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Old 15-07-2010, 02:59   #31
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Originally Posted by RunningFish View Post
Do engine manufacturers that provide fuel burn vs RPM assume full load?

I'm pretty certain that if I motorsail at 1800 RPM my fuel burn would be less than if I motored into the wind at the same engine speed.
Motorsailing doesn't count because you unload the drive train.

If you are motoring with no other means of propulsion, the burn rate per hour (NOT per mile) will closely match the factory fuel consumption curves, assuming your prop is reasonably well pitched.
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Old 15-07-2010, 03:02   #32
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Originally Posted by Astrid View Post
At a given rpm, an engine will burn x amount of fuel per hour. In calm water and with a clean bottom and neutral current and wind that rpm will produce a certain speed. Change any of the parameters, and though engine rpm and fuel bun remains the same, speed will be affected and you will be burning the same amount per hour but not going as far in that time (or if you have a current in the same direction as you are headed, as in down a river, you might actually go faster at that rpm and fuel burn rate).
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G'Day,

If this was true, then running your engine in neutral at, say 2000 RPM would burn the same amout of fuel as running in gear and loading the engine. I don't think so...

Cheers,

Jim
No, she is exactly right, and said it better than any of the rest of us. Motor sailing doesn't count, and neutral gear doesn't count.

Motoring in gear will provide constant fuel burn rate per hour at any given RPM. Burn rate per mile will vary according to conditions, but not per hour.
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Old 15-07-2010, 03:11   #33
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Originally Posted by Jim Cate View Post
G'Day,

If this was true, then running your engine in neutral at, say 2000 RPM would burn the same amout of fuel as running in gear and loading the engine. I don't think so...

Cheers,

Jim
I fully agree with that. Yes, diesel engines are governed. So the rpm will not change at a certain throttle setting when the load changes. What will change is the amount of fuel being squirted in the cylinder per 2 strokes (assuming 4 stroke) to maintain the revs. Maintaining 2000 rpm when tied to the dock will use more fuel than motor sailing at 2000 rpm or indeed running the engine in neutral at 2000 rpm. All that is taken care of by the governor; it maintains a certain engine speed by varying the amount of fuel injected.
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Old 15-07-2010, 03:18   #34
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Originally Posted by Jim Cate View Post

No, she is exactly right, and said it better than any of the rest of us. Motor sailing doesn't count, and neutral gear doesn't count.

Motoring in gear will provide constant fuel burn rate per hour at any given RPM. Burn rate per mile will vary according to conditions, but not per hour.
Sorry, I have to disagree with you. Fuel consumption will vary with the loading of the engine. Depending on weather the engine will need more or less fuel to run at a certain rpm. Motoring against the wind at say 2000 rpm will burn more fuel per hour (higher resistance on the prop since it has to work harder) and will result in reduced speed as well when compared to motoring on flat water with no wind. More slip at the prop and more HP to maintain 2000 rpm means more fuel both per mile and per hour.
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Old 15-07-2010, 05:14   #35
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Originally Posted by sigmasailor View Post

Sorry, I have to disagree with you. Fuel consumption will vary with the loading of the engine. Depending on weather the engine will need more or less fuel to run at a certain rpm. Motoring against the wind at say 2000 rpm will burn more fuel per hour (higher resistance on the prop since it has to work harder) and will result in reduced speed as well when compared to motoring on flat water with no wind. More slip at the prop and more HP to maintain 2000 rpm means more fuel both per mile and per hour.
When you say that fuel consumption will vary with the loading of the engine, you are exactly right.

But loading of the engine is an almost direct function of RPM if you are motoring, no matter what are the conditions, as long as you are indeed motoring, and not idling or motor-sailing.

