Diesel fuel consumption?

[jviss]

Does anyone watch their fuel consumption carefully? I noted that I burnt 0.44 gallons per hour while motoring at about 5 knots last week, 1800 to 2000 engine RPM, over a six hour period (wind right on the nose). This is a Catalina 36, 6 tons or so, Universal M25, 21 HP three cylinder direct injection, 2.1:1 gear, 13x15 two blade Sailor prop.I'm curious about fuel use idling, charging batts, motoring. Thanks,jv

 

[Stu Jackson]

It doesn't matter much

The overall consumption is what you need to know, since you'll be using the engine in a variety of uses over time. Our M25 uses exactly what you said, 0.44 gph, measured over eight years. Our fuel gage is kaput and will stay that way. Every 20 hours, we fill up with 9 to 10 gallons. Very simple.

 

[Ross]

Generally diesel engines use about 9 ounces

of fuel per hour per horsepower. That seems to hold true for all size engines. If you are using .44 gallons per hour that is about 56 ounces per hour that comes out to about 6 horse power delivered by the engine. I checked Boatdiesel calculator and their numbers are close to these.

 

[mortyd]

fuel consumption

stu has it right, the only reliable fuel gauge is the one on your wrist.

 

[jviss]

Fuel gauge

My gauge died, so I started sticking hte tank with my tape measure and noting hte engine hour meter.

 

[Benny]

JVISS you have the figure ...

Like Stu says you just need the overall or average burn rate. Do not concer yourself with variations over the RPM range as they will be small and inconsequential. Wind, current, bottom and prop condition do not play into this formula, they will affect how far you travel but that is a different issue. From your burn rate you can calculate the maximum number of hours you can get from your tank capacity. You are looking for a good safe figure wich will allow you to plan usage and schedule refueling. Take the size of your tank in gallons and multiply by 80%, the 20% being held as an emergency reserve (100% of the tank capacity is not usable as depending on sea conditions you will be sucking air well before the tank is empty), and divide the 80% figure by your burn rate of .44 and you will get a maximum number of hours that you can safely plan around before refuelling. Create a simple log to record the hours from your hour meter every time you fill the tank up and that way you will always know how much fuel you have used and how much you have left.

 

[tom]

Load Affects Burn Rate

As I understand a diesel the fuel injectors are controlled by a govenor. When you set the throttle you are setting a RPM not a fuel rate. Unlike a gasoline engine where you are setting a fuel rate. With the gasoline you speed up or slow down as the load changes. But with a diesel the govenor gives more fuel or less fuel as the load changes to maintain a constant RPM. It would follow that a head wind or tail wind or clean or dirty bottom would affect the burn rate. I scrubbed my hull saturday and noticed better performance at 1500 RPM. I guess that distance traveled is separate but related to burn rate.

 

[jviss]

I think Tom is right

On these diesels, thso-called throttle in not a throttle at all, but an engine speed control. The governor will increase the fuel supplied to keep the speed at the governor setting. For example, no load at 1500 RPM will use a lot less fuel than driving the boat at 1500. And a dirty bottom will require more power to move through the water at a given speed than a clean one, more fuel will be used. I have an original M25 data sheet, a two sided glossy sheet, that has the power v. RPM, torque v. RPM and fuel consumption per B.H.P. It turns out that you get maximum torque, about 13 HP and 0.5 GPH at 1800 RPM according to these sheets. (Assuming, of course, that something connected is actually absorbing the 13HP). Since I can cruise at 5 knots at 1800 RPM and 0.44 GPH, it must be taking less than 13HP to propel my boat, with its relatively clean bottom, through fairly flat water and a 12 knot headwind.

 

[Benny]

Tom, with all due respect...

