Publish Date: Thursday, October 22, 2009
Location: Beaufort, South Carolina
Coordinates: 32° 25.792′ N 80° 40.445′ W
We just completed a run from Beaufort, North Carolina to Beaufort, South Carolina (spelled the same, but pronounced completely differently: Bo-fort vs. Bew-furd). This cruise covered about 280nm, which took about 42 hours (two nights at sea). We ran about 30 miles offshore in the Atlantic, where we had very calm seas for the first 18 hours, and then 1-2 foot following seas for the remainder of the run — it was a nice ride!
In preparation for our Pacific Ocean crossing next spring, we conducted a series of fuel-burn tests while we were underway. The stated range of our Nordhavn 43′ trawler is 3000nm, but the actual range is quite variable, based on a variety of factors. Some factors are under our control, like how fast we run the engine, and how much we use the generator underway. Other factors are out of our control, like how much wind or current we encounter. We conducted the fuel-burn tests to help us make decisions about those factors that are under our control. It’s about 2700 miles from the Galapagos Islands to the Marqueses Islands in the South Pacific, and you really don’t want to come up short on fuel during this crossing!
The first thing we did to prepare for the tests was to establish a method to accurately measure fuel consumption. The supply tank to the engine (also called the “day tank”) has a small reservoir (approximately 2 gallons) at the top of the tank, with a sight-gauge. Under a normal running configuration the supply tank is fed by one of the two main fuel tanks, so it always stays full. But if we turn off the main feed valve, the supply tank level drops as fuel is burned (the supply tank is also configured as the “return tank”, so fuel being returned from the engine and generator goes back into the supply tank). Before we left port, using a measuring cup, I carefully drained fuel from the supply tank, marking an exact one-gallon scale on the sight-gauge. Once underway, we did a series of fuel-burn observations with the engine running at various RPMs. The exact procedure was the following:
- Set the engine speed to the desired RPMs (e.g., 1800).
- Close the main fuel tank valve to the supply tank, after which the fuel level in the supply tank will begin to drop.
- When the fuel level reaches the top of the one-gallon scale, start a timer, and note the trip-log mileage.
- When the fuel level reaches the bottom of the one-gallon scale, stop the timer, and note the trip-log mileage. It took 15-40 minutes to burn one gallon of fuel, depending on the current RPMs.
- Open the main fuel tank valve to the supply tank.
This procedure accurately measured how long it takes to burn one gallon of fuel at the given RPMs. With some simple math we can convert this into the standard “gallons per hour” number for each speed. The procedure also measures how far we travel on one gallon of fuel, but this can be a highly variable measurement, depending on the present sea conditions (wind, waves, and current). One of the reasons we ran these test on this cruise was because the seas and wind were calm, and we estimated that we had a very small and consistent current, which gave us relatively accurate distance results. Below are several graphs of our fuel-burn tests. Our usual cruising “sweet spot” is about 1800 RPMs, so we ran tests for 300 RPMs on either side of 1800 (i.e., 1500-2100 RPMs). You can see larger versions of the graphs by pressing the “Full Screen” link at the lower right corner of the album.
What’s the bottom line? By managing our speed, we should be able to get across the Pacific Ocean with only our primary fuel tanks (i.e., no supplementary fuel bladders on deck). The general approach to a crossing like this is to run slow (1600 RPMs) for the first half of the trip, and then gradually increase speed as the distance to the destination shrinks. If we have sea conditions like we had on this last run, it will be an enjoyable 16-18 days at sea. 😉
How have you accounted for increase fuel burn due to using your stablizers more in the rough Pacific?
Hi Kent,
It’s a good question. Stabilizer drag is part of how different sea states can affect our “mileage”, and we’ll have to build in a cushion for these kinds of variables. In the end, we may carry extra fuel on deck just to be safe — we’ll see.
Thanks,
David
What an eye opener. I never realized a genset would effect your range as greatly as it does. Can you run the Pacific using only your alternator? If not, can you calculate the amount of time you will be forced to use your generator and therefore your range?
Can you rig up a sail? That may solve the generator issue. I wouldn’t want to think of that passage with no ice. In fact, you could take our boat and we’ll pick it up in Auckland.
Hi John,
We do not need to run the generator all the time. The only essential function we need it for is to make fresh water. We can make about 25 gallons per hour with the water maker, so we would not have to run the generator very long each day. We’ll also start the trip with 300 gallons in our tanks.
We would also run the generator to do laundry and use air-conditioning. The laundry can be done when the generator is running to make water. Air-conditioning is not essential to get across the ocean — it would just make us more comfortable if it’s unbearably hot and humid.
Thanks,
David
Hi Marianne,
Don’t worry: We can make ice without running the generator! Your favorite food will be in abundance in the middle of the ocean. 🙂
David
David;
As you well know strange things can happen at sea. I strongly urge you to have extra fuel on deck for any emergency that may cause you to burn extra fuel. Its your family, better safe than sorry.
Stan
Dave, I think I would do more rougher water computing before setting off at 1600 RPM. A good start when leaving to build a fuel reserve would be 1350 rpm. You will be heavy with full fuel, provisions and additional spares so you will be slow. Neverthless you will still average near 6 knots. As the fuel lightens you will naturally pick up speed after the half way point. If at half way you see your way clear to increase speed you may. Another thing to think about, if you do the math, an additional half knot doesn’t make that much difference in time. Once you are in your routine you will be comfortable to go forever, not like the testing two nighter. AND, when you get Pacific pricing on fuel you will be glad to have saved an additional 100 gallons or whatever vs a few less hours under way.
