Stretching the Fuel Range to Cross the Pacific

Stretching the Fuel Range to Cross the Pacific

2-9-2011

REDUCE GENSET HOURS AND REDUCE RPM.

When Starr made her run from Seattle to San Francisco and on to Honolulu we chose to run the Northern Lights 20 kw gensets only when we needed to make water and or the air conditioning. The total main engine hours for Seattle to Honolulu were 352hrs. Total Genset hours were 75hrs. (We needed A/C for the last few days prior to Honolulu). Fuel consumed Seattle to Honolulu was 3,000 gallons for 3,000 nm.

Starr’s fuel consumption over the past year and a half:

Nautical miles run from Seattle to Japan : 9,000nm

• Total fuel used: 9,400 gallons.

Total main engine and genset hours:

• Main engine hours: 1,051 hrs
• Genset hours: 1,382 hrs at an average of 1gph. (Lots of hours due to air conditioning use in the Marshalls, Micronesia and Marianas.)

So deducting 1,382 gallons for the genset fuel burn:

• Starr’s main engine used 8,018 gallons, so for 1,051 hrs, we averaged 7.6 gph.
• Starr’s average boat speed over the 9000 nm (divided by 1051 hrs) was 8.5 kts.

We need to remember that Starr had prevailing winds behind us all the way to Japan, but we cannot expect that to be the case for the passage from Japan to Honolulu. Heading back to Honolulu, we’ll likely have winds from ahead of the beam all the way. The Japan-to-Honolulu passage is approximately 3,768 nautical miles, with no fuel stops along the way. Our goal is to reach Honolulu with 25% of our fuel left unless weather conditions require either higher RPMs or a longer route.

With Starr’s normal fuel capacity of 3,700 gallons and our four new intermediate bulk containers (264 gallons each), our total onboard fuel for the passage will be approximately 4,756 gallons. I’m estimating that wind, seas and currents on the nose will cost us about ½ knot on the average, meaning Starr should be able to make good about 8.0 knots over the bottom at a main engine fuel burn of approximately 7.6 GPH. With no generator fuel burn and no weather diversions, based on the figures above Starr should require approximately 471 main engine hours @ 7.6 PGPH for the trip, for a total main engine burn of about 3,580 gallons. That leaves us (4,756 minus 3,580) about 1,176 gallons, a comfortable 25% safety margin, that can be used for any weather diversions and for generator time to make water and keep the boat air conditioned. In the unlikely event that we operated the generator for all 471 hours (at 1 GPH) our safety margin would be just over 700 gallons or about 15%–still a quite acceptable safety margin.

Starr is mostly a “battery boat”. The main engine has a 175-amp, 24-volt Leese-Neville alternator power to provide power for almost all the ship’s electrical systems. The largest power demands come from the Sub-Zero refer and freezer units. The other big draw is the 4,000cfm Delta-T engine room blower. This is a 240-volt 3-phase blower that draws 10 or more amps depending on its speed setting. When Starr is underway with our genset off, our main engine alternator puts out a constant 95 to135 amps, and the FloScan meter tells us that’s costing us just under ½ GPH.

My friend Milt Baker discussed this with his diesel mechanic guru, Bob Senter of Northern Lights, and here’s what Bob said:

“It doesn’t matter if the HP comes from a generator’s diesel or the main, it still takes fuel to make kilowatts. The difference is in the conversion losses. 130 amps x 27 volts (approx charge voltage of a 24 volt alternator) = 3,510 amps, roughly 3.5 KW. You lose about 30% in the inverter making AC from the DC and another 10-15% in losses at the belt drive to the alternator. Remember, you don’t suffer those losses making AC directly from the generator. Based on the losses, I’d expect the fuel burn from the alternator to be about .5 GPH extra when heavily loaded. You can make 5 KW with the generator using the same fuel burn it takes to make 3.5 KW with the alternator. Up to about 2.5 KW, it’s probably more efficient NOT to run the generator, since it is much less efficient at light loads. (I calculated KW because it’s the easiest way to understand power, regardless of whether it’s AC, DC or differing voltages. Just multiply amps x volts for KW.)”

All the above is good to know but perhaps not relevant to Starr’s upcoming passage to Hawaii. Here’s my thinking:

• Even if I apply Bob’s data to our fuel calculations, I think we are better off not using the 20KW genset because our tiny 3.5KW load is just too low for efficient use of the generator. Also,, I would prefer to load up the main and keep the exhaust temperature above 600 degrees if possible.
• We will do some tests when we are underway and see what the Floscan shows with the genset on and or off.

THE OTHER MAJOR FUEL SAVING CAN COME FROM REDUCING THE MAIN ENGINE RPM’S;

When we need to further economize on fuel we have reduced the rpm’s from the usual 1500 RPM down to 1250 RPM. The fuel burn goes down from 8gph down to 5.5gph which can extend our range closer to 6000nm.

For this run I will assume we keep the power up and get to Hawaii as quickly as possible.

BOTTOM LINE: If the weather gods are kind to us and my calculations are correct, it’s my guess that Starr will arrive in Honolulu with about 1,000 gallons of diesel, roughly 21% of the fuel she departs Japan with. By any cruising yardstick, that’s a most satisfactory safety margin.