Motoring against the wind at 2000 rpm will give you less speed but almost exactly the same fuel burn per hour, as motoring at 2000 rpm in flat water. I don't know why you think the prop has to "work harder". It moves a certain amount of water at a certain RPM, and this moving water creates thrust just like a jet engine, or more precisely, like the propellor on a prop-driven airplane. Look behind your boat sometime and you can see the stream of water shooting out behind the boat, moved by your prop. The thrust which is produced by this stream of water will result in different degrees of speed depending on how much the boat is resisting forward motion. There may be some tiny variation based on the relative speed between the prop and the mass of water ahead of it, but it makes little difference.

If it were otherwise, then you would see black smoke and engine overloading in tough conditions at a given RPM, but not under milder conditions. We don't see this; we see black smoke and overloading when a prop is overpitched, and an overpitched prop will produce this effect in all conditions, tough or mild.
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Old 15-07-2010, 05:23   #36
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Originally Posted by sigmasailor View Post
Any decent diesel should get something like 1 HP for an hour using 200 grams. Suppose you use 10 HP to cruise that would need 2000 grams or about half a gallon per hour.
RPM, rated HP alone do not say a lot; it all depends on how heavily you load your engine.
Remember that the speed in water is a third power function for the required power and thus fuel consumption. Ignoring changing efficiencies if you can maintain 6 knots using 10 hp you would need 9/6 ^3 = 3,8 * 10 = 38 HP (2 gallons per hour) for 9 knots.
If I can quote myself quoting Vigor:

"The 1/2gal/10hp is not what you HAVE but what you are using. So that will account for some of the difference.

So if you have a 40hp engine, but are running it at a 20hp rate your usage would be about 1 gal/hour.

That is the rule of thumb, so your usage should be thumbing like that."

Sounds pretty close to me. Results may vary.
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Old 15-07-2010, 05:47   #37
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We all forget about torque, max torque is usually the rpm that the engine is running at peak efficiency. This will usually give you maximum fuel economy. The more load applied to the engine the more fuel required to maintain set RPM/torque. So correct prop is a lot more important than many realise as the prop provides most of the load. HP is a very over rated form of power measurement.
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Old 15-07-2010, 06:09   #38
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Same Yanmar

My 27hp Yanmar uses less than .5 gal.per hr.[.4gph] but that is a good number to use for 'insurance'.
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Old 15-07-2010, 06:18   #39
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After following the thread, I'll bet David is sorry he asked the question. I just finished my second trip on the ICW. I was told my Universal 50 would get .75 gal per hour at 2,000 rpm and it turned out to be more like .90. For purposes of "preparation" David, you will probably get between .50 and .80. However, you will be able to calculate your individual performance within a couple days. Carry a couple 5 gal jerry cans for peace of mind but the fact is, you will find more than sufficient refueling along the way. Enjoy your voyage and hope to see you out there.
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Old 15-07-2010, 07:28   #40
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1/2 gal

I have a 3gm30 in my 33' Beneteau that displaces 14,000#.
Turning the engine at 2800rpm and doing 6.5 knots it burns 1/2 gal per hour.
This has been consistantly measured over more than 800 hours of motoring.
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Old 15-07-2010, 08:12   #41
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Originally Posted by Dockhead View Post
When you say that fuel consumption will vary with the loading of the engine, you are exactly right.

But loading of the engine is an almost direct function of RPM if you are motoring, no matter what are the conditions, as long as you are indeed motoring, and not idling or motor-sailing.

Motoring against the wind at 2000 rpm will give you less speed but almost exactly the same fuel burn per hour, as motoring at 2000 rpm in flat water. I don't know why you think the prop has to "work harder". It moves a certain amount of water at a certain RPM, and this moving water creates thrust just like a jet engine, or more precisely, like the propellor on a prop-driven airplane. Look behind your boat sometime and you can see the stream of water shooting out behind the boat, moved by your prop. The thrust which is produced by this stream of water will result in different degrees of speed depending on how much the boat is resisting forward motion. There may be some tiny variation based on the relative speed between the prop and the mass of water ahead of it, but it makes little difference.

