Fuel burn rate

[SFS]

I agree with Stu and Benny, current and wind have nothing to do with fuel burn rate.

That being said, those of us that chart average burn rates (I use Stu's method, and keep a spreadsheet) are taking into account a wide variety of throttle settings (idle, docking speed, no wake speed, sail raising speed, low cruise, high cruise, emergency ahead). I can see how on an inland lake there would not as many times where higher RPM are developed. The numbers Ed gave for his 3GM30 are the same numbers I've been getting for my engine over the year we have had the boat, BUT I have a 3YM20 with 10 fewer horsepower. The scenario makes sense if he rarely asks the motor to deliver more than 20 of his 30 hp.

To the OP: Most of us are talking about fuel consumption rates, because that is what you asked about. As several posters have pointed out, if you are trying to figure out the range of your boat, that is a whole different ball of wax. Range IS affected by winds and tides and currents and bottom condition and boat trim. For safety's sake, do not forget that. Also, do not forget that some of the fuel in your tank may not be useable, for a variety of reasons.

 

[jssailem]

I think the question was average burn rate. Benny and Stu are right, In the simplest form that is Gallons and Hours.

 

[Johann]

The chart I have for the 3GM30F shows .78 gal/hr at 2800 rpm, and .62 gal/hr at 2500 rpm. I cruise at 2800-2900 rpm and estimate .75 gal/hr which seems to be pretty accurate. The prop can make a big difference... I'm using a 3 blade CS.

 

[Benny17441]

Stu, in my opinion I think it is due to peop

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, I think what happens is that people "overthink" this or try to equate it to their road motor vehicles behavior. The "good enough concept" seems to get lost in the shuffle. It is indeed a challenge in communication to try an explain this. Let me give it another try; the object of the average burn rate for your boat calculations is to be able at any time to determine approximately how much usable fuel is left in the tank and nothing more. A more accurate result involving the complexity of additional factors does not really improve on the results yielded by the simple average burn rate formula. We may all run our engines at idle, slow speed or cruising RPM for different lengths of time and this will account for the need to calculate and have your own average burn rate. The average rate will already have factored in changes in throttle and RPM and will reflect your preferences as the helmsman. Some may set the cruising RPM and seldom change it while others may be constantly fiddling with the throttle. Realize that we have only been talking about fuel burned in a measure of time. RPM is a basically a measure of time. Forget about winds, currents, drag as these are components for a different set of calculations to determine speed and range. At this point all we want to know is for how many hours will our engine run. How far can we go and how fast can we get there depends on the prevalent conditions. We may go 1 nautical mile in one hour or we may go 6 nautical miles in hour while using the same amount of fuel. Fuel management practices requires that we keep an amount of fuel in reserve. This reserve is used to prevent the engine from sucking air as not all fuel in the tank is usable, it also provides a safety margin for variances in the ABR calculations as well as extra fuel to account for unanticipated events. ABR calculations with practice can be done in the head to periodically update estimates of range as the boat advances towards its destination and it is this simplicity of not needing a calculator that really makes it very useful. The results are simply obtained and good enough to fulfill our needs. A more accurate figure which might require additional collection of data and or the use of a calculator would be more cumbersome and could actually detract from the simplicity of readily available info. I can look at the hour meter and determine that we have ran 8 hours since our last fill. Simple math tells me I have burnt approximately under 4 gallons of fuel and that I should have a tad more than 22 gallons of unreserved fuel left for another 44 hours of motoring. If we are on the water and moving at 4.5 knots I calculate that I have a safe range of around 200 nautical miles. Updating the calculations every hour that passes will provide a fresh estimate. Changes in speed may reduce or increase the range. If you are just piddling around in your sail area then it gets simpler as the only issue is whether you have enough fuel for the day or not. This has nothing to do with optimizing fuel efficiency and all about calculating available fuel.

 

[Keith13315]

Yes my question has been answered. Yes I understand all of the variables involved. I wanted an idea, a starting point if you will, and I have that. In the spring I can start my own log and get my averages. Based on my sailing/ motoring style and the local conditions which is Lake Michigan no tide no real currents just wind and waves. Thanks for all the input it was very helpful.

 

[Alan Gomes]

Yes my question has been answered. Yes I understand all of the variables involved. I wanted an idea, a starting point if you will, and I have that. In the spring I can start my own log and get my averages. Based on my sailing/ motoring style and the local conditions which is Lake Michigan no tide no real currents just wind and waves. Thanks for all the input it was very helpful.

The wind, waves, and current will have no effect on your burn rate. Now, how fast/far your boat travels in a given period of time is another matter.