Fuel economy data

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.

Comments

Looking forward to your recap Dan. It seems that I will be continually able to live vicariously through you guys. I'd love to hear a little more about how the kids faired, and if the Boat schooling was any different than home schooling for them.

 Grant T  4/24/2011

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Grant, the kids did well, although not in equal amounts. My son (age 10) took to the boat like a fish to water. My daughter (age 8) missed home a lot more. But the schooling continued pretty much as we did it at home, except at a slower pace due to all the amazing activities we undertook. It is fair to say, after 5 years of home schooling, that we know things go better if we double the amount of time we think we'll need in any one place. Half of the time will be for sightseeing, and the other half for simply living - which includes the schooling. Dan

 Dan Freedman  4/24/2011

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SUBJECT: RE: Fuel economy data Dan: Great information, and a great report. I’m very jealous of your stats. -Ken Williams

 Ken Williams  4/25/2011

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Ken - I believe your boat displaces more than twice mine, and that you and I have identical engines but you have two of them. I presume that your engines will idle at the same fuel flow as mine (but twice). Out of curiosity, what do you get for speed at idle? Since your boat has a slightly longer waterline length than mine and also has twice the engine HP, I suspect your top end is higher than mine (10.8 knots with 2/3 fuel, 11 knots with 1/3 fuel). Am I remembering correctly from one of your posts that you get about 9 knots at 12 gph or something like that? Given the extra beam and height of your boat, those aren't bad numbers at all! Dan

 Dan Freedman  4/25/2011

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Glad to hear it all went so smoothly - a tribute to your exceptional planning skills, among others. Welcome home!

 Bob B.  4/25/2011

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Hi Dan, Very informative post, as always. In reviewing the Sea Spirit against competitors I believe the trade-off for the slippery hull and shallower draft would be less room down below. What are your main reasons for considering a larger boat? Do you need more room for guests, for the kids, or both? After spending the season on-board what improvements would you suggest over the current layout? All The Best, Ken H

 Ken H  4/26/2011

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It's an interesting question, Ken. Sea Spirit doesn't have the forward "basement" space that the Nordhavn 55 has. In the N55, the master and VIP cabins are only a couple of steps down from the galley, creating a massive storage "basement" space underneath. In Sea Spirit (as in most other boats), the staterooms are down a flight of stairs. The result is that the N55 pushes the floor of the foredeck very high up, whereas Sea Spirit has a very protected foredeck. Aft, there seems to be more room in the lazarette than in deeper-but-rounder hulled trawlers. The cockpit floor is high enough that the laz has a lot of headroom (but not stand-up), although as a result, the cockpit is too far above sea level to reach over and grab a fish. Fish grabbing has to be done from the swim platform which, thankfully, is fairly large. Except for the fuel tanks, which take up a LOT of space port and starboard in the engine room, all other tankage is centerline under the floor, which seems to make sense. I believe the keel itself is not as deep as on some other trawlers, and as a result, the propellor is a few inches less in diameter. Initially, I was worried that the prop might not be as efficient, and I'm sure that's true. But the overall hull shape is obviously (from the numbers above) very efficient, so it appears they made a good trade off here. In terms of changes, we'd opt to slightly re-work the third cabin in order to get a better use of the space in there, probably turning the two existing bunks so that they are 90 degrees to each other rather than vertically above each other as they are now. We also feel the lack of a "utility room", which some Selenes have, and which always struck me as a good idea. The master cabin is so massive that probably it could be shaved down to make a utility room next to (or perhaps even inside) the engine room area. Finally, remembering that we are a family of 4 with visitors, we are looking at the next size up of Sea Spirit (the 75), and would be very happy to buy from the same manufacturer again. Having said that, there are many good boats being built, so competition is stiff! But overall, we really like the particular set of decisions that was made with this boat.

 Dan Freedman  4/26/2011

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Winter 2011: Training from Lauderdale to Georgetown and back!
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