Engine torque or power

[trimix]

Hi,

I was wondering which parameter is important for a marine engine which drives a propeller.

For a given boat when do I consider the torque of the Engine and when do I consider the power ?

(for a car I know that the torque allows the car to accelerate and the power to maintain a high speed)

Thanks for your help

[rasorinc]

You need torque to push a boat through water. High power low torque is of less value then high torque less power

[apex1]

In our Marine Diesel engine world, you may simplify,
high speed, high rpm, small prop diam. high power
low speed, low rpm, larger prop, high torque

Regards
Richard

[FAST FRED]

For a high speed boat HP is the game.

For most of us that pay for our own fuel the ability to match and not UNDERLOAD the engine to the prop at the speeds we cruise is the name of the game.

Most cruisers are overengined by 50% to 300% , at the cost of fuel economy and reduced engine life.

Shoot to use 70% 80% of the engines 24/7 rating at your favorite speed , hopefully near the torque peak on the Mfg chart.

FF

[Rick Willoughby]

Hi,

I was wondering which parameter is important for a marine engine which drives a propeller.

For a given boat when do I consider the torque of the Engine and when do I consider the power ?

(for a car I know that the torque allows the car to accelerate and the power to maintain a high speed)

Thanks for your help

The relationship between power and torque is:
Power = Torque X Rotational Speed.

Rated power for any engine is the peak power it can achieve. Normally is it close to the peak rpm.

If a motor has a flat torque curve it is easier to match to the torque demand from a prop over a range of operating conditions. With most boats using typical IC engine there is little to no advantage in having variable gearing. If you have the gearing and prop best suited to the boat and motor then it usually works well through the range of speed.

Designing the propulsion system for a boat and given motor gets down to what is more important or what compromise you want to make. You can usually gain better fuel economy by overpropping the motor, which may not then deliver full power before it is torque limited at the top end. Having two gears would enable both conditions to be achieved but then there is extra weight and complexity so the benefit is likely to be very small.

If you give more idea on the type of boat and motor along with what objective you hope to achieve you might get more useful input.

Also your statement about the car could be misleading. The more important aspect for acceleration is for the motor to develop high torque at low rpm. You could have different motors with identical peak torque and peak power that perform differently. The motor that has the wider torque band will out accelerate the motor with a narrow torque band.

Rick W

[Ad Hoc]

trimix

"..I was wondering which parameter is important for a marine engine which drives a propeller..."

It really should be said the other way around.

Without knowing a single bit of info regarding your "problem", one can summarise as thus:

The RPM, delivered power, diameter and the speed of advance all dictate the design of the prop and hence its performance. In addition, you need to check the cavitation no., so you'll need to know, or, estimate the static pressure and dynamic pressure.

So given that all you have is an engine with "delivered power", and possibly a gear box to obtain the some kind of RPM, the prop still has many other variables that affect the overall performance.

That's before getting in BAR ,number of blades and so on...It is all a balancing act.

However a quick rough and ready guide has been posted by apex1 above.

[Mark Emaus]

trimix

"..I was wondering which parameter is important for a marine engine which drives a propeller..."

It really should be said the other way around.

Without knowing a single bit of info regarding your "problem", one can summarise as thus:

The RPM, delivered power, diameter and the speed of advance all dictate the design of the prop and hence its performance. In addition, you need to check the cavitation no., so you'll need to know, or, estimate the static pressure and dynamic pressure.

So given that all you have is an engine with "delivered power", and possibly a gear box to obtain the some kind of RPM, the prop still has many other variables that affect the overall performance.

That's before getting in BAR ,number of blades and so on...It is all a balancing act.

However a quick rough and ready guide has been posted by apex1 above.

What does BAR stand for? What does it mean?

Mark

[Ad Hoc]

Mark

BAR = Blade Area Ratio.

It is the total, projected or developed area of the blades on the prop. Where the developed area is the sum of the face area of all the blades, and projected area is the projection of the blades (like normal dwg projection one may have done at school or uni) onto a plane normal to the prop axis.

