Fuel burn rate
- Thread starter Keith13315
- Start date
There are some threads about fuel consumption, so a search may be fruitful. To get helpful responses, you may want to provide information about your engine.
A general rule of thumb is that diesels burn roughly a liter (which is about a quart) per hour for every 10 horsepower being developed. My Yanmar 3ym20 seems to support that notion.
If the boat is new to you, you may want to start charting your average fuel use across all usage modes - warmup, undocking, docking, slow cruise, normal cruise, WOT. This is easy to do, but requires the ability to fill the tank to the same level at each refueling. Simply chart the number of engine hours since the last fuel fill up versus the amount of fuel bought. Divide gallons by hours of engine time to get burn rate in gallons per hour. Over a period of time you will know your average fuel burn. When you have collected enough data, you can infer an approximate burn rate at cruising speed by looking at those stretches where a significant amount of engine hours were spent in cruise.
If you are trying to calculate range, remember that some portion of the fuel in the tank may not be usable due to the locations of fittings (i.e., it may not be possible to empty the tank to dryness).
A general rule of thumb is that diesels burn roughly a liter (which is about a quart) per hour for every 10 horsepower being developed. My Yanmar 3ym20 seems to support that notion.
If the boat is new to you, you may want to start charting your average fuel use across all usage modes - warmup, undocking, docking, slow cruise, normal cruise, WOT. This is easy to do, but requires the ability to fill the tank to the same level at each refueling. Simply chart the number of engine hours since the last fuel fill up versus the amount of fuel bought. Divide gallons by hours of engine time to get burn rate in gallons per hour. Over a period of time you will know your average fuel burn. When you have collected enough data, you can infer an approximate burn rate at cruising speed by looking at those stretches where a significant amount of engine hours were spent in cruise.
If you are trying to calculate range, remember that some portion of the fuel in the tank may not be usable due to the locations of fittings (i.e., it may not be possible to empty the tank to dryness).
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1/2 gallon (2 quarts, or about 2 liters) would fit the general rule of thumb for a motor putting out 20 hp.
To the OP: your engine's owners manual will likely have fuel consumption curves. If not, a google search will let you find them somewhere.
To the OP: your engine's owners manual will likely have fuel consumption curves. If not, a google search will let you find them somewhere.
"cruising speed" can have varying definitions, but at 70-75% max RPM, say around 2800, you will burn about a half gallon an hour, as others have said..(RPM is accurate RPM, not necessarily what shows up on a Yanmar tachometer) A dirty bottom and prop will increase that a little.. any other drag will increase that a bit to, like towing a dink, or motoring into a stiff wind or significant chop.
For early planning, you could use 5/8 gallon per hour and modify as you get more familiar with the boat and how you like to motor it..
For early planning, you could use 5/8 gallon per hour and modify as you get more familiar with the boat and how you like to motor it..
The engine is a 3GM30F.
Thanks for the responses my previous 31 with a 2GM20F burned about 2 quarts per hour at 2800-3000 RPM (on the tach) I am planning the trip home with the 340.
Regards
Keith
Thanks for the responses my previous 31 with a 2GM20F burned about 2 quarts per hour at 2800-3000 RPM (on the tach) I am planning the trip home with the 340.
Regards
Keith
I have a 2000 H340 with the 3GM30F engine turning a three blade Campbell Sailer prop. I run my engine at ~2,800+ RPM most of the time I am motoring. Over the past five years my fuel consumption has been 0.61 Gal/hr on average.
The burn rate pertains mostly to the engine size and not to any particular model boat. The easiest to calculate and which yields a good enough result is the average burn rate based on the average way an individual uses his boat. As a starting point for a 3GM30F you could use .75 gallons per hour. With further calculations every time you fill up that figure could be brought down to a more realistic level. I see that John reports a burn rate of .61 which sounds quite reasonable but you may use your boat differently than John so I would advise you calculate your own. I'm sure you know that speed and range are a factor of weather and other considerations. When day sailing or weekending we motor sail very little but when we go on an extended trip we motor sail quite a bit to keep up speed. We still use our calculated burn rate and any savings induced by the help of the sails is just gravy for our fuel reserve. We calculate range on the water before and during the trip according the encountered weather conditions. These calculations will indicate whether we have adequate fuel to reach our destination or if we need to divert to an alternate destination to refuel.
Yes, I was looking for estimate only the boat is new to me and therefore I have no experience with the fuel consumption. I have it down to the OZ. with the H 31 with the 2GM20F
Remember the "average burn rate" is a simple formula with only two variables; gallons and hours. Do not complicate with factors like varying RPM, wind , currents, condition of the hull, etc. The effect of some of these factors are already built in into the average and the others just affect speed and range calculations. This is a case where we just need a good enough figure and no effort to obtain a more accurate figure will significantly improve its usefulness. Have calculated and used the average burn rate for decades and have not ran out fuel a single time. That is good enough for me.
Benny is absolutely right. I've been doing calculations for 18 years and am quite assured that the math is correct. I even built a spreadsheet 18 years ago, and I used it when coming up the coast from SF to Vancouver Island. It is more accurate than a fuel gauge.
http://c34.org/bbs/index.php/topic,3841.msg21571.html#msg21571
Average is ALL you need, and in reality it doesn't vary much unless you compare idling (which you shouldn't do for long periods of time anyway) to WOT (which most of us don't do for long periods of time anyway, too).
Range is all that is affected by wind and current, 'cuz the engine hours will be longer against opposition. That's all, simple math.
I get 0.35 - 0.40 gal/hr very reliably. '99 H340 with 3GM30F and 3 blade MaxProp. Inland lake so no current, and this is calculated when I fill the tank, maybe once every 30 months
The direction and strength of currents has no bearing on the engine burn rate. The formula is simple with just two variables "gallons" and "hours". Thirty months is quite an interval to be able to attain a reliable average burn rate figure. It usually takes a minimum of around 15-20 calculation cycles to start narrowing down a reliable rate figure. Your reading of 0.35- 0.40 GPH seems quite low for the 3GM series engine but I'm not entirely discounting it as it may be due to the way you use your boat. I just would not go below 0.60 GPH with that engine to use in calculations for an offshore passage where range projections might be critical in reaching the intended destination.I get 0.35 - 0.40 gal/hr very reliably. '99 H340 with 3GM30F and 3 blade MaxProp. Inland lake so no current, and this is calculated when I fill the tank, maybe once every 30 months
Benny, I continue to find it fascinating that folks simply don't "get this." Maybe it was the high school math teacher not being good enough.
Stu, an opposing current might cause you to use a higher throttle setting to maintain SOG. But what do I know, we have no current.
And my math skills are just fine.
And Benny, this boat in this lake might not hit your threshold for quite a few more years. I'll stand by my calculations based on about 5-6 fillups over the last 13 years.
And my math skills are just fine.
And Benny, this boat in this lake might not hit your threshold for quite a few more years. I'll stand by my calculations based on about 5-6 fillups over the last 13 years.
Everything you ever wanted to know about how fuel efficiency in boats is calculated - courtesy Dave Gerr.