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Simple speed calculation

Apr 24, 2013

To the editor: As an shipboard engineer and sailor, I enjoyed reading Michael Robertson’s article about balancing his speed and fuel consumption to determine the range of his Fuji 40 (“Balancing speed with fuel consumption,” March/April 2013, issue 208). While the calculations that Robertson completed are certainly valid and can provide a solution to the question, “How fast can I go and still make it to my destination?” I think there is a very simple permanent calculation that can help as well.

The trade-off between fuel economy and speed are shown by the two curves. At speeds greater than four knots, fuel economy begins to suffer.

Mike Cilenti determined that four knots was the best fit for matching speed to fuel economy for Michael Robertson’s Fuji 40.

The calculation is speed (knots) divided by fuel consumption (gph) at that speed, gives you fuel economy (nm/gal).

I have attached a spreadsheet where I used the information displayed on the card Robertson created (and photographed for inclusion in his article) to calculate fuel economy (i.e., nautical miles per gallon). The spreadsheet shows that throttling all the way down is not the most efficient, since as he points out, the engine is not at its most efficient, and by going slower, it takes you longer to get where you’re going (and you therefore burn more fuel).

Additionally, the information (and included chart) also illustrates that by throttling up to 5.5 knots his fuel economy is still relatively high, but rapidly drops off above this speed. As Robertson points out, this drop is caused by the increased power required as the resistance of the hull increases as the boat approaches hull speed.

A simple card with the information on the spreadsheet can help any boat owner know where his boat performs best under power without the need to perform multiple calculations while at sea. Of course, additional information can be added to the spreadsheet to plug in the distance to your destination to calculate how long it would take at each speed and how much fuel you would burn.

—Mike Cilenti is a Lieutenant Commander in the U.S. Coast Guard and an avid sailor. He’s also a licensed professional engineer and has several upper level merchant marine licenses (both deck and engineering).

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