If it were otherwise, then you would see black smoke and engine overloading in tough conditions at a given RPM, but not under milder conditions. We don't see this; we see black smoke and overloading when a prop is overpitched, and an overpitched prop will produce this effect in all conditions, tough or mild.
Sorry, but I have to disagree once more. As soon as your prop starts to slip more (for instance when motoring upwind in bad weather) it will run less efficient (more slip) and cause more drag on the engine and use up more HP to maintain the 2000 rpm. The extremes are 2000 rpm in neutral (or motor sailing where the sails deliver all or more of the the power) and 2000 rpm trying to move a dock. This could come to a point where the governor starts to squirt so much fuel in the engine that it will no longer burn all the fuel (overloading causing black smoke and reduced rpm).

It's easy when you compare it to a car (where your foot acts as the governor): going uphill you have to press harder (and use more fuel) than when going downhill if you maintain your speed and rpm.

It has to to with torque (HP = rpm x torque or HP=Watt=Nm/s = /s (RPM) x NM (foot.pound); more fuel means bigger bang on the piston, harder push on the arm of the crankshaft (= torque): you will see a rise in HP at equal rpm.
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Old 15-07-2010, 08:27   #42
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Originally Posted by sigmasailor View Post
Sorry, but I have to disagree once more. As soon as your prop starts to slip more (for instance when motoring upwind in bad weather) it will run less efficient (more slip) and cause more drag on the engine and use up more HP to maintain the 2000 rpm. The extremes are 2000 rpm in neutral (or motor sailing where the sails deliver all or more of the the power) and 2000 rpm trying to move a dock. This could come to a point where the governor starts to squirt so much fuel in the engine that it will no longer burn all the fuel (overloading causing black smoke and reduced rpm).

It's easy when you compare it to a car (where your foot acts as the governor): going uphill you have to press harder (and use more fuel) than when going downhill if you maintain your speed and rpm.

It has to to with torque (HP = rpm x torque or HP=Watt=Nm/s = /s (RPM) x NM (foot.pound); more fuel means bigger bang on the piston, harder push on the arm of the crankshaft (= torque): you will see a rise in HP at equal rpm.
But it's not like a car, not at all.

In a car you have a direct mechanical connection to the earth. So a given RPM in a given gear will always produce the same speed, but very different amounts of load depending on whether you are driving into a headwind, uphill, etc. The road is fixed to the earth and doesn't move under the influence of power transmitted through your tires.

In a boat, you do not have any mechanical connection to the earth. Your prop engages the mass of water and creates a stream of water behind you which creates thrust. So in the exact opposite situation from a car, at a given RPM you always get a constant load, but a different speed. The "road" in this case is a moving mass of water, which is not fixed, but rather is thrown out behind the boat by the prop. It doesn't "slip" to any great extent. It's not like a car's tires.

Your idea of a propellor's "slipping" is a false analogy to cars and tires. It doesn't "slip". It moves a nearly constant mass of water in a stream flowing aft of your boat. This stream of water produces a constant amount of thrust at a given RPM. Constant thrust requires constant amount of power. But 500 pounds of thrust, say, will give you very different speed, depending on how much resistance the boat has to moving forward. But the propellor doesn't feel that resistance; it just keeps putting out that stream and generating a constant amount of thrust. And that's why the power required to maintain a certain RPM, and thus fuel consumed to produce that power, is constant, well, very nearly constant (there will be a slight variation due to the speed differential between the basic mass of water and the prop).

I hate to be argumentative, but I'm right about this. It's basic hydrodynamics.
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Old 15-07-2010, 08:48   #43
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But it's not like a car, not at all.

In a car you have a direct mechanical connection to the earth. So a given RPM in a given gear will always produce the same speed, but very different amounts of load depending on whether you are driving into a headwind, uphill, etc.