The thrust developed by the prop varies directly with the surface area.

[alan white]

A thing that is pushed around in a circle with one pound of pressure at one foot from the axis covers 6.2918 feet per revolution (2 x radius x pi). If it takes a minute to go around, the force required to push it is said to be one ft lb at one RPM.
This can be used to figure what that means in horsepower.
Really, the circle can be straightened, so you can also say that raising a pound 6.2918 feet vertically is the same thing, as the pressure exerted is in addition to gravity.
A horsepower is the power required to raise 33,000 lbs to a height of one foot in one minute.
If we are raising one lb to 6.2918 feet in one minute, we need much less than a horsepower. We can also say we can raise 6.2918 lbs one foot in one minute. 33,000 divided by 6.2918 is 5,252.
One horsepower is then equal to one ft lb @ 1 RPM x 5,252.
Also, since 1 RPM x 1 ft lb x 5,252 = 1 hp, and the square root of 5,252 is 72.5, then 72.5 ft lbs @ 72.5 RPM = 1 hp. So if the RPM is 50 times as much, at 3625 RPM, at 72.5/50 lbs (1.45 lbs) you still have 1 hp. A diesel boat engine might run at 3625 RPM, and if it had 100 hp at that speed, it would have 72.5 ft lb of torque. Or, it could have 72.5 HP and 100 ft lbs of torque @ 3625 RPM.
I think.

[Mark Emaus]

lol....That is a lot of info....but it now gives me a place to start to figure what I need to know. One of the ideas we ave is to make the mudmotor a hydraulic system. I can see where this will help. I saw a BAR spread sheet and I was wondering what it stood for, (not any more).

Mark

[alan white]

A lot of the time, engine power is expressed in kilowatts, since both measurements describe actual power over time (They could be expressed as kilowatt hours (KWH), or horsepower hours when applied to continuous output like engines..

Deriving HP from torque and RPM also gives Kilowatts. The simple way to instantly remember how many KW are equal to a horsepower is to recall that two HP are equal to 1492 watts, the date Columbus "discovered" America
(1.492 KW). So one HP is half that, 746 watts (.746 KW).
One hundred HP is the same as 74.6 KW.

[kroberts]

At what size/scale of boat, either power level or length of boat, does it become practical to use a motor-generator as a transmission, the way a modern train does?

Sorry if this sounds like an off-topic post, but the whole idea in this case is to more accurately match the torque and power generated by the engine to the needs of the propeller in the water.

[Rick Willoughby]

At what size/scale of boat, either power level or length of boat, does it become practical to use a motor-generator as a transmission, the way a modern train does?

Sorry if this sounds like an off-topic post, but the whole idea in this case is to more accurately match the torque and power generated by the engine to the needs of the propeller in the water.

The power demand of the propeller is well under the power produced by an IC engine right up to the design point. There is no need for gearing unless you want to aim for optimised fuel efficiency. There is a size where the fuel efficiency gained by using an electric transmission would offset the losses in the electrical system plus the higher capital costs involved.

There are other factors that might come into the equation such as the boat may have a high electric demand because of the function it serves and propulsion is only one of the main loads. There could be space restrictions to mount the IC engines. This is not uncommon in SWATH type vessels so electric is a good choice for the smaller vessels here. There could be need for distributed thrust with a number of side thrusters locate around a vessel. These could be electric.

For a simple cruiser I would expect you would be looking at 1000kW or more before the economics of diesel electric come into play. There have been businesses set up doing smaller systems but I think they have withered through lack of interest.

Rick W

[kroberts]

That's exactly what I wanted to know, Rick. Thanks.

[alan white]

Good question, and while I could maybe guess what efficiencies are served in each scenario, I'd like to hear what folks have to say on the subject. My feeling is there are many many factors at play, maybe the least of which would be a slight difference in fuel consumption.

