TROUBLESHOOTER

Passage Maker - - Contents - Steve Zim­mer­man

Cal­cu­lat­ing Fuel Burn

If you cruise on a power­boat, you pay at­ten­tion to fuel con­sump­tion. De­spite the im­por­tance of this topic, con­fu­sion and false as­sump­tions abound. Many as­sume that an en­gine of mod­er­ate power runs more eco­nom­i­cally than a high-out­put ver­sion, but is that al­ways true? Should you pre­fer a sin­gleengine trawler over a twin-screw, semidis­place­ment boat be­cause of fuel econ­omy? If you have a twin-en­gine boat, can you run on one en­gine to re­duce fuel burn?

Let’s be­gin by agree­ing on the stan­dard. While many boaters fo­cus solely on gal­lons per hour (GPH), this is a mean­ing­less num­ber on its own. For ex­am­ple, here’s a ques­tion: Which is more ef­fi­cient, Boat A burn­ing 11 GPH or Boat B burn­ing 22 GPH? With­out cal­cu­lat­ing miles per gal­lon (MPG) it is im­pos­si­ble to say. Gal­lons per hour can be help­ful when cal­cu­lat­ing range and de­ter­min­ing whether you have enough fuel to get to your des­ti­na­tion. If you are con­sum­ing 20 GPH and you will be run­ning for five more hours, then you know you will burn 100 gal­lons of fuel be­fore ar­riv­ing. But that does not speak to fuel econ­omy. When we com­pare cars, we all agree that MPG is our stan­dard. So we will adopt the same stan­dard for this ar­ti­cle.

Let’s re­turn to the ques­tion of which boat is more ef­fi­cient, Boat A burn­ing 11 GPH, or Boat B burn­ing 22 GPH. Let’s in­clude the speed and look at the math.: Boat A: 10 knots (nau­ti­cal miles per hour) / 11 GPH = 0.9 miles per gal­lon

Boat B: 22 knots (nau­ti­cal miles per hour) / 22 GPH = 1.0 miles per gal­lon

In this par­tic­u­lar ex­am­ple, we see that al­though the dif­fer­ence is mi­nor, the boat burn­ing more gal­lons per hour achieves bet­ter mileage. For the pur­poses of this dis­cus­sion, we will con­cen­trate on nau­ti­cal miles per gal­lon, which we will ab­bre­vi­ate to nMPG.

FUEL BURN BA­SICS

A given hull will re­quire a cer­tain amount of en­ergy to move it through the wa­ter. The fuel con­tains the stored en­ergy and we can de­scribe that in terms of horse­power ( hp), kilo­watts ( kW), or Bri­tish ther­mal units ( BTU). One gal­lon of diesel fuel stores about

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