All else being equal, which would get better fuel economy, a 30 hp motor running at full speed or a 60 hp motor running at half speed?
[ 03-27-2003, 06:32 PM: Message edited by: Rocky ]
All else being equal, which would get better fuel economy, a 30 hp motor running at full speed or a 60 hp motor running at half speed?
[ 03-27-2003, 06:32 PM: Message edited by: Rocky ]
All else being equal, which would get better fuel economy, a 30 hp motor running at full speed or a 60 hp motor running at half speed?
[ 03-27-2003, 06:32 PM: Message edited by: Rocky ]
All else being equal, which would get better fuel economy, a 30 hp motor running at full speed or a 60 hp motor running at half speed?
[ 03-27-2003, 06:32 PM: Message edited by: Rocky ]
Ummmm.. What?
The 30 horse. This is the reason why all new cruise ships built are diesel-electric. A diesel-electric power plant is actually less efficient, horsepower for horsepower, than one where the engine has a hard connection to the prop shaft. The difference, however, comes where you vary the load (not to mention the fact that an electric plant can be put wherever you want it. One Royal Carribean ship has a gas turbine generator up in the stack). Most cruise ships never run at full power, or if they do, not for long. When running at partial speed, they shut down one or more generator and run the remaining ones at full load, which is inherently more efficient. The QE2 repowered from steam to nine diesel gen-sets and easily covered the cost of conversion in the fuel savings.
You have to remember that you're getting at most a third of the available energy of the fuel as generated power out of the engine. The rest is lost through the cooling system, auxiliaries, and the general inefficiency of a piston engine. Even though you're generating only thirty horses out of the sixty horse engine, you're still running a larger cooling pump, alternator, bigger engine itself, and operating below the designed peak efficiency.
That help?
Jeff
[ 03-27-2003, 08:00 PM: Message edited by: JeffH ]
Ummmm.. What?
The 30 horse. This is the reason why all new cruise ships built are diesel-electric. A diesel-electric power plant is actually less efficient, horsepower for horsepower, than one where the engine has a hard connection to the prop shaft. The difference, however, comes where you vary the load (not to mention the fact that an electric plant can be put wherever you want it. One Royal Carribean ship has a gas turbine generator up in the stack). Most cruise ships never run at full power, or if they do, not for long. When running at partial speed, they shut down one or more generator and run the remaining ones at full load, which is inherently more efficient. The QE2 repowered from steam to nine diesel gen-sets and easily covered the cost of conversion in the fuel savings.
You have to remember that you're getting at most a third of the available energy of the fuel as generated power out of the engine. The rest is lost through the cooling system, auxiliaries, and the general inefficiency of a piston engine. Even though you're generating only thirty horses out of the sixty horse engine, you're still running a larger cooling pump, alternator, bigger engine itself, and operating below the designed peak efficiency.
That help?
Jeff
[ 03-27-2003, 08:00 PM: Message edited by: JeffH ]
Ummmm.. What?
The 30 horse. This is the reason why all new cruise ships built are diesel-electric. A diesel-electric power plant is actually less efficient, horsepower for horsepower, than one where the engine has a hard connection to the prop shaft. The difference, however, comes where you vary the load (not to mention the fact that an electric plant can be put wherever you want it. One Royal Carribean ship has a gas turbine generator up in the stack). Most cruise ships never run at full power, or if they do, not for long. When running at partial speed, they shut down one or more generator and run the remaining ones at full load, which is inherently more efficient. The QE2 repowered from steam to nine diesel gen-sets and easily covered the cost of conversion in the fuel savings.
You have to remember that you're getting at most a third of the available energy of the fuel as generated power out of the engine. The rest is lost through the cooling system, auxiliaries, and the general inefficiency of a piston engine. Even though you're generating only thirty horses out of the sixty horse engine, you're still running a larger cooling pump, alternator, bigger engine itself, and operating below the designed peak efficiency.
That help?
Jeff
[ 03-27-2003, 08:00 PM: Message edited by: JeffH ]
Senior Member
Not sure I agree with you there, if you have a look at the specific fuel consumption figures in any of the engine spec pamphets you will see that there is a point at which the fuel consumption per hp/hour graph is well down on the peak power consumption. At the risk of sounding repetitive this is grams per KW Hour or oz per HP hour. On the Gardner that is going into my own boat that figure is at 1200 rpm, the motor develops peak power at 1700 rpm.