[Ad Hoc]

Alan
In a word...weight.
I designed such a SWATH about 7 years ago. The whole "trasmission" package was extremely heavy. Was about twice that for a 'conventional' transmission package. But, that is what the customer wanted!

[apex1]

Alan
In a word...weight.
I designed such a SWATH about 7 years ago. The whole "trasmission" package was extremely heavy. Was about twice that for a 'conventional' transmission package. But, that is what the customer wanted!

Right Ad Hoc,
we have to pay hefty penalties if we go Diesel Electric, weight is just one. Nevertheless there are many applications in the commercial field where that is the only senseful way to go. SWATH are one, we have them on duty since about ten years now in the mouth of the Elbe river near Hamburg, as Pilot commuters. Outstanding good and reliable craft BTW.
Cruise ships, Pontoon cranes, Fire fighting boats,with their heavy El. loads are another field, and Icebreakers are propelled by Diesel El. since more than 65 years now, due to the high torque low rpm ability.
But if we are asked to install such system in a yacht, there is only one possible statement, regardless of: power range, intended use, cost, comfort, engine load and the other 28 different issues. NO, that is not sensible! Ok I had to edit that, Ad Hoc is watching my spelling.

This was said by one who was quite recently looking for a Diesel Electric propulsion for his own vessel, me. And I must not rake in the mist to receive real world information, as you know.

Regards
Richard

[Ad Hoc]

Apex
Yes those are the SWATHs built by A&R. Their designs are somewhat different to ours, namely speed.

When weight is not an issue, then yes, they have their place on board. However, all my designs are weight sensitive owing to their speed. So this DE method of transmission is not an option at all. Unless, like in the example I gave, the client wants it and is prepared to "pay" for the consequences

By the way the word is "sensible". But don't ask me how to spell that in German...you're way ahead of me on that score! :)

[Mark Emaus]

The power demand of the propeller is well under the power produced by an IC engine right up to the design point. There is no need for gearing unless you want to aim for optimised fuel efficiency. There is a size where the fuel efficiency gained by using an electric transmission would offset the losses in the electrical system plus the higher capital costs involved.

There are other factors that might come into the equation such as the boat may have a high electric demand because of the function it serves and propulsion is only one of the main loads. There could be space restrictions to mount the IC engines. This is not uncommon in SWATH type vessels so electric is a good choice for the smaller vessels here. There could be need for distributed thrust with a number of side thrusters locate around a vessel. These could be electric.

For a simple cruiser I would expect you would be looking at 1000kW or more before the economics of diesel electric come into play. There have been businesses set up doing smaller systems but I think they have withered through lack of interest.

Rick W

Rick,

Are you saying that you could use up to 1000kws before you need to put an IC engine with a generator attached to it on a boat? If so, using the kw figure mentioned before, are you saying that you could run an equal size electric motor, (instead of an IC engine) of around 120hp for 10 hours and be able to have some sort of battery pack set up before needing a generator to keep the system running? If that doesn't make sense, let me ask you this. What size electric motor would it take to run as an equal to a say 100hp IC engine, and what size battery pack would it take to run that E Motor for an hour without having it charged. I am bad with electronics, but I am guessing that having 6, 12volt batteries in serries, (making a 72 volt system) along with a 3 phase system would make the draw on the system much less. And If you wanted to have this system run forever, You would have to put a IC engine and generator on the boat, what size engine and generator would it take to keep it running? wow! One more senerio Rick.....If you made it a 24 volt system, (to get rid of the 4 extra batteries weight) and just had the IC engine charge the system, How much IC Engine HP would be required to keep it charged?

Mark

[Mark Emaus]

Rick,

The above post only is for the motor driving the prop and nothing else.

Mark

[kroberts]

Mark,

The idea here is a diesel-electric, the same as a train engine. The motor and generator are the power transmission, nothing else. All the power of the engine goes into the generator, and the only connection between the prop and the engine is the electrical wires.

It gives you a very wide gear range. It lets you run the engine at the speed which barely provides enough power to turn the prop at the desired speed.

