "two cents" from some 35 years of doing propeller calculations...

I use power as the focus of my calculations because it is a better way to describe the energy transmitted to the propeller. Let me give you an example with two engines.

Engine A: 200 rated kW at 2400 rated rpm
Engine B: 200 rated kW at 1800 rated rpm

Same power, but different engine torque.

Push Engine A through a 2:1 gearbox, delivering 1200 shaft RPM. Size the propeller.

Push Engine B through a 1.5:1 gear box, delivering the same 1200 shaft RPM. Size the propeller and it is exactly the same as for Engine A. Power absorption is the same, thrust delivery is the same, shaft torque at the propeller is the same.

Engine B may have more engine torque, but it does not deliver any more energy or any difference in the propeller or its performance. This is why I use power to express the potential energy of an engine, and why engine torque is often misleading.

Also, for what its worth, I haven't used a Bp-Delta curve since college. For the reasons stated by others previously, the KT-KQ curves are the foundation of all of my calculations. Again, these are two sides of the same coin. One just is easier to use as a manual design chart (Bp-Delta), the other has greater scope for all applications (KT-KQ). You can create the Bp-Delta chart from KT-KQ data, and there are many numerical optimization methods that could be used to employ the KT-KQ data to solve for optimum propeller parameters.

Don MacPherson
HydroComp, Inc.

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### FAST FREDSenior Member

On most well done lobster boats the engine can be R&R overnight.

This means a NEW \$10,000 Crusaider gas engine or \$25,000+ diesel is seldom required.

By choosing the traditional work boat keel cooling abd dry exhaust stack, no marinization , just a marine tranny is required. Twin Disc is a good choice.

The savings in using an auto gas engine or a used skool bus diesel will make up for decades of fuel.

The International DT 360 or 466 is my favorite if a big diesel is required.

Under 15,000 miles the DT from a wrecked skool bus should be \$3000 to max \$5000 with its required electronic injection box and wiring.

New is great for deep pockets folks but when the only risk is an extra nights work, why bother?

After all, that new engine is a used engine after the first start up.

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### AluminationJunior Member

This is a really good explanation. I've tried to simplify my understanding of the HP/TQ relationship over the years.

This can be compared to electricity or water in that ....

Pressure - Voltage or Torque
Flow- Amperage or RPM
PxF- Power or HorsePower

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### arcwiNew Member

I would humbly disagree, at least with 2) and 3) - in the above context, i would say

2) the torque applied to the prop, and
3) the shaft RPM, so one can calculate the power delivered to the prop.

IMHO

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### gonzoSenior Member

You don't need shaft RPM to calculate the power applied to the propeller. The engine power minus losses gives you that.

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### arcwiNew Member

Granted, but only if one knows the engine power figure.

My assumption was that the applied torque is known and the engine power not - hence, by using the torque figure and factoring the RPM, one can calculate the power.

The idea here is that the torque figure is measured, while the power figure is calculated and is always a function of the torque and the RPM.

The reverse relation is valid also - one can take the calculated figure, the power, and by reverse calculation arrive to the input value, the torque, if the particular RPM is known...

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### gonzoSenior Member

If you don't know the engine power, why would you know the power? Either get the power rating from publish data, or hook it up to a dynamometer; you would need to do that to get torque at various RPMs anyway.

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### JoakimSenior Member

It is just the same! You can calculate the power from torque and RPM or the third one when two is given (at the same point of course).

So it really doesn't matter, if you use torque or power as long as they are at the same known RPM.

Mostly engines are marketed for maximum power or maximum torque. These are at different RPM and from those maximum power tells mutch more about the cababilities of the engine.

Especially diesels are marketed for very high maximun torque without telling that the maximum power RPM is much lower and thus you need different gear ratio and will not get more torque at the prop shaft or wheels.

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### gonzoSenior Member

Proper specifications for calculations should use the torque/RPM curve graph.

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### FAST FREDSenior Member

To pick an engine ,what is missing from this interesting discussion is any concept of efficiency.

This can be found only on the engine mfg fuel map or BMEP graph.

This looks like a series of clouds , one inside the other.

In the center is the best fuel for power the engine can do.

As you go out further the efficiency , the ability to create power per gallon of fuel goes down.

The art is to select the right amount of power and prop the boat to operate in the bulls eye the most amount of time.

For efficiency the usual mfg provided garbage of a Hp vs phony prop graph is .,,,

BOOB BAIT FOR THE BUBBAS!

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and if its VW or MAN diesel dont believe the graphs

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### gonzoSenior Member

The target of economy is not always what the customer wants. They may want top speed, for example.

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### philSweetSenior Member

Usually, efficiency is going to be pretty similar between similar cost options. However, this boat is bimodal, it can do maybe 30 knots, but will spend a LOT of time operating at much lower power. As long as that low power cruise is, say 30% of the engine's rated cruise power (including house loads), again, there isn't much to choose between. But if you go for a 400hp engine and spend 90% of your time at 50hp, then you do want to seriously look at the engine efficiency considerations and how all the auxiliary equipment on board is being powered. That's getting pretty fussy, but worth doing. The fuel map of a poor 400hp motor and poorly thought out ancillaries at 50 hp may be very different from a good 280hp motor at 50hp.

This is something the builder would have a good feel for. Holland's a gearhead, he's been at it for a long time, and he has 160+ boats in the water.

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### FAST FREDSenior Member

Not cheap but ZF has 2 speed transmissions that could help with vast under loading.

When only minor HP is required shift ,to speed up the shaft speed to help load the motor at above idle rpm.

A CPP would also help, but both these methods much cost more than simply accepting a shortened engine life .

An 8000 hour motor that only went 4,000 hours in a 300 hour a year boat should not be a huge problem.

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### BarrySenior Member

"In the center is the best fuel "

Are you saying fuel consumption??say gallons per hour??

"for the power the engine can do"

Do you mean what the engine is rated at??

I looked at a couple of these graphs and the center of best efficiency occurred at around 2000 rpm, which is much slower than where most max horsepower can be produced in at least a gas engine.

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