Fuel consumption and navionics

Hello Below · May 12, 2019

jngotasail said:
Hi,
I am setting up the boat info in Navionics and I have come to the fuel consumption part. I have a 2007 Hunter 33. I know that asking what fuel consumption would be is like asking how long is a piece of string but I have to put a figure in. So any suggestions? I am new to this boat and at this point have not a clue.
Of course a real sailer would just enter 0 but.........

The fuel consumption chart for your particular engine would be a good start. These will be available from the manufacturer's website. Or if you let us know the make and model of your engine, someone will help out.

Terry Cox · May 12, 2019

jngotasail said:
Of course a real sailer would just enter 0 but.........

A good round number in place of zero would be one gallon per hour. Any other more specific number will require an extended discussion/opinions, testing under various conditions, experimenting, endless calculations, and so forth and so on. Yours could be slightly more or slightly less. It largely depends upon how precise you want the number.

Stu Jackson · May 12, 2019

1 would be a good start, but for his smaller boat, Terry, I'd start with 0.5.

Here's why:

Fuel Log

It's not hard to figure out, just try it for a few fill-ups and you'll know how much fuel your engine uses.

Top it up. Run for two hours. See if it will take a gallon. Done, for starters.

The reality is that OVERALL fuel consumption rarely varies. Of course, theoretically it would be between idle, cruising speed and WOT. But that's NOT how we use the engines in our boats. It really all averages out. And THAT'S the ONE number you need to know.

SFS · May 12, 2019

Agree with Stu. We kept the same sort of spreadsheet for our Yanmar 3YM20, and it varied a bit, but not a lot. Our number, after lots of hours cruising at 2800 RPM mixed with lower rpm settings for docking, anchoring, speed reductions for bridges and traffic, etc., was almost always 0.45 gallons per hour, plus or minus 0.05.

Another rule of thumb if you have a diesel: Diesels burn about 1 liter per hour for every 10 horsepower being developed. This was borne out by our experience. Our Yanmar was rated at 23 hp at 3600 rpm. Therefore, at 2800 rpm I suspect I was developing about 17 hp, which would be 1.7 liters per hour. That converts to 0.449 gallons per hour.

What engine do you have?

Stu Jackson · May 12, 2019

SFS said:
Another rule of thumb if you have a diesel: Diesels burn about 1 liter per hour for every 10 horsepower being developed. This was borne out by our experience. Our Yanmar was rated at 23 hp at 3600 rpm. Therefore, at 2800 rpm I suspect I was developing about 17 hp, which would be 1.7 liters per hour. That converts to 0.449 gallons per hour.

Very cool information. Thanks. I'm going to add it to my link, with attribution, of course.

SFS · May 12, 2019

The notion is not mine. I read it in an article on fuel consumption. The figure only works for diesel engines - the number would likely be quite different for gasoline engines.

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SFS · May 12, 2019

Here is some language from a Boating magazine article (see link below):

"Likewise, a well-maintained diesel engine burns about 0.4 pounds of fuel per hour for each unit of horsepower it produces."

So 10 hp would use 4.0 pounds an hour. Diesel weighs 7.2 pounds per gallon, so 4.0 pounds/7.2 pounds per gallon = 0.555 gallons, every hour (for every 10 hp being developed). Pretty close to the "rule of thumb" I read, but have been unable to find again.

https://www.boatingmag.com/calculating-fuel-consumption

JamesG161 · May 12, 2019

SFS said:
The notion is not mine.

But fairly close.
Also RPM is directly related to "HP used" under load, not in neutral.
1 liter per 10 HP used.
But even with any navigation type app, they are trying to be like a "power boat" and estimate "Time" left on fuel tank.
Never rely on it as your only way to know the fuel reserves.
Jim...

SFS · May 12, 2019

jngotasail said:
Great information
I have a Yanmar 29hp

Okay, so at WOT, getting 29 hp, that's 2.9 liters per hour, or .77 gallons per hour. Your are likely not running at WOT, so Stu's suggestion of 0.5 gallons an hour is a better (though less conservative) estimate than a gallon an hour. You should be able to get a fuel consumption curve from Yanmar for your specific engine. Or google "fuel consumption for Yanmar X" where X is your complete engine model number.

However, as many people have already said, start a log, and over time that will be your overall number. Fuel dock hands were always astonished that I could tell them how much we were going to need to fill up to within about half a gallon. It's not hard if you are keeping track of engine hours. We had an 18 gallon tank, and considered 15 usable, so after 30 hours, we were taking on fuel.