This is not an efficient setup if you consider the losses in the motor and in the generator, but a normal transmission setup the engine is usually turning much faster than it would have to to develop the torque necessary to turn the prop that fast.

In other words, you save money because your engine never turns faster than it has to, and is always fully loaded.

There is a very large start-up cost though, the motor and generator cost a bunch of money and the weight of the rig is prohibitive if your boat is sensitive to that. Very worthwhile for big ships, not worthwhile for small boats.

[Rick Willoughby]

Rick,

Are you saying that you could use up to 1000kws before you need to put an IC engine with a generator attached to it on a boat? If so, using the kw figure mentioned before, are you saying that you could run an equal size electric motor, (instead of an IC engine) of around 120hp for 10 hours and be able to have some sort of battery pack set up before needing a generator to keep the system running? If that doesn't make sense, let me ask you this. What size electric motor would it take to run as an equal to a say 100hp IC engine, and what size battery pack would it take to run that E Motor for an hour without having it charged. I am bad with electronics, but I am guessing that having 6, 12volt batteries in serries, (making a 72 volt system) along with a 3 phase system would make the draw on the system much less. And If you wanted to have this system run forever, You would have to put a IC engine and generator on the boat, what size engine and generator would it take to keep it running? wow! One more senerio Rick.....If you made it a 24 volt system, (to get rid of the 4 extra batteries weight) and just had the IC engine charge the system, How much IC Engine HP would be required to keep it charged?

Mark

To be clear on what I am saying. You can use and electric transmission in place of a simple forward reverse mechanical transmission on whatever size craft you choose.

There is an advantage offered by electric transmission in that you can decouple the diesel from the prop so the diesel works at constant rpm while the electric provides speed control. At a certain size or specific application the advantage of running the diesel in its ideal economic rev range will provide fuel efficiency in the diesel that can offset the extra losses in the electrics compared with the simple mechanical system.

To do the analysis you need to have a specific application in mind. The ice breaker is a good example because it can have very wide load demands on the prop. There may be times when the prop requires very high torque at low rpm. Using the electric transmissions offers this advantage. A diesel engine swinging a big prop, able to generate the force to extricate the vessel, could be lugged down at low rpm. If you designed a mechanical system for this case then the engine would be underloaded in normal cruise.

I do think small electrical propulsion systems are practical where you have means of collecting and storing electrical energy other than using a diesel generator. There are lots of nice low voltage electrics on the market that make this viable now and something I have been dabbling with. At the present state of play you need to start from square one with the overall vessel design but there are a few examples around trying different solutions.

Rick W

[Rick Willoughby]

R..... What size electric motor would it take to run as an equal to a say 100hp IC engine, and what size battery pack would it take to run that E Motor for an hour without having it charged. I....
Mark

On this specific question. You would need a 100HP electric motor to do the same as a 100HP diesel motor providing the diesel rating is measured at the gearbox offtake. The diesel engine probably rated more like 120HP to have 100HP at the offtake.

Running a 100HP electric motor for 1hour will require about 90kWh of energy. A VRLA type battery would need to be around 200kWh to delivery that sort of power at acceptable efficiency and to give a reasonable cycle life. Most VRLA batteries are quoted at the 10 hour rate (0.1C). So 90kW on 200kWh is about 2 hour rate - not ideal. Energy density for these batteries is around 40Wh/kg. So will require a battery of roughly 5 tonne. VRLA batteries cost around $0.5/Wh so the battery will set you back about $100k.

The new lithium batteries have much better power density than the VRLA type so they would be worth considering in an application like this.

Rick W

[FAST FRED]

The new issue of Professional Boat Builder (free online) has a great article by N Calder about a fellow attempting to get "really" efficient.

Happily (for him) he has a $20,000,000 grant to play with so it could be interesting.

Basically he wants to run a diesel a 15 min every hour at full output to charge a bat set to run the boat , till the next 15 min blast.