I do agree with the situation on motors where the load varies a lot, but in VLCCs where the cruise speed varies only a little they run direct drive low speed diesels, and usually only one of them in a 50 000 ton plus ship. the specific fuel consumption per hp hour by the way is hugely better than the figures that we are used to in our smaller motors and they make car engines look like total spendthrifts ( let alone 2stroke outboard motors).
JohnW
An expert is but a beginner with experience.
Senior Member
Not sure I agree with you there, if you have a look at the specific fuel consumption figures in any of the engine spec pamphets you will see that there is a point at which the fuel consumption per hp/hour graph is well down on the peak power consumption. At the risk of sounding repetitive this is grams per KW Hour or oz per HP hour. On the Gardner that is going into my own boat that figure is at 1200 rpm, the motor develops peak power at 1700 rpm.
I do agree with the situation on motors where the load varies a lot, but in VLCCs where the cruise speed varies only a little they run direct drive low speed diesels, and usually only one of them in a 50 000 ton plus ship. the specific fuel consumption per hp hour by the way is hugely better than the figures that we are used to in our smaller motors and they make car engines look like total spendthrifts ( let alone 2stroke outboard motors).
JohnW
An expert is but a beginner with experience.
Senior Member
Not sure I agree with you there, if you have a look at the specific fuel consumption figures in any of the engine spec pamphets you will see that there is a point at which the fuel consumption per hp/hour graph is well down on the peak power consumption. At the risk of sounding repetitive this is grams per KW Hour or oz per HP hour. On the Gardner that is going into my own boat that figure is at 1200 rpm, the motor develops peak power at 1700 rpm.
I do agree with the situation on motors where the load varies a lot, but in VLCCs where the cruise speed varies only a little they run direct drive low speed diesels, and usually only one of them in a 50 000 ton plus ship. the specific fuel consumption per hp hour by the way is hugely better than the figures that we are used to in our smaller motors and they make car engines look like total spendthrifts ( let alone 2stroke outboard motors).
JohnW
An expert is but a beginner with experience.
Twelve years and counting
Well I agree with both of you!
A marine diesel is usually optimised for 90% of maximum continuous rating, but the current generation of big crosshead two strokes have remarkably flat power/consumption curves.
Even so, you are only going to recover the maximum waste heat through the exhaust gas boiler, shaft generator if fitted, and so on at 90% MCR which is also the point at which the turbocharger is going to feel happiest.
Jeff is dead right about cruise ships; they have to spend all day at anchor pretending to be a power station and then sprint overnight to the next port, whilst a VLCC has to poodle along at 14 knots from one week to the next.
I was part of a team who looked into diesel-electric VLCCs because we wanted to abolish the boiler, which is only needed to run the cargo pumps, and put the engines somewhere more sensible, but we could never quite get it to add up.
IMAGINES VEL NON FUERINT
Twelve years and counting
Well I agree with both of you!
A marine diesel is usually optimised for 90% of maximum continuous rating, but the current generation of big crosshead two strokes have remarkably flat power/consumption curves.
Even so, you are only going to recover the maximum waste heat through the exhaust gas boiler, shaft generator if fitted, and so on at 90% MCR which is also the point at which the turbocharger is going to feel happiest.
Jeff is dead right about cruise ships; they have to spend all day at anchor pretending to be a power station and then sprint overnight to the next port, whilst a VLCC has to poodle along at 14 knots from one week to the next.
I was part of a team who looked into diesel-electric VLCCs because we wanted to abolish the boiler, which is only needed to run the cargo pumps, and put the engines somewhere more sensible, but we could never quite get it to add up.
IMAGINES VEL NON FUERINT
Twelve years and counting
Well I agree with both of you!
A marine diesel is usually optimised for 90% of maximum continuous rating, but the current generation of big crosshead two strokes have remarkably flat power/consumption curves.
Even so, you are only going to recover the maximum waste heat through the exhaust gas boiler, shaft generator if fitted, and so on at 90% MCR which is also the point at which the turbocharger is going to feel happiest.
Jeff is dead right about cruise ships; they have to spend all day at anchor pretending to be a power station and then sprint overnight to the next port, whilst a VLCC has to poodle along at 14 knots from one week to the next.
I was part of a team who looked into diesel-electric VLCCs because we wanted to abolish the boiler, which is only needed to run the cargo pumps, and put the engines somewhere more sensible, but we could never quite get it to add up.
IMAGINES VEL NON FUERINT
Senior Member
In the case of a small yacht, won't the answer depend on the design of the hull, and the amount of power needed to drive it to hull speed?
