You never know your boat’s real range until you start to make substantial ocean passages. Theoretical range in flat water with no current and little wind can be surprisingly optimistic so we probe the bounds conservatively. The 3,023 nm Indian Ocean crossing from Dampier, Australia to Rodrigues, Mauritius is the furthest we have ever gone between fuel stops but, still, it’s only a small and fairly safe extension over our previous longest run where we did 2,600 nm between Hawaii and the Marquesas Islands. On that 2,600 nm run we stopped and visited both Palmyra Atoll and Fanning Island en route, so the actual open ocean crossing distance wasn’t longer than previous trips. Prior to our Indian Ocean crossing, the longest unbroken offshore run we have completed was 2,000 nm from San Francisco to Hawaii. So the latest 19-day, 3,023 nm open ocean run is, by far, our furthest between both fuel stops and landfalls.
Every crossing brings new data and more experience. On this last run the most interesting learning was that our range is much further than we previously understood. Or, for a given distance, we will be able to travel it faster. We use the engine control unit (ECU) reported fuel burn numbers to control our speed and help ensure we arrive with the intended safety margin. We’ve been told these numbers are very accurate and we have tested them and verified numerous times that they are indeed incredibly precise.
We have hand-calibrated sight gauges where we have filled the tanks up from the bottom to the top in 25-gallon increments, marking the sight gauges on each increment, so they are now quite accurate. In addition, we have installed a Maretron FPM100 Fluid Pressure Monitor that we use to measure hydraulic system pressure, transmission system pressure and also tank levels. To measure hydraulic pressure we use a Setra 0-5000 PSI pressure sensor. To measure tank levels we use, depending upon the fuel depth in that particular tank, either a Setra 0-2 PSI or a Setra 0-3 PSI pressure sensor. These also required calibration where the tank is filled, this time in 1/16 of a full tank increments. But, once calibrated, the FPM100 provides approximately +/-3% accuracy. On large tanks, that’s an impressive degree of precision.
With a combination of the sight gauges and the Maretron FPM100 electronic gauges, we have a fair degree of confidence in the quantity of fuel being carried. This tank level data is far more precise than we have had in past crossings. What we have learned over the last year using these fairly precise tank level indicators is that the Deere ECU-reported fuel economy is very accurate. Prior to this trip, we have been able to detect no variance between the sight gauges, the electronic tank level sensors, and the ECU-reported fuel burn. But, on the Indian Ocean trip, the ECU-reported burn was 13% higher than the actual burn. The cause of the surprising discrepancy is that in coastal cruising, where we are operating at higher engine loads, the ECU-reported fuel burn is very precise. But, when operating at nearer to 25% load, rather than the 75% load and above we usually run when coastal cruising, the ECU is just over 13% conservative which is to say our actual range is 13% further than we thought.
It took nearly half the trip to conclude this was occurring and be confident that we were not seeing measurement error in tank levels. Each fuel level check ended up with a bigger number than expected. Early in the trip, it was only 10 to 20 gallons, so this was lost in the measurement noise. But, as the trip progressed, the number just kept going up. Eventually we had 160 gallons “too much”. At that point, the data was inarguable and we adjusted our speed to run much faster and enjoy the couple of hundred gallons of newly “discovered” fuel. This is one of the reasons why the second half of the run was so much faster than the first half.
This is great news because it means our maximum range is actually well in excess of 3,500 nm and, even better, we can run faster than we have been running on longer runs. It’s like we just added more than 300 gallons of fuel tank capacity to Dirona.
Another learning from a 19-day stint at sea is that, if the weather isn’t rough, it’s really not at all hard. It’s a bit more work and it takes a bit more motivation to get jobs done but, generally, it’s not a big thing. After 19 days at sea, we arrived feeling refreshed. It was not a strenuous or difficult passage even though there were only 2 people on the boat and we man the helm in shifts. Years ago we ran 4 hours on 4 hours off but we just kept evolving our shifts and ended up with Jennifer going to sleep at 6pm for 4 hours and getting up at 10pm, taking the helm from 10 to 5am, then sleeping from 5am to 10am or whenever she wakes up naturally. I take the helm for most of the day. It seems to work for us and, in good weather, it’s not much more tiring than a normal work day.
We have gotten comfortable with our shift schedule and don’t find it particularly tiring in good conditions. In rough weather, it’s tiring no matter what you do. It’s hard to sleep as soundly in rough water and it’s just more work to move around. So far, we haven’t found a solution to that other than bad weather avoidance by aiming for the statistical best weather time of year.
We have always operated Dirona by driving to the required fuel economy rather than the more common practice of setting a specific RPM for the trip. The reason we prefer to drive to fuel economy is that this really is the limiting factor on longer trips. We have found that current, winds, and swell can have a tremendous impact on fuel economy. When we first got Dirona, like many new owners, we carefully measured our fuel economy on flat water and were amazed at how good it was. Then we went out in the open ocean and were amazed at how much different the results were. Swell can take 3/4 of a knot off the top and, there are times we it can be as much as a full knot can be lost. This is huge because the fuel burn stays the same, the RPM stays the same, but the distance traveled can be reduced by upwards of 15%. The effective “loss” of 15% of the distance could yield some unfortunate surprises when crossing an ocean.
Arguably the negative impact of swell isn’t that surprising but what has been an eye opener for us is the power and speed of open ocean currents. It seems strange that the 1,000 nm from shore it’s possible to find current running a full knot or more, but it certainly does happen. What is perhaps even more surprising is how much it can change over very short distances. The overall ocean current predictions probably are reasonably accurate on average but the hour to hour changes in currents are not predicted and there have been trips where we have seen currents for days at a time that were running opposite to the predictions. I’ve jokingly concluded that ocean current predictions are only there to make weather prediction look comparatively accurate.
Because actual fuel economy can vary so greatly due to the force of winds, currents, and swell, you really only have two choices. You can allocate a very large reserve to account for the fact that the actual consumption can vary by 15 to 20% in adverse conditions or you will need to just periodically check point on the fuel load and distance left, and adjust speed to fit. The latter allows the boat to run faster and it seems the safer approach as well, so we run to the trip-required fuel economy.
Now that we have accurate fuel tank level data, I have written software that tracks distance left and fuel left on the trip and keeps an up-to-date fuel economy goal. The program uses this data to compute the needed fuel economy and the current rate of consumption and shows two