Boating By Numbers–Tracking Fuel Burn

One of the worst feelings in boating is the one you get when you realize, after running around all day, that you don’t have enough fuel to get home. It happens. The general rule of thumb when you leave the slip or launch at the ramp is to abide by the rule of thirds: use one third [...]

8th March 2010.
By Pete McDonald

Photo courtesy of www.chaparralboats.com

One of the worst feelings in boating is the one you get when you realize, after running around all day, that you don’t have enough fuel to get home. It happens.

The general rule of thumb when you leave the slip or launch at the ramp is to abide by the rule of thirds: use one third of your tank to get out, one third to get home, and keep one third in reserve. Why that extra third? Changing sea conditions could mean you’ll need to burn more fuel to cover the same distance going one way versus the other.

Watching the fuel gauge is important, but how many fuel gauges out there are accurate enough to fully trust?

A far more reliable gauge would be an in-dash digital fuel flow monitor. Many new boats today come standard with electronic fuel management systems such as Mercury’s SmartCraft gauges. If not, it’s worth the upgrade. Some brands have both miles per gallon (mpg) and gallons per hour (gph) settings; others have just one or the other.

Toggle through the modes on the gauge. Using SmartCraft as an example, you can get an instant digital readout of a lot of vital material–fuel to waypoint, fuel level, low fuel level warning, fuel used, fuel flow, and fuel range. You can also look at instant and average mpg.

SmartCraft can display instant and average mpg.

If you are so inclined, you can calculate your boat’s estimated range on your own with the same formulas that magazines use for performance data charts. For gasoline inboards and outboards, record the speed and gph burn at 1000 rpm, then proceed through the rpm band at 500 increments until you reach wide-open throttle.

Builders and magazines typically estimate range based on 90 percent of the fuel capacity, with the following quick formulas:

Miles Per Gallon = MPH/GPH.

Range = .9(Fuel Capacity) x MPG

So if you record 3 mpg at 3500 rpm, and you have a 100 gallon tank, your range–if you cruised the whole time at that speed–would be around 270 miles.

There are a lot of other variables involved–tide, current, wind, sea state, added weight from fuel, gear, and people–but if you tried to keep the throttle close to 3500 rpm and follow the rule of thirds, you know you could head out around 90 miles before turning back.

Or, if you have them, you could let  the algorithms in your fuel management gauges do the work. Or you could hope there’s an easily reached fuel dock between you and home. Or you could bring aboard a sturdy tow rope and a membership card to Sea Tow.


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About the author:

Pete McDonald

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Pete McDonald is a contributing editor to Power & Motoryacht. Previously, he spent 11 years on the editorial staff of Boating. He has won multiple writing awards and holds a master’s degree from the Columbia University Graduate School of Journalism.

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