In other words, if you're talking about a displacement hull that takes 50 hp to get to hull speed, you may get a different answer than you would for a planing skiff.
Senior Member
In the case of a small yacht, won't the answer depend on the design of the hull, and the amount of power needed to drive it to hull speed?
In other words, if you're talking about a displacement hull that takes 50 hp to get to hull speed, you may get a different answer than you would for a planing skiff.
Senior Member
In the case of a small yacht, won't the answer depend on the design of the hull, and the amount of power needed to drive it to hull speed?
In other words, if you're talking about a displacement hull that takes 50 hp to get to hull speed, you may get a different answer than you would for a planing skiff.
Ummmm.. What?
Yep, but the question was "all things being equal." Since marine diesels are more my line of familiarity, I unconciously assume that's what we're talking about. They are often designed to be most efficient running at or near full power for long periods of time, and as such, all things being equal, an engine running at designed power will have less fuel consumption than an engine twice the size running at half power. In the real world, of course, all things are never equal. Is the 60 horse a turbocharged version of the 30 horse? In that case, the engines would probably share the same block, and both would have wildly different fuel consumption curves. Gas or diesel? Gas engines derived from standard automobile blocks of course aren't designed to be most efficient at peak power (how many people drive around with the gas pedal floored all the time? Outside of New Jersey, that is). Or would the 30 horse be a four cylinder and the 60 six? There's another answer. Two stroke? Four stroke? Direct injected two stroke?. Is this a boat that has a max rating for 30 horse, and you want to put in a 60 horse at half power in the hope of better fuel consumption (bad idea)? Comparing a boat designed for a 30 horse running at full power to a boat designed for 60 horse running at half power is, of course, apples and oranges. This could become an insanely complicated answer if you really wanted to get into engine design and power curves and so forth and depends entirely on what the engine is designed to do.
Jeff
Ummmm.. What?
Yep, but the question was "all things being equal." Since marine diesels are more my line of familiarity, I unconciously assume that's what we're talking about. They are often designed to be most efficient running at or near full power for long periods of time, and as such, all things being equal, an engine running at designed power will have less fuel consumption than an engine twice the size running at half power. In the real world, of course, all things are never equal. Is the 60 horse a turbocharged version of the 30 horse? In that case, the engines would probably share the same block, and both would have wildly different fuel consumption curves. Gas or diesel? Gas engines derived from standard automobile blocks of course aren't designed to be most efficient at peak power (how many people drive around with the gas pedal floored all the time? Outside of New Jersey, that is). Or would the 30 horse be a four cylinder and the 60 six? There's another answer. Two stroke? Four stroke? Direct injected two stroke?. Is this a boat that has a max rating for 30 horse, and you want to put in a 60 horse at half power in the hope of better fuel consumption (bad idea)? Comparing a boat designed for a 30 horse running at full power to a boat designed for 60 horse running at half power is, of course, apples and oranges. This could become an insanely complicated answer if you really wanted to get into engine design and power curves and so forth and depends entirely on what the engine is designed to do.
Jeff
Ummmm.. What?
Yep, but the question was "all things being equal." Since marine diesels are more my line of familiarity, I unconciously assume that's what we're talking about. They are often designed to be most efficient running at or near full power for long periods of time, and as such, all things being equal, an engine running at designed power will have less fuel consumption than an engine twice the size running at half power. In the real world, of course, all things are never equal. Is the 60 horse a turbocharged version of the 30 horse? In that case, the engines would probably share the same block, and both would have wildly different fuel consumption curves. Gas or diesel? Gas engines derived from standard automobile blocks of course aren't designed to be most efficient at peak power (how many people drive around with the gas pedal floored all the time? Outside of New Jersey, that is). Or would the 30 horse be a four cylinder and the 60 six? There's another answer. Two stroke? Four stroke? Direct injected two stroke?. Is this a boat that has a max rating for 30 horse, and you want to put in a 60 horse at half power in the hope of better fuel consumption (bad idea)? Comparing a boat designed for a 30 horse running at full power to a boat designed for 60 horse running at half power is, of course, apples and oranges. This could become an insanely complicated answer if you really wanted to get into engine design and power curves and so forth and depends entirely on what the engine is designed to do.
Jeff
Senior Member
Engine efficiency and fuel economy are two different things on a boat. Even if all things are equal.