your definition of burn rate is not 100% correct. The formula which defines burn rate is, Burn Rate = Gallons Consumed/ hours elapsed. The only components to it are fuel volume and time in hours. This formula yields an average figure which encompasses average consumption from start up, idle, slow speeed and high speed cruising. This is not meant to render an exact precise figure but a practical usable guideline to calculate fuel consumption. It is a very common misconception to try to enter loads and speed to factor in but they are not part of the formula and are not needed to calculate burn rate. JVISS engine will burn on the average .44 gallons of fuel per hour irrespective of wind and current strength and direction or whether the bottom is clean or not. In an automobile the engine is mechanically engaged to the the road through the tires that is why fuel consumption is measured in miles per gallon. In a boat water is fluid medium with varying degrees of slipage and that is why the consumption is measured in hours it being a constant. The formula for speed as you know is time * distance and I'll show you how you can use burn rate together with it. The reason I'm taking the time in elaborating this for you is that I truly find that knowing and understanding your burn rate will add to your safety and succesful trip execution.Burn rate can interact with speed and distance by adding some additional variables. If .44 gallons give you an hour of running time and you are making 5knots over the prevalent conditions at the end of your run, you will have travelled 5 nautical miles; if on the other hand if you are bucking a current and are only able to do 3 knots at the end of the hour you will have only travelled 3 nautical miles. Taking this further it allows you to calculate the approximate amount of fuel you will consume in a leg of a trip and your approximate time of arrival. Should conditions change through the trip you can recalculate from that point forward and as many times needed. Obviously if the bottom of the boat is clean the boat will travel faster and influence your time distance calculations but your burn rate will remain the same. You can plan a trip ahead of time and figure out your refueling points by an assumption about what the conditions will be. I will usally calculate my distance figures at two posible average speeds 3 knots and 5 knots knowing that if the actual conditions are anywhere in between i'm fine but like it has happened that I find myself bucking a strong current at 1 knot it forces me to recalculate and perhaps chenge the strategy of the trip or leg. On long trips we carry extra diesel in jerry cans where availability may be a factor. Simple calculations on the water will allow us to calculate the availability of motoring time and give us the required time that we must move under sail to reach our destination.Calculate your burn rate and the maximum safe number of hour your tank capacity will yield and you have two important pieces of information which will help in your navigation. Good luck.

 

[tom]

Wind Affects Burn rate

Burn rate is affected by the wind. Going into the wind puts more of a load on your engine and also increases "slip". It is kinda like going up a hill. Current doesn't affect burn rate as you are moving through the water at the same rate and there isn't a change in load. Drag from a dirty hull increases the load on the engine as will pulling a dinghy. If you don't believe that wind affects the load get a bicycle and ride it into a 20 knot wind then turn around with the wind on your back. Your mind might not notice the difference but your legs will!!!! Even as fat as I am my boat has a lot more windage than I do on my bike.  I figure on a conservative rate of 1/2 gallon /hr and about 10 miles through the water / gallon. I do a little better usually. We used about 50 gallons to motor about 500 miles from Mobile to Wheeler Lake. There was a current most of the way but it varied from strong at the lower end to very weak as we aproached Pickwick Lake. I don't worry too much about motoring except in a river/lake where I can't sail. Usually we use the motor as little as possible and sail as much as possible. We carried 10 gallons of diesel on the deck as I was worried about our 20 gallon tank and the about 200 miles from Mobile to Demopolis but we made it there without needing the deck fuel. We burned very little fuel from Panama City to Pensacola as we sailed all night with a cold engine. We are back to using less than a gallon/trip to the boat as we motor out of the marina and raise the sails.

 

[Ross]

Benny, for all of your carefully considered

figures it still comes down to Horsepower/hours. A foul hull will require more power than a clean hull, head winds require more power than following winds. Running at hull speed requires 3 times as much power as running at 75 per cent of hull speed. Because a diesel runs with an unrestricted air intake, the fuel is metered into the engine to maintain a set rpm. With no load there won't be a large demand for fuel, increase the load and more fuel is metered into the cylinder(s) to maintain speed. You can continue to increase the load and the engine will maintain speed while belching great clouds of black smoke. Finally if you increase the load enough you will exceed the power of the engine and stall it. Motor sailing would be one way of increasing your range because the sails would carry some of the horsepower load.

 

[Benny]

I give up,

I don't need to know how many horsepower my engine is putting out at any particular time; it is just not useful information. I just need to know fairly accurately how much fuel I have left at any given instance. The simple formula of burn rate=gallons/hours gives me that information in a more than adequate manner.

 

[jviss]

Benny:

First, you so-called "formula which determines burn rate" is not a formula at all, it is simply the definiton of the unit called burn rate, i.e., it is an identity. This point hurts the credibilityy out of what else you say on the topic. You then go on to say "this formula yiels an average figure...," etc., which is simply more definiton of a metric on your part, not an observation of the reality of fuel consumption. The simple reality of this system is that power requires fuel; more power requires more fuel. For a speed regualted system, increasing the load on the constant speed output results in increased fuel consumption. At maximum power output available and increasing load the engine will begin to slow down, ie., go out of speed regulation. For a fixed-pitch prop in a given viscosity fluid at constant prop speed, the boat will achieve a particular constant speed if there is enough engine power available. Increase the resistance of the hull in the water (like growth on the hull) and it will use more power at this speed than before. Said another way, the burn rate,ie., the GPH, is a function of the efficiency of the powerplant and the power required for the operating condition. You can actually calculate this pretty closely by knowing hte force required to pull your boat through the water at the operating speed, the pitch of the propeller, the slip factor of the propeller at that operating point, and the GPH to BHP curve for the engine. People used to literally tow new boat designs with a spring balance in the tow line to learn the force requried for that hull at certain speeds, in order to evaluate the hull design, select a propeller, etc. (Power=Force*Distance/time, or in this case, the spring balance force times speed). It should be intuitive that the more resistance there is on the hull and rigging, due to a rought botom or wind, the more force will be required to more it through the water at a given speed.You would have to add alternator loads to this, too. All told, I don't htink it will vary enough to make a big difference in calculations. I'll just assume .5 GPH always.