Hi Scott,
Thanks very much for the suggestions. This is the first time we will have done this, so we can use all the help we can get. Your successful crossings are an inspiration to us, and we’re happy to learn from your experience.
Safe cruising!
David
Is there a small reliable wind driven marine generator that can keep a batteries set/inverter set-up available for minor needs, so as not to run the main gen-set so often?
Eli
Hi David
What model Lugger do you have?
Ours is an L1066T.
Whilst underway with a slight head current, a bit over 1/3 fuel, full water tanks and no wind, we were using at :-
1500 rpm 1.7 U.S Gallons Per Hour /6.43 L 6.3 Knots 1 USG = 3.78 L
1600 rpm 1.9 “ / 7.19 L 6.6 Knots
1700 rpm 2.3 “ / 8.70 L
1800 rpm 2.7 “ /10.22 L
1900 rpm 3.4 “ /12.87 L
2000 rpm 4.0 “ /15.14 L 7.6 Knots
2100 rpm 4.7 “ /17.79 L
2200 rpm 5.5 “ /20.81 L 8.0 Knots
2300 rpm 6.6 “ /24.98 L 8.3 Knots
Now the majority of the time we cruise at 1500 rpms.
We will keep watching your travels and investigations in readiness for our ocean crossings in the future!
Travel Safe
Karen
Cheers Karen
Hi Karen,
Thanks for sharing your data with us. It’s helpful to see the results that others are getting.
Our engine is an L668D, which is naturally aspirated. Our boat was delivered in 2006.
We still have a lot to learn before we cross the Pacific, but we’re looking forward to the process, and to the crossing. We’ll look you up when we get to Australia.
Thanks again,
David
Hi Eli,
Our main engine alternator keeps our batteries charged while we’re underway, and the inverter provides 120-volt service to the boat. While underway, we would only run the generator to (a.) make water, (b.) do laundry, or (c.) run air-conditioning. If we’re at anchor, we also have to run the generator to (d.) charge the batteries, and (e.) heat water.
Thanks,
David
Hi david,
very interesting data as we work up our own 43 towards longer passages!
To compare with Karen’s data, the best info I have to hand is as follows, based on the short passage making we currently do:-
1800 rpm 10 L /hr
1900 rpm 12 L/hr 7.4 knots
This is with the boat in the calmest sea conditions I’ve ever seen, no wind but with the Stabilisers on ( doing nothing!)
The boat had 400 USG of fuel and half the water load turning 25.5 tonnes on the boat lift and a “just cleaned” bottom.
With somewhat more waves and wind, we had a result of :-
2200 rpm 22 L/hr 8.1 knots
so the results seem to correlate reasonably well with yours and Karen’s.
Our 43 is #34 with the electronic L1066T and all the results are taken from the Murphy Gauge so the fuel delivery is theoretically what the fuel pump thinks it’s injecting into the cylinders. Call me cynical, but this might not be what is actually going in, which might be more reliably measured by your method…
Love your Blog, by the way – areal inspiration to those of us early in “the process”
best regards,
Colin Ingram
Suilven N43#34
Hi David,
We did some fuel computations travelling from Florida to the BVIs. This was done with 10-12 kt headwind and a nearly full load (fuel, water and supplies). The boat speed is approximate from our paddle wheel speed log and not GPS so any current was not a factor.
1400 RPM 5.2 kts 1.6 gph
1500 RPM 5.5 kts 1.8 gph
1550 RPM 5.7 kts 2.0 gph
1600 RPM 5.9 kts 2.2 gph
1650 RPM 6.1 kts 2.4 gph
1700 RPM 6.3 kts 2.6 gph
1750 RPM 6.5 kts 2.8 gph
1800 RPM 6.6 kts 3.0 gph
We often cruise at 1750 rather than 1800 as at 1800 we feel that our fuel burn becomes less economical. For long range work we would definitely go slower. Levity is hull #5 and after hull #8 PAE altered the propellor pitch slightly so you may be more efficient than us. The 43s appear to have less efficient hulls than the 46s although they can carry more fuel. There is no way we would get 6.0 kts at 1350 RPM. As Scott Flanders says I think the boat’s load makes a big difference to its performance. What was your approximate load when you measured your fuel consumption? On a Pacific crossing we would be taking extra fuel in the form of fuel cans so that we would also be able to fill up from local service station when there is no fuel dock available.
Cheers, Bill and Rosemary
i was wondering if u were going to use solar panels to try and reduce fuel consumption
Have you tested speed and fuel consumption on the wing engine? Wouldn’t you need to plan its fuel consumption from the half way point to insure an arrival under your own power?
You should read Ken Williams blog. The hydraulic alternators cause a higher fuel burn. I can’t remember what the rate was but if you contact him, I’m sure he will tell you what it was on his 68 footer. An extra fuel bladder as a safety measure on the rear deck to be used first is a good idea or convert your used oil tank into a temporary fuel tank to use first. Just want you to be safe and be able to make the passage. A little reserve is a safety item that I would stress.
Here is something to keep in mind w/diesels and fuel burn..RPM is controlled by a governor, which supplies the amount of fuel to maintain a set RPM.. As load increases or decreases, fuel burn will change…In land equipment, this can vary a great deal…With boats, I would think that the load would be pretty much the same..I don’t know how much climbing swells and riding down their backsides would effect the loading on the engine..I would assume some..A pyrometer, would give a good indication of engine load, and if it was steady fuel burn would likewise be constant..If it was varying up and down several hundred degrees, fuel burn would also be going up and down…My guess would be that at cruising RPM’s that NA engine would be seeing exhaust temps of 600-700 degrees F. just food for thought, Great site..