Has a bunch of it worked out on paper , and now the cash to attempt it, so it will be interesting to read in a couple of years how it went.

FF

[Joakim]

Hi,

I was wondering which parameter is important for a marine engine which drives a propeller.

For a given boat when do I consider the torque of the Engine and when do I consider the power ?

(for a car I know that the torque allows the car to accelerate and the power to maintain a high speed)

Thanks for your help

For both a car and a boat the maximum power of the engine is vastly more important than the maximum torque of the engine.

If you only know the maximum power of the engine, you can already estimate the top speed and acceleration of your car/boat. If you only know the maximum torque of the engine, you can not estimate anything unless you know the rpm (thus power) or gearing/propeller.

Let's look at an example. You have two engines:

1. 3 liter engine providing 300 Nm torque and 50 kW power (at 1800 rpm).

2. 1,5 liter engine providing 150 Nm torque and 80 kW power (at 5300 rpm).

Both engines have a properly selected gearing/propeller. Which engine will provide better acceleration? Which engine will provide better top speed?

Joakim

[pilotdude]

Hmmm...I've spend 15 min utes on this very important topic, and I already got a headache!

Trying to address the original question, let me chirp with a slightly different angle:

hulls (shape, length)
|
v
displacement
|
v
resistance (total)
|
v
-=== VELOCITY === -
^
|
thrust
^
|
torque
^
|
power

Look at the hmmm... tabulated text above:

First you determine the hulls shape and length, then you get the displacement, which in conjunction with the hull parameters will give you the total resistance trough the water (frictional, wave, etc).

Those are the factors that prevent the boat form achieving the velocity you want.

So to counter the resistance, you will need some thrust (very important, applied in the right direction!), which thrust is created by the propeller, spinning on a shaft, which shaft is turned by a motor and it is the moment created by the motor called "torque" that has to be able to turn the said propeller.

So now you have how much torque one needs to get into to the propeller so it can create the thrust needed to overcome the resistance so the hulls could be pushed at certain velocity question answered.

What about the power, I hear - well, the power is a derivative number, derived from the torque and the RPM at which a particular shaft rotates. Nothing more. Really not important!

That's an engineer's perspective though. If you ask a diesel engine (or boat) salesmen, you may get another answer. Or a bullshit. Or both.

[gonzo]

The total HP needed is the basic specification. You can adjust the torque output with the gear ratio

[pilotdude]

I would like to add something to my previous post.

The HP is an economic parameter. If you are interested in how much work will be done and how much (in resources) that will cost you, then you should ask for the HP parameter.

One big problem with quoting HP as a solution for a force problem is that one never knows the losses in the system, hence one can never be certain what exact force that quoted HP produces.

The vendors of diesel engines quote max HP, sometimes rated HP (rated for certain time), not that often they mention recommended HP settings (i.e. for cruising). But one has to look in their engine graphs to try to find the actual force (torque) the engine will produce at the actual HP settings, the RPM being the usual indicator of the HP settings.

To conclude, if you are interested in determining the amount of power you have on board (irrespectively if you can use it all), then the HP is your question, if you are interested in how much of that power is converted into motion, then the torque is your question.

[TollyWally]

I was under the impression that discounting subs the original reason for going diesel electric was that a certain large size it was cheaper than building a gear that could handle the load. On another note I believe that on a dyno the horsepower recorded is actually mathmatically derived from the actual measured torque and rpms. I am interested on others opinions on these factoids lurking in the recesses of my memory.

[Ad Hoc]

"...The vendors of diesel engines quote max HP, sometimes rated HP (rated for certain time), not that often they mention recommended HP settings (i.e. for cruising)..."

When ever i pick up my MTU/MAN or other booklet, clearly shows the rating of each engine and hence the duty, beit: cruise, workboat, fast ferry, long voyage etc.

[gonzo]

Engines are rated as 1,2,3 etc. duty. Each has a % of top RPM duty.

[pilotdude]

I am sure you differentiate between HP rating and engine rating.