Here's my point. All things being equal, assume you have a boat that requires 15 hp to get to hull speed. And assume you have a 30hp diesel.
The results:
1. You will get better fuel economy running the engine at "half-speed", just enough throttle to squeeze out 15 hp and get the boat to hull speed.
2. The engine, taken alone without the boat, will produce hp more efficiently at full or cruising throttle.
My point is that most small displacement boats are overpowered, so running the engine at its most efficient speed will not get you the best fuel milage. You would end up with a lot of wasted hp, and burnt fuel, trying to push the boat faster than hull speed.
Like all seemingly simple questions, the answer is "depends."
[ 04-27-2003, 08:26 AM: Message edited by: Scott Rosen ]
Senior Member
Engine efficiency and fuel economy are two different things on a boat. Even if all things are equal.
Here's my point. All things being equal, assume you have a boat that requires 15 hp to get to hull speed. And assume you have a 30hp diesel.
The results:
1. You will get better fuel economy running the engine at "half-speed", just enough throttle to squeeze out 15 hp and get the boat to hull speed.
2. The engine, taken alone without the boat, will produce hp more efficiently at full or cruising throttle.
My point is that most small displacement boats are overpowered, so running the engine at its most efficient speed will not get you the best fuel milage. You would end up with a lot of wasted hp, and burnt fuel, trying to push the boat faster than hull speed.
Like all seemingly simple questions, the answer is "depends."
[ 04-27-2003, 08:26 AM: Message edited by: Scott Rosen ]
Senior Member
Engine efficiency and fuel economy are two different things on a boat. Even if all things are equal.
Here's my point. All things being equal, assume you have a boat that requires 15 hp to get to hull speed. And assume you have a 30hp diesel.
The results:
1. You will get better fuel economy running the engine at "half-speed", just enough throttle to squeeze out 15 hp and get the boat to hull speed.
2. The engine, taken alone without the boat, will produce hp more efficiently at full or cruising throttle.
My point is that most small displacement boats are overpowered, so running the engine at its most efficient speed will not get you the best fuel milage. You would end up with a lot of wasted hp, and burnt fuel, trying to push the boat faster than hull speed.
Like all seemingly simple questions, the answer is "depends."
[ 04-27-2003, 08:26 AM: Message edited by: Scott Rosen ]
Here's another one Scott, 60' tug; 550hp@1200, 175hp required for hull speed. On long run,(Seattle to Alaska) fuel comp increased until I increased power for short time, then returned to normal. Know why?
Terry
Here's another one Scott, 60' tug; 550hp@1200, 175hp required for hull speed. On long run,(Seattle to Alaska) fuel comp increased until I increased power for short time, then returned to normal. Know why?
Terry
Here's another one Scott, 60' tug; 550hp@1200, 175hp required for hull speed. On long run,(Seattle to Alaska) fuel comp increased until I increased power for short time, then returned to normal. Know why?
Terry
Senior Member
Clogged fuel injectors?
Senior Member
Clogged fuel injectors?
Senior Member
Clogged fuel injectors?
Lion
Terry,
My guess is an increase in operating temp to near optimum which held on for a while after the burst of high power which would normally improve fuel consumption. Thermostat might have needed tweaking, or maybe the higher operating temps were adverse to longevity and reliability.
This issue is not always as simple as it seems.
Lion
Lion
Terry,
My guess is an increase in operating temp to near optimum which held on for a while after the burst of high power which would normally improve fuel consumption. Thermostat might have needed tweaking, or maybe the higher operating temps were adverse to longevity and reliability.
This issue is not always as simple as it seems.
Lion
Lion
Terry,
My guess is an increase in operating temp to near optimum which held on for a while after the burst of high power which would normally improve fuel consumption. Thermostat might have needed tweaking, or maybe the higher operating temps were adverse to longevity and reliability.
This issue is not always as simple as it seems.
Lion
When was the last time all things were equal? I missed it.
No individual rain-drop thinks it\'s responsible for the flood.
When was the last time all things were equal? I missed it.
No individual rain-drop thinks it\'s responsible for the flood.
When was the last time all things were equal? I missed it.
No individual rain-drop thinks it\'s responsible for the flood.
Lion:
Your right! Engine is 8-268A GM Cleveland and runs cold at hull crusing speed, possibily loading up. I am installing piros to keep track and adjust engine while running. Keeping engine at the correct temps decreases fuel burn.
Good thinking!
Terry
PS Headed to Port Angeles to work on boat, no internet till I get back, see you guys next week.
