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Old 14-07-2010, 18:40   #16
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Yes, the diesel engine is a constant RPM device, but the whole thing, engine, prop, boat is more like constant HP. Slowing the boat a couple of knots does not seem to load up the engine significantly more, or use more fuel, like it would in say an automobile.
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Old 14-07-2010, 18:45   #17
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Engine manufacturers provide in the the manuals that come with the engine (More so PDF's now in their websites) the curves that show gallons per hour versus RPM. There is no need to guess at this.
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Old 14-07-2010, 18:51   #18
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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.
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Old 14-07-2010, 18:53   #19
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Our marine diesels are constant RPM engines, right? So fuel burn is really a product of load and RPM? So, disregarding prop slip for a minute, 2800 Rpm should give you a constant speed, regardless of a clean or dirty bottom, but a dirty bottom would burn more fuel as the engine governor supplies more fuel to maintain the 2800 RPM due to the extra load caused by the resistance?
I had a conversation about this with a Yanmar rep at a boat show a bunch of years ago, when I was deciding how to fit out a boat. His point was to show me the chart on the manual that showed fuel consumption at various RPMs. He said, "If you put this engine on a crab crusher and run it at 2,000 RPMs, it's gonna get .5 GPH. And then if you rip it out of that crab crusher and put it into an ultra-light, at 2,000 RPMs it's still gonna get .5 GPH. The engine don't care how fast the boat's going, or how much the boat displaces, or how many hulls you have. it just cares about RPMs."
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Old 14-07-2010, 18:55   #20
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There is no need to guess at this.
They don't show gallons per hour while cruising long distances which was the OP's original question.
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Old 14-07-2010, 18:55   #21
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The engine manufacturers assume (rationally)that the propeller is properly pitched so that when the engine reaches maximum RPM, it is very close to its maximum horsepower. Some engine manufacturers will not honor the warranty of an engine with a propeller that is over pitched where the engine is incapable of reaching its maximum RPM...(for reasons of lugging an engine and making it impossible to burn the carbon out occasionally).

The manufacturers curves do show gallons per hour versus RPM. Its up to the owner to make the basic calculation to determine which RPM achieves which speed through the water. With this knowledge and knowing how much fuel capacity the boat holds, range can be determined. Its the owners responsibility to use this data to determine range. This involves plotting your own curves which eventually can get a skipper an RPM versus range curve for his boat. Of course in reality to determine range, one would have to figure for current, hull fouling, windage and sea state...or just look at the GPS and see what your SOG is.
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Old 14-07-2010, 19:00   #22
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You've heard a variation of 1/2 to 1 gph. If that nets you 7 knots you have a range of 175-350 miles. I suspect the ICW has pretty frequently spaced facilities.

I'd carry a couple of 5 gallon jugs and carry on.

You should get an accurate measure of your consumption at the earliest opportunity.
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Old 14-07-2010, 21:15   #23
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Can you "stick" the tank?

I have a stick....graduated in gallon increments.....fill tank.....run engine for two hours....stick tank....voila.

I like it simple
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Old 14-07-2010, 23:00   #24
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Can you "stick" the tank?

I have a stick....graduated in gallon increments.....fill tank.....run engine for two hours....stick tank....voila.

I like it simple
I like the simple stick method too. Problem is my tanks are not rectangular. They are five sided with a small flat spot at the bottom of the hull then they angle up the hull and finally turn up parallel to the inboard side of the tank. I have a stick that is tiked off for every five gallons. The first five gallons takes 4", second and third take 2 3/8" of stick, fourth and fifth 2 1/8", then 1 7/8" and finally 1 3/8". It would have taken me a long time to do it for each gallon but maybe it is worth it.
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Old 14-07-2010, 23:24   #25
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wow, really? are we all getting that small a return on the published HP specs for our engines? (not calling you a liar - it's an honest question)

No, he's right. Rated horsepower is peak horsepower, which occurs at wide open throttle and near or at redline. You won't cruise wide open, hence you will rarely use the full rated power of your engine.

We cruise at 7.5 knots or so in calm conditions, at about 2400 rpm or 2/3 of rated continuous speed. I reckon like that we're using maybe 30 horsepower out of rated 100. And indeed we burn just about 6 liters an hour or 1.5 gallons, just like what the formula says. Iif we kick it up to 3000rpm, we gain a knot of speed but fuel consumption doubles to about 3 gallons/hr. That's because as you approach hull speed the amount of power required to gain a little increment of speed goes up exponentially.
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Old 14-07-2010, 23:32   #26
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They don't show gallons per hour while cruising long distances which was the OP's original question.
The curves will be very close if the prop is reasonably well pitched.

For motoring, under most conditions, clean hull, dirty hull, calm, stormy, head wind, tail wind. The curves don't tell you MILES, because speed will vary depending on those factors. But time versus rpm will gibe
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Old 14-07-2010, 23:43   #27
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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). If you want to maintain the same speed as you would have with a clean bottom or favorable current or sea conditions, you have to increase your rpm, and therefore your fuel usage (except in the down river scenario where you could maintain speed by reducing rpm and fuel burn, but who wants to do that)..

The governor for most boat diesels controlled by a throttle would be of the variable speed type and controls the fuel flow to maintain the rpm set by the operator using the throttle and tachometer, as opposed to a constant speed or load-limiting governor. A diesel generator would usually have a constant speed type to maintain the optimum rpm for fuel efficiency, while a diesel driving a pump at a pumping station would probably have a load limiting type governor to maintain rpm above minimum but below maximum load.

In a boat diesel where the operator controls speed through a throttle, the governor is normally a variable speed type but at the same time is also a load limiting type in so far as it has minimum and maximum limits--minimum being idle speed and maximum being the highest rpm set by the manufacturer. The throttle controls any speed between those two points, making the operator the ultimate governor himself.
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Old 15-07-2010, 00:01   #28
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...a throttle...the throttle..a throttle...The throttle
You probably meant the fuel control?
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Old 15-07-2010, 01:49   #29
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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.
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Old 15-07-2010, 02:49   #30
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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).
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
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