176 seamanship secrets
Each run consists of 1 hour at a specifi c RPM. Record in your log: RPM,
speed, GPH, wind, current, sea condition. Th en bump the speed up or down
to the next RPM, and again run for a full hour. When complete, plot your
fi ndings onto your graph. Note the weather conditions on the graph.
If you made reciprocal runs, you’ll need to make two graphs. Label each
graph with the conditions encountered (i.e., “Bow seas”; “Following seas”; etc.).
When cruising, use the graph that most closely matches your current conditions.Alterations and Fuel Provisioning
Remember that any alterations to your engine or boat—such as adding tur-
bochargers, changing props, or provisioning for extended cruising—might
affect the accuracy of your data.
Even without alterations, it’s wise to overestimate fuel needs by 30% or
more. This compensates for fighting wind, current, or seas, and for passenger
and provision weight or running generators.
A FUEL CONSUMPTION GUIDE FOR SLOW BOATS
Sailboats and vessels with slow speeds needn’t go through the process outlined
above. Th e method outlined below gives a rough estimate at cruising RPM. Sailing
vessels in particular need to run as oft en as possible at maximum throttle. Diesels
hate babying, and serious problems crop up when a diesel is run continuously
at idle speeds. For an accurate consumption calculation, get the engine up to
maximum speed when you clear the no-wake zone. Your iron genny will reward
you with longer life, better performance, and fewer problems.
- To p o ff the tank. Fill your tank to 90% capacity. Mark the position of the
fuel fl oat gauge. Use a felt-tip pen and mark on or near the gauge. - Log entries. Make a log entry showing the date and time of fi lling. Describe
the mark location on the fuel gauge in case it rubs off. - Run at cruising RPM. Run the boat at cruising RPM. Burn the fuel down to
the quarter-tank mark on the fl oat gauge. - Find your consumption. Return to the fuel dock. Record the date and time.
Fill the tank back to the exact level shown by your mark on the fuel fl oat
gauge. Divide gallons taken on by the hours run under power between fi lls.
Round the result up to the next higher tenth of a gallon.
You have finished the test and return to the fuel dock. It takes 15.4 gallons to
fill your tank to the mark you made on the fuel gauge. Total time run was 33.8
hours. Divide the total gallons of fuel by the total hours run:
15.4 gallons ÷ 33.8 hours = 0.456 GPH
Rounding to the next higher tenth gives you a result of 0.5 GPH at cruising RPM.