We completed the 17th-Annual Caribbean 1500 Rally from Hampton, Va., to Tortola in the British Virgin Islands in November of 2006. The event featured less wind than any of its 16 predecessors and challenged each skipper's understanding of his boat's fuel capacity and consumption.
When we heard skippers ask questions about most efficient engine and boat speeds on the SSB radio during the daily chat, we were reminded that, as with the other elements of passage planning – food and water – the strategy for fuel management must be developed before leaving the dock and reviewed each day along the way.
While there are many electronic answers to fuel calculation (see sidebar), electronics can quit. To develop a bulletproof fuel management strategy, you must quantify the "usable" fuel capacity of your boat and understand the rate at which fuel is consumed by your engine when your boat motors in the expected range of operating conditions. This information is not available in your operator's manual; it only comes from experience with your boat. And it takes time.
"USABLE" FUEL
Your boat manufacturer provides capacity specifications for each tank on your boat. That information is helpful as a starting point; "usable" fuel in each tank can be up to 15 percent less, depending on the positioning of the pickup tube.
To determine your usable fuel capacity, run each tank until it is close to empty (or, run the tank dry if you can switch to a full tank before the engine shuts down and don't mind bleeding the engine fuel system to restore power). When you refill the tank, mark it in your log or post a note of each tank's usable fuel next to the fuel gauges.
While you have the tank empty (or as close to empty as you care to go), it is a good time to do some tank maintenance like cleaning and checking all of the tank fittings and joints for corrosion and leaks. When cleaning the tank you may find debris that could be churned up from the bottom of the tank and clog the fuel pickup tube and filters during a passage. We experienced a clogged pickup tube when a piece of caulking used to seal the inspection plate in the manufacturing process was sucked in.
Many offshore cruisers add fuel capacity with fuel containers stowed on the rail or in lockers safely before leaving the dock for a long passage. Extra jerry cans provide additional usable fuel for a long passage.
EFFICIENT ENGINE OPERATION
To determine the most efficient speed to run your engine when your boat is equipped and loaded for an offshore passage, calculate your boat speed at various engine RPM.
First, ensure that your knot log is correctly calibrated according to the operator's manual for your equipment. Then, on a calm day with flat seas, motor at the lowest recommended operating RPM for your engine. Note your boat speed. Increase your engine RPM in 250-RPM increments until you reach the maximum continuous RPM listed in your engine operator's manual. Record your boat speed at each RPM setting once the boat speed has stabilized.
The next step is to add fuel consumption to the speed matrix for your boat. Start with the fuel consumption curve in your engine manual. By examining this data, you can pinpoint the engine speed at which you can motor the greatest distance on a gallon of fuel and the amount of fuel you will consume at any given engine and boat speed.
The next step is to verify your assumptions. With a full fuel tank, motor for several hours at your cruising RPM. Note your distance and speed. At the end of the run, refill the fuel tank. Divide the fuel used by the motoring time to give you the fuel usage per hour. Now compare that to the calculated figure and see how the actual fuel consumed matches the calculated rate. Pick three or four other RPM levels and repeat the process.
