Stretch Your Fuel Dollars


Hand holding gas nozzle inserted into deck fill for fuel tank

By Brian Gordon; Last updated 6/25/2021

Let’s face it, when most of us bought our first boat, we were not particularly concerned about fuel economy. Instead we were looking forward to the fun and excitement that boats provide. But now, with the radical increase in fuel prices, most of us have a far less blasé attitude toward the cost of operating our boats.

Gallons per Hour versus Miles per Hour

For those unfamiliar with the cost of operating a boat, it is astounding how much fuel marine engines consume. While most anyone who drives a car is familiar with “miles per gallon” as a way of measuring fuel efficiency, for boats think “gallons per hour”. To give you some perspective, a rough approximation of what this means for a gasoline outboard powered boat is to take the horsepower of the motor and divide it by 10. So for example, a 225 horsepower outboard running at full throttle will burn about 22.5 gallons per hour. Now ask yourself, when is the last time your car burned a tank of gas (about 20 gallons) in just one hour?

Of course, as experts point out, throttling back to 75 percent of full throttle can result in up to 50 percent less fuel burned. Throttling back even more will further reduce fuel consumption, but at a certain point, reduced boat speed may result in an unacceptable time to arrival. So the issue becomes, how much fuel are you willing to burn in order to arrive at your destination within a time frame that is acceptable to you? That is the proverbial “sweet spot”. We will discuss how to find your “sweet spot” shortly but first, let’s take note of what the widely acclaimed marine writer Don Casey recommends.

Man transporting fuel down dock

Eight Ways to Cut Your Fuel Cost

In our West Advisor article Lowering Your Fuel Cost (which we encourage you to read), noted marine writer Don Casey suggests eight practical measures to reduce fuel consumption each and every time you go out. Here is a condensed version of what he has to say:

  1. Reduce drag by keeping the bottom clean.
  2. Tune your engine and follow the manufactuer's engine maintenance recommendations, since things like cooling systems and fuel systems maintenance are every bit as important and impact fuel mileage as well.
  3. Handle throttles smoothly, applying only as much power as circumstances require.
  4. Don’t push water. Proper trim is essential to good fuel economy.
  5. Check and repair or replace your prop as necessary. Consider pitching up or pitching down if your boat is over- or under-revving at WOT.
  6. Lighten the boat. Carry only what you need for your trip.
  7. Install a fuel flow sensor. (We elaborate on this below.)
  8. Repower with a more fuel efficient engine.

DIY: Determine Your Boat’s Most Efficient Cruising Speed

To figure the optimum RPMs at which to run your engine that will get you to where you want to go at a cost and within a time frame you are willing to accept, create a four-column table like the one that appears below.

RPMMPHGPHMPG
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
5700

Next, take your boat out and run it at each of the RPMs that appear in the left hand column. When making your runs, make sure you always run the boat in the same direction. Then, using a GPS and your boat’s fuel flow gauge, fill in the MPH and the GPH for each of the RPM entries. After finishing your runs, you will need to do some, math. For each row in the table, divide MPH by GPH and note the answer under MPG. Following is an example of the table filled in with values taken from a Yamaha Performance Bulletin for single Yamaha F300XSB2 outboard motor mounted on Pursuit DC 246 Boat. To this table we have added additional columns (with values italicized) where we assume a fuel cost of $4.00 per gallon:

RPMMPHGPHMPGFuel Cost/GCost/HRCost/20 M TripTime to Arrive
10005.21.3445.220.023.85 HR
15007.12.13.3848.423.682.82 HR
20008.63.82.26415.235.262.32 HR
25009.35.51.6942247.32.15 HR
300011.17.11.56428.451.121.80 HR
350015.49.41.64437.648.881.30 HR
40002312.41.85449.643.150.87 HR
450033.615.72.14462.837.050.59 HR
500039.220.51.9148241.820.51 HR
550043.826.51.65410648.760.46 HR
57004526.71.694106.846.990.44 HR

From the table above you can see that at planing speed the best fuel economy is 2.14 MPG at 4500 RPM and that the fuel cost to arrive at a 20 mile destination would be $37.05. The table also indicates that the travel time would be 0.59 of an hour. Of course the results that you get will vary with your boat type, boat weight, prop pitch, motor type and environmental factors such as wind speed, wind direction and wave action.

Garmin GFS 10 Fuel Flow Sensor

Garmin's GFS 10 Fuel Flow Sensor.

Fuel Flow Sensors

If constructing a table like the one above seems a bit tedious, there is another approach—one that will help you determine your fuel efficient “sweet spot” regardless of wave action or conditions. That approach is to install a fuel flow sensor. Currently, West Marine offers two: Garmin’s GFS 10 Fuel Flow Sensor and Navico’s Electronic Fuel Flow Probe. When connected to a NMEA 2000 multifunction display or instrument, these sensors keep you abreast in real time of your fuel use in gallons per hour, how much fuel you have used and your remianing range based on current RPMs and how much fuel you have left. The dynamic nature of the algorithym enables you to accurately adjust throttles and trim for maximum economy even as wind and sea conditions change.

The Rule of Thirds

GMI 10 Garmin display showing gal/hr, mpg, fuel remaining, range

Fuel use in gallons per hour, miles per gallon, fuel remaining and range as seen on multifunction display.

Running out of fuel is a leading cause of boater distress. To avoid running out of fuel, when planning for a trip, always follow the Rule of Thirds. This rule dictates that you reserve one-third of your available fuel for arriving at your destination and one third of your fuel for the return trip. The remaining third should be held in reserve for unforeseen circumstances such as unexpected weather or towing another boat.

So for example, if you own the Pursuit DC 246 boat powered by the Yamaha outboard referenced above and plan on making a 20 mile trip while traveling at your “sweet spot” (33.6 MPH while turning 4500 RPM where you will burn 2.14 GPH), the minimum amount of fuel you will need to carry is 28.03 gallons. The easy way to calculate this is to triple the distance and divide that by 2.14. (3 x 20 = 60 miles/2.14=28.03 gallons)

Keep Your Engine Tuned Up

Sonar engine parts finder banner

As mentioned above, keeping your engine tuned up and following a program of regular engine maintenance will help to contain your fuel costs. For do-it-yourselfers and professionals alike, West Marine offers a complete selection of outboard motor, inboard and inboard/outboard parts. To find the parts you need, check out West Marine's interactive SONAR Parts Finder.