 

[Stu Jackson]

Why complicate something simple?

Sure, you're all correct about the factors that influence the amount of time it will take you to get to your destination.Benny's point was simpler: for any given engine, figuring, as he said, the variables of idling, full power, cruising speed, etc. that any engine runs during the course of any day, that there is always an average fuel consumption, in this case for our M25s engine's, it is 0.44 gph.More headwinds? Foul or fair tide? Clean or dirty bottom ONLY affect how far you go in any given time span. But, DURING THAT TIME, the engine is STILL ONLY consuming that 0.44 gph.So if it takes you four hours to get there, based on "conditions" you'll burn 2 gallons or less. If it takes you twice as long to get there, you'll burn twice as much fuel.I've been measuring this stuff for eight years, with anywhere from two to eight fill 'er ups per year, and I gotta tell ya, I've been motoring through all sorts of stuff,, here on the Bay and out in the ocean and through the Golden Gate Bridge, up and down from the Delta, with headwinds, tailwinds, tides comin' and goin', wind either way, clean or foul bottom.Hours used times use per hour still equals fuel consumed.Stay with it, Benny, sometimes it's harder than it needs to be.Stu

 

[jviss]

You don't get it either

Stu, in practical terms you are right - 0.44 GPH is close enough for jazz. But it's tough to take such a pedantic post that simply states that A equals A; or Fuel consumption is gallosn used divided by hours, or miles per gallon is miles used per gallon. It's just a little too silly. Or to quote you, "Hours used times use per hour still equals fuel consumed." Here's a link:http://en.wikipedia.org/wiki/Tautology_%28rhetoric%29

 

[Ross]

Jviss in answer to your original question,

Yes I do. And the faster I try to go the faster I burn fuel. When I am out on the water in my boat the only destination that I have is Tranquility. I know that I must return but I am not in a hurry to do so. My best fuel comsumption rates are at about 4.5 kts. and if I run full throttle I can get about 5.7 and burn twice as much fuel. The extra time spent motoring simply charges the batteries. When we are on/in the boat we are "there" and not in a hurry to be someplace else.

 

[Stu Jackson]

Tautology, um?

jviss - just trying to answer your question, based on "practical" terms and real world use. Yes, as I said, all the other things are thrown in during a finite analysis, but for regular daily use, and what one needs to know I'm not sure those enter into it. And Ross is quite correct whn he says your fuel use will go up if you redline the throttle all the time. Someone else answered your question about the difference between idling, etc. in your original post. Sure, you could idle until you ran your tank dry, and I'm sure that it'd last a lot longer than 0.44 gph, but I was under the impression that we were after some useful material. Speaking of which, that's the first time I've every been dissed by wikipedia, so thanks for the reference. ;DMy point was: regardless of all those factors, all of which are correct in terms of the effect on the time needed to get somehwere, is really IS simple.Oh, sorry to be tautologic again.See ya.Stu

 

[Big Windy]

Confusing

This whole thread is out of control, with several good folks telling us more than they actually know. Burn rate is burn rate is burn rate...the engine does not know it is fighting a head wind or the current or the wind...it's still using the same amount of gph. If it burns 1/2 gph at 2500 rpm...that's it period, regardless of ground speed, through water speed or anything else. Has nothing to do with how far you go (or don't go). It is simply how much fuel is used during a certain time. Period.

 

[Ross]

Bert, My engine will run 2500 rpm in neutral

and will run 2500 rpm down wind, and will run 2500 rpm while towing another vessel and will run 2500 rpm while motor sailing. In each situation the rate of fuel comsumption WILL BE different.

 

[tom]

Bert you are Flat Wrong

The burn rate depends upon the load in a diesel. At a given RPM a diesel adds more fuel as the load increases. At a given burn rate a gasoline engine will change RPM's as the load changes. You might have noticed that you have to give a car more gas as you go up a hill to maintain the same speed. Going up a hill increases the load on the engine. It takes more horsepower to go up a hill. If you have ever pulled a trailer you might have noticed that your MPGs dropped. My truck gets about 22 MPG normally but pulling my old sailboat it only gets about 14 MPG....that is because pulling the sailboat increases the load. When calculating you fuel use in an airplane you have to figure the amount used to get to altitude separately from the amount used while cruising at a given altitude because while climbing you either use more fuel or have lower RPM's. Most people climb at wide open throttle then back off once they level off and have reached cruising speed. Even in airplanes the fuel burn rate depends upon speed. My old cessna 172 burned about 25% less fuel at 80 knots as compared to 100 knots. The procedure when you were low on fuel was to cruise at about 70 knots where the plane had the best MPG assuming no wind.