CHAPTER 5
FUEL SYSTEM
- 25 -
34z
boat by recording the speed and fuel rate at various RPMs. We’ve already seen that the lighter
Cummins Mercruiser sterndrive installation with 320 HP 6 cylinder QSD4.2 diesels on hull #3
burns approximately 2 gallons per hour less (18 gph) than the Volvo 370’s at cruising speeds,
achieving 1.4 nmpg at 25 knots. It’s anticipated that the standard sterndrives and/or lighter Volvo
D4 or the Cummins QSD4.2 engines will improve the efficiency by a tenth or two from those
shown below.
Range of Efficient Operation
It’s interesting to note that it doesn’t particularly matter whether
you are going 9 knots or 27 knots on a 40z, nautical miles per gallon remain fairly constant.
Cruising Speed
Given suitable conditions, 10% below wide-open throttle is generally the top
end of the cruising speed range. The data below would indicate that 2900-3000 RPM in the 28-
knot range would be a good efficient range.
Sour Spot
The 40z seems to have a huge “Sweet Spot” and just one small “Sour Spot” to avoid
at about 1600-1700 rpm where the most power is applied in overcoming resistance prior to the
boat jumping up on a plane at just over 10 knots. You can see that the boat is no more efficient at
that point than at 30 knots. These rpm numbers will vary by engine size and drive type.
FUEL EFFICIENCY
Volvo 370s IPS
RANGE
ACCELERATION (secs)
RPM GPH
KTS NMPG NMrng* 0-10 0-20 0-30
600 0.6 4.8 7.6 2392
1000 2.3 6.7 3.0
937
1200 3.5 7.6 2.2
684
1300 3.9 8.0 2.1
646
1400 4.7 8.2 1.8
555
1500 6.8 9.3 1.4
428
1600
7.8
9.5
1.2
384
1700
9.5
10.0
1.1
333
2.5
1800 9.7 13.1 1.4
425
1900 10.6 14.3 1.3
425
2000 12.7 15.1 1.2
374
2200 14.8 17.9 1.2
380
2400 17.6 20.9 1.2
374
5.3
2600 20.5 24.0 1.2
370
2800 22.9 26.9 1.2
370
3000 26.4 30.0 1.1
357
9.5
3200 30.5 32.8 1.1
339
3400 35.7 36.1 1.0
319
3500 38.5 37.7 1.0
308
Remember that fuel level readings when underway, with the fuel pushing back
in the tank where the fuel level sensor is located, could be reading ¼ tank more than what’s really
there. So, when you get down to 1/3 tank, it’s time to top off.... not roll the dice on finding another
fuel dock open later in the day.
