There are all sorts of ways of calculating fuel burn and range under power.
You can use just the power requirements of the boat, add in (or not) requirements for electrical and hydraulic, consider adverse weather (or assume smooth water), and allow for the boat getting lighter as it burns its fuel load (or keep it heavy as a fudge factor).
The data which follows is from our preliminary sea trials with FPB number one. The boat weighs 90,000 pounds/40,800 kg based on the trim marks, wind is calm, sea state is smooth, water depth is 30 feet / 9 m. There is no allowance for hydraulics or electrical, nor rough water drag. The data is based on a usable fuel load of 3400 US gallons (12,800 L).
The column heads stand for RPM on the engine, Speed Over Ground (SOG) from the GPS, Speed Through the Water (STW) calculated by deducting half knot of current from SOG, and then Liters/hour and Nautical Miles/liter, followed by US Gallons/hour and Nautical Miles per Gallon. The next column is the Speed Length Ratio at which the boat is operating.
The last three columns are the range in Nautical Miles based on three different fuel load considerations.
Fuel burn data is from the John Deere CPU readout (the Deere folks tell us they think this is accurate). We are using full load – half would show up a lot better – because most ocean crossing is done this way if you have sufficient fresh water tankage to make up for the fuel burned.
Now let’s plug in some added power loss for hydraulics and electrical.
In the spreadsheet above we’ve added 0.7 gallons/hr (2.6 L/hr) to the fuel burn. This is what we think we’ve averaged with Wind Horse over the last couple of years (it is hard to differentiate exactly what fuel is being used for propulsion vs. what fuel is going to auxiliary requirements). Hydraulic loads are light with the load sensing hydraulic pump being used, and the fact that it does not normally take much energy to keep the boat upright in waves. Electrical fuel consumption depends primarily on air conditioning usage. However, when AC power is being used this is efficient in terms of fuel consumed as the big alternators on the engine provide AC through inverters (which is much more fuel efficient than using a genset at the same time the engine is running). The data on SOG has been dropped so the spread sheet will fit on this page.
You can see that at 8.2 knots we’ve got a range of 8200NM (right hand column) and at 9.7 knots range is just over 5700NM. These would improve substantially if you were running at half load or you didn’t replace consumed fuel with fresh water on a really long passage.
OK – here’s how we look at range with an eye towards crossing oceans. We’ve reduced the theoretical range by 5% to 20% to accommodate extra drag from wind, rough water wave drag, or maybe a dirty bottom. Our estimate for Wind Horse on rough water drag is around 15%, so the 85% of range column would be, on average, our choice. Of course if the wind and waves are behind you fuel burn is reduced and range is increased.
Even in this worst case scenario we’ve still got over 6000 nautical miles of range at a 200-mile-a-day clip (8.5 knots), and 4600 NM at 232 miles a day (9.7 knots).
A final word on range. It is worth restating that this data will vary with conditions. If you are running with following wind and waves, you can maintain speed, drop engine load and RPM, and substantially reduce fuel burn, sometimes by 10-to 20-percent. Going uphill we’ve seen fuel burn increase from 15% to as much as 30% (the latter maintaining 11 knots into 30 knots of breeze and big waves). The primary range data presented is based on smooth water and calm winds. We think it is realistic based on what Deere have told us about the reported fuel burn from their instrument panel.
As already mentioned, bottom and prop condition have a lot to do with efficiency and fuel burn. A thin crust of scale on the prop will reduce efficiency by 10- to 20-percent. A dirty bottom likewise will have a big effect on consumption. You have plenty of power to maintain cruising speed, but the dirty bottom/prop will cost you fuel.
In figuring how far to go between pit stops you also have to consider fuel burn at anchor for the genset and diesel heater. On Wind Horse, for heating, we averaged about 4.5 US gallons/17 liters per day during early summer in Alaska, with night time temperatures in the mid-30 degree F (0 to 2C) range and mid 40s F (7C) during the day. We typically average three quarters of a gallon/one liter per day for the genset when at anchor.
You can, of course, substantially improve range by reducing or eliminating air conditioning usage under way, and allowing the boat to move a bit in the waves (by de-tuning the gain settings on the stabilizer control). The biggest variable is boat speed. Backing off from 9.7 knots to 8.6 results in a 32%% improvement in range.
For another article on this subject click here.