I thought I'd interject a quick blog entry with some fuel economy numbers for Sea Spirit. As you may recall, she's a very slippery Sparkman & Stephens hull design, so we knew she'd get good fuel economy. But we were so busy learning how to run the boat that it took us a couple of months to get around to truly measuring it.
In order to do the measurements, we picked a calm day (of which we had plenty in the Exumas), and made sure that our GPS speed and our speed-thru-the-water numbers were confirming that there was little to no current. We then warmed up the boat to normal operating temperature, and ran her at various RPMs in order to measure the fuel usage.
We took the fuel usage from the most accurate source we have aboard, which is the engine's Murphy PowerView gauge (pictured here).
This is an amazing little device that takes J1939 data emitted by the engine, and displays that data on many different pages. Although not shown in the above picture, it is possible to arrange the display to show fuel flow in gallons per hour, engine RPM, engine workload as a percentage of total workload, and so on. I used these numbers to construct a fuel economy chart.
One question we had was how accurate are the fuel flow numbers? I called Northern Lights, makers of our Lugger L1276-A2 engine, and asked them. The answer: Extremely accurate, since the data come from tables constructed by the manufacturer in order to control the flow of fuel through the fuel injectors.
So, here are the results (you may need to click on the image to make it large enough to read):
In this diagram, engine RPM is along the bottom, and the vertical scale is either knots, NMPG, or GPH, depending on which bars you are looking at. The green bars represent Sea Spirit's speed, the yellow bars represent fuel consumption, and the thin line represents fuel economy in nautical miles per gallon. I have another diagram that also shows percentage of engine load, but omitted it here to avoid too much clutter.
Several things stood out to me upon examining this data:
1) At this fuel load, our envelope of possible speeds on Sea Spirit is from 5.8 knots at idle to 10.8 knots at full throttle. If we want to go over 11 knots, we need to be lighter on fuel (we had about 2/3 full tanks , or 1,400 gallons, for this test).
2) Our envelope of possible fuel usage in terms of gallons per hour ranges from 1.4gph at idle to 17.3gph at full throttle and 1885RPM.
3) Our envelope of possible fuel economies ranges from a high of 4.1 nautical MPG at idle and 5.8 knots, to a low of 0.6 nautical MPG at full throttle and a shade under 11 knots. Frankly, this entire range is pretty darned good for a 63 foot boat.
4) We seem to troll for fish at around 7 knots, which would get us 2 GPH or about $10/hour at $5/gallon fuel prices. This seems pretty darned good to us. This is 3.5 NMPG, or about $1.43 per NM.
5) When we're not in a hurry, we often cruise at around 8 to 8.5 knots, at around 4 GPH, getting 2 NMPG, spending $20/hr or $2.50/NM for fuel. This is the speed we use when we're moving the boat a short distance and just want to enjoy the ride.
6) When we're trying to cover a larger distance in a shorter time, we often choose to cruise at 9.5 knots, burning around 9.5GPH. I can see from my chart that if I slowed down only 0.3 knots from 9.5 to 9.2 knots, I would lower my fuel usage from 9.5 to 7.5 GPH or 1.2 NMPG. While I don't usually think too hard about fuel prices, if range were an issue, this would be worth doing. It seems that this is right around the point where fuel usage goes through the roof without much improvement in speed.
7) When we really need to get somewhere far away before dark, we open up the throttle a bit, and go at 10.5 knots. Still, this is only 1.3 knots faster than our normal "distance" speed, but uses 13.4 GPH rather than the 7.5 GPH we'd get at 9.2 knots. Clearly, we're well into the area of diminishing returns here.
8) While we have not validated the slow-speed numbers in less smooth water, we have had some experience with the higher-speed numbers in 4 to 6 foot seas. We seem to lose about a knot or, put another way, if we want to go at 9.2 knots, we'd need to use 1500 RPM rather than 1400, and put up with the higher fuel burn. The rougher seas make for more water pushing back against the bow, and more "climbing" of the boat over the waves, which accounts for this difference.
9) From these measurements, it's pretty clear that the boat has the range to cross any ocean on this planet, if you pick the right speed, even with copious generator usage (which for us is about 0.5GPH most of the time, and can be roughly calculated as 1.0 GPH for each 10KW of power used).
Postscript: Sea Spirit is now in Fort Lauderdale for about the next 3 months, as the Freedman family prepares for our next cruising season. We are at home in Honolulu, and recovering from jet lag. I'll be writing a final entry for this season soon, where I'll describe our voyage back up through the Exumas, and how we turned our captain into a very bored lady (because she had successfully taught us to do absolutely everything by ourselves). I'll also talk a little about our thoughts on Sea Spirit herself (plot killer - we're exceptionally happy with the boat, but are talking to the manufacturer about building the next size up). In that next posting, I'll also ruminate a little on what I've noticed about trawlers versus "go fast" boats, and also about the reality versus the perception of the differences between various brands of boats.
All the best in the mean time, and thanks for the wonderful and very helpful comments so many of you have made over the past several months.