In a boat, you do not have any mechanical connection to the earth. Your prop engages the mass of water and creates a stream of water behind you which creates thrust. So in the exact opposite situation from a car, at a given RPM you always get a constant load, but a different speed.

Your idea of a propellor's "slipping" is a false analogy to cars and tires. It doesn't "slip". It moves a nearly constant mass of water in a stream flowing aft of your boat. This stream of water produces a constant amount of thrust at a given RPM. Constant thrust requires constant amount of power. But 500 pounds of thrust, say, will give you very different speed, depending on how much resistance the boat has to moving forward. But the propellor doesn't feel that resistance; it just keeps putting out that stream and generating a constant amount of thrust. And that's why the power required to maintain a certain RPM, and thus fuel consumed to produce that power, is constant, well, very nearly constant (there will be a slight variation due to the speed differential between the basic mass of water and the prop).

I hate to be argumentative, but I'm right about this. It's basic hydrodynamics.
I agree with you on two of the above three points:
1) the car was a bad example, should have used planes but know nothing about those;
2) it is about hydrodynamics.

Hydrodynamics dictate that more power is used to turn a propeller when slip goes up (bad weather, towing a boat, growth, etc.). Propeller efficiency depends on how the water flows around the propeller. More slip means more drag and more loading on the engine.
You could carry out the following experiment: put out a stern anchor and put the engine on 2000 rpm (look at how far the governor pushes the fuel pump); now let go of the anchor and keep looking at what happens at your fuel pump. As soon as the speed pick up you can probably feel and hear the engine relaxing.

Don't worry about being argumentative; it makes my brain work and thats only good.
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Old 15-07-2010, 09:30   #44
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It's a little different with airplanes, because their very high speed compared to boats means that there is a big proportional difference of speed between the stream of air pushed back by the propeller and the airplane's airspeed, at different airspeeds. So the thrust of the propeller falls off as the airspeed of the plane approaches speed of the stream of air.

Speed of boats varies from 0 knots to only to hull speed, which is peanuts, so speed through the water has relatively little effect on thrust-power generated by the stream of water thrown out by the prop, proportionately. That's why thrust-power is almost constant for a given RPM. I've got the formulas somewhere.
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Old 15-07-2010, 09:36   #45
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I agree with you on two of the above three points:
1) the car was a bad example, should have used planes but know nothing about those;
2) it is about hydrodynamics.

Hydrodynamics dictate that more power is used to turn a propeller when slip goes up (bad weather, towing a boat, growth, etc.). Propeller efficiency depends on how the water flows around the propeller. More slip means more drag and more loading on the engine.
You could carry out the following experiment: put out a stern anchor and put the engine on 2000 rpm (look at how far the governor pushes the fuel pump); now let go of the anchor and keep looking at what happens at your fuel pump. As soon as the speed pick up you can probably feel and hear the engine relaxing.

Don't worry about being argumentative; it makes my brain work and thats only good.
You're still assuming that the the propeller is acting in water which sits still so that the propeller is kind of screwing the boat through the water. That would make the whole scenario more like a car. But the water in which the propeller acts does not sit still. It is impelled out in a stream behind the boat. So the interaction between prop and water stream will not vary too much according to boat speed.

Your experiment seems to me to be correctly designed. I run my engine alongside in gear from time to time, which is pretty analogous to your anchor example. You can see the stream of water shooting out behind the boat, even though the boat is fixed to the dock. The engine is stressed just about the same as it is underway at the same RPM. That's because the behavior of that stream of water does not change dramatically according to boat speed. It keeps shooting merrily out behind, even though the boat is not moving; it is not sitting there and straining against the water like a car being run too slow in a high gear. That's because even though the boat is standing still in relation to the larger mass of water, the prop is still running at speed through that stream of water which it is inducing, producing pretty much the same amount of thrust it would be underway, and thus consuming about the same amount of fuel.

But it's a good idea to check the fuel pump -- a good objective confirmation of all of this.
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