Lion:
Your right! Engine is 8-268A GM Cleveland and runs cold at hull crusing speed, possibily loading up. I am installing piros to keep track and adjust engine while running. Keeping engine at the correct temps decreases fuel burn.
Good thinking!
Terry
PS Headed to Port Angeles to work on boat, no internet till I get back, see you guys next week.
Lion:
Your right! Engine is 8-268A GM Cleveland and runs cold at hull crusing speed, possibily loading up. I am installing piros to keep track and adjust engine while running. Keeping engine at the correct temps decreases fuel burn.
Good thinking!
Terry
PS Headed to Port Angeles to work on boat, no internet till I get back, see you guys next week.
Best fuel effiency is at peak torque rpm. On a gas engine, this is usually about 65% or 70% of max hp rpm; on a diesel, it's usually about 85% to 90% of max hp rpm.
Designers can design engines all over the place, but usually that's where they tend to like to end up.
FWIW, for a *typical* recreational marine diesel (or recreational gas engine), a 30 hp engine run WFO won't last begin to last as long as a 60 hp engine run at 30 hp, assuming engines designed to the same hp/displacement dimensions. Designed to the same piston speeds (at 30 hp vs 60 hp), the 60 hp will last longer, though not by all that much.
FWIW-2, many ultra-long life industrial diesels, are detuned industrial engines, are detuned recreational engines, are detuned performance engines.
Designed life of an engine is a design criteria going into the design, not coming out.
Want long engine life? Cubic inches/cubic pounds. Nothing beats a warm, well-tuned engine with low piston speed and good bearing, lubricated well.
Best fuel effiency is at peak torque rpm. On a gas engine, this is usually about 65% or 70% of max hp rpm; on a diesel, it's usually about 85% to 90% of max hp rpm.
Designers can design engines all over the place, but usually that's where they tend to like to end up.
FWIW, for a *typical* recreational marine diesel (or recreational gas engine), a 30 hp engine run WFO won't last begin to last as long as a 60 hp engine run at 30 hp, assuming engines designed to the same hp/displacement dimensions. Designed to the same piston speeds (at 30 hp vs 60 hp), the 60 hp will last longer, though not by all that much.
FWIW-2, many ultra-long life industrial diesels, are detuned industrial engines, are detuned recreational engines, are detuned performance engines.
Designed life of an engine is a design criteria going into the design, not coming out.
Want long engine life? Cubic inches/cubic pounds. Nothing beats a warm, well-tuned engine with low piston speed and good bearing, lubricated well.
Best fuel effiency is at peak torque rpm. On a gas engine, this is usually about 65% or 70% of max hp rpm; on a diesel, it's usually about 85% to 90% of max hp rpm.
Designers can design engines all over the place, but usually that's where they tend to like to end up.
FWIW, for a *typical* recreational marine diesel (or recreational gas engine), a 30 hp engine run WFO won't last begin to last as long as a 60 hp engine run at 30 hp, assuming engines designed to the same hp/displacement dimensions. Designed to the same piston speeds (at 30 hp vs 60 hp), the 60 hp will last longer, though not by all that much.
FWIW-2, many ultra-long life industrial diesels, are detuned industrial engines, are detuned recreational engines, are detuned performance engines.
Designed life of an engine is a design criteria going into the design, not coming out.
Want long engine life? Cubic inches/cubic pounds. Nothing beats a warm, well-tuned engine with low piston speed and good bearing, lubricated well.
Senior Member
Jax got it right on. Each engine has its own performance curves, and best efficiency is achieved at the peak of the torque curve. On many engines the torque curve drops fast past its peak, giving lousy efficiency at full throttle. Ideally a displacement boat will be powered/propped to achieve about 90% of its theoretical hull speed at the RPM that coincides with the torque peak.
Senior Member
Jax got it right on. Each engine has its own performance curves, and best efficiency is achieved at the peak of the torque curve. On many engines the torque curve drops fast past its peak, giving lousy efficiency at full throttle. Ideally a displacement boat will be powered/propped to achieve about 90% of its theoretical hull speed at the RPM that coincides with the torque peak.
Senior Member
Jax got it right on. Each engine has its own performance curves, and best efficiency is achieved at the peak of the torque curve. On many engines the torque curve drops fast past its peak, giving lousy efficiency at full throttle. Ideally a displacement boat will be powered/propped to achieve about 90% of its theoretical hull speed at the RPM that coincides with the torque peak.
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