Thinking about fuel efficiency

I swapped a couple of emails this morning with another Nordhavn owner talking about fuel efficiency.

 

Overall, I’m very satisfied with the fuel consumption of my boat. That said, given current fuel prices, anything that can be done to increase fuel efficiency is a good thing.

 

Following is a table I put together based on fuel consumption observed on my boat:

 

3000 gallons of fuel

300 gallon reserve (10%)

2700 Usable fuel

Twin Engine

RPM Speed GPH GPH GPH Range Miles/

Port Starboard Total Gallon

1000 6.7 3.8 3.5 7.3 2,478 0.92

1100 7.3 4.5 4.3 8.8 2,240 0.83

1200 7.8 5.9 5.5 11.4 1,847 0.68

1300 8.6 7.4 6.8 14.2 1,635 0.61

1400 9.3 9.5 8.7 18.2 1,380 0.51

1500 9.9 12.0 11.5 23.5 1,137 0.42

1600 10.6 14.1 13.6 27.7 1,033 0.38

1700 10.6 16.6 13.6 30.2 948 0.35

Single Engine (Running with one prop in neutral)

1100 5.2 0.0 4.9 4.9 2,865 1.06

1200 6.7 0.0 6.0 6.0 3,015 1.12

1400 7.5 0.0 10.5 10.5 1,929 0.71

 

These number should not be considered as accurate. It is virtually impossible to get reliable data. Sea conditions, currents, how clean the bottom is, the amount of fuel and water on board, and many other factors can change these numbers materially. However, these numbers do give a broad sense of what the consumption is, and how it varies by speed.

 

Generally, I average around 9 knots, and average around 5,000 miles a year. Looking at the table above, this implies that I can go about .56 miles for each gallon consumed. In other words, I should budget to buy around 9,000 gallons of fuel a year. At $5 a gallon, that’s $45,000 per year! A 5% savings would be $2,250 in my pocket annually. Of course, I could save significantly more, simply by taking my time whenever I cruise, but what fun would that be?…

 

Also importantly, I have several long distance passages planned. I usually think in terms of the crossing to Hawaii, which is 2,100 miles. Or, a more critical distance for me is from the Aleutian Islands to Japan, a crossing we may be making next summer. Our plan is to stop in Siberia, but it is far from certain that we’ll be able to do this. If we can’t stop I need to be prepared to run 2,000 miles non-stop, and want to do so as quickly as I can. It will be typhoon season, and “hanging out” is not advised. Looking at the chart above you can see that I need to run at 7 knots to make the crossing safely. Yuck.

 

I do believe that these numbers were captured unscientifically, and are low. On a major passage I absolutely believe I could outperform these numbers. But, I don’t know by how much, and would hate to be wrong.

 

Our focus, in the discussion this morning, was on whether or not swapping the prop to one optimized for long range cruising might give the extra efficiency needed to extend range. Most boats, mine included, have fixed pitched props. Many airplanes, and some boats, have variable pitch props, which allow you to tune the pitch of the prop (the angle of attack) to maximize fuel efficiency under different conditions. My thought this morning was that it might be better to optimize the prop around the cruising that is done 99% of the time, rather than the 1% of the time when making a major passage.

 

I did consider putting in variable pitch props when the boat is built, but decided to go with the simpler solution. Anything I put onto the boat I need to get repaired in third world countries. A “simple” prop seemed the smarter move.

 

The bottom line: None .. no decision was made. It was just fun discussion…

 

-Ken W

 

5 Responses

  1. Hello Ken-
    I am an aspiring Nordhavn owner, looking forward to early retirement at 45.
    I’ve fallen for the lines of the N62, though love what you’ve come up with in the unique design of the N68. I was curious if it was possible to add a fly bridge to a N62? My thought is that a N62 without a dry stack could be easily refitted to support hardtop for all electronics and hardtop over aft upper deck lounge. Perfect scenario would to include a hot tub like your beautiful N68. You had mentioned in your book that you had explored the idea of refitting San Souci #1 with upgrades, was a fly bridge one of them.
    Any recommendations or suggestions would be greatly appreciated.
    Best
    RF

  2. Good catch David! I should have mentioned that.

    Here’s the issue:

    I have a hydraulic system that runs the stabilizers. It runs off of a hydraulic pump which is at the back of each of the two engines (I have two pumps). There is a button that engages either the port pump, or the starboard pump. The pump puts a lot of load on the engine, so I alternate every few hours between the two engines. The stats indicate that I was running the port engine.

    One thing I don’t like about the pump is that it throws out a lot of heat. The transmission, on the side with the pump engaged, runs 10 to 20 degrees hotter than the other side. It’s not hot enough that it is indicative of a problem, but I generally like symmetry. I would like it if the temperature gun gave the same reading from both engines, but it never can work out that way.

    The hydraulic pump is needed for the thrusters, and for the stabilizers. When running the thrusters (parking the boat), I run both punps together, to compensate for the low engine rpm, and to provide maximum horsepower for the thrusters.

    -Ken W

  3. John… It is interesting to contrast our two boats. At 7 knots, you burn 3 gallons an hour, not counting the generator, and I’m burning 8 gallons per hour. You weigh 22 tons, and I weigh 100 tons. In other words, I weight 4.5 times as much as your boat, but only burn about 2.5 times as much fuel. That doesn’t seem that bad a comparison. As to “Length at Waterline”, I am 63′ and your boat is 25′. So.. I am nearly twice as long, and burning nearly triple the fuel. Of course beam has to factor into the equation. I am 21′ wide, and your boat is 15′.

    To add a bit more complexity to this discussion: I believe that my boat is about 10% less efficient than a single engine boat. I say this based on running alongside an N64 (Samurai) off of Mexico and comparing fuel burn at the same speed. The twin skegs on my boat seem to add friction to the bottom.

    -Ken W

    PS … I’m not sure what Nordhavn is telling people about the range on the N68. My guess is that they do their calculations based on a much lighter N68 than mine, with a single engine — and, in totally calm conditions (which I never seem able to find).

  4. Ken, interesting to see the large difference between our 40 and your 68. your fuel burn is much greater than I would have expected for a larger full displacement boat. we run on average at 1750 RPM, providing 7 knots and burn around 3 gallons per hour. Our numbers are in line with the 40 taken around the world and others posted.

    A few observations that you may want to look at:
    You run much lower RPM then us
    Our water line is 35′ yours is ?,
    we weigh around 45KLBS you weigh ?

    how does your actual burn rate compare to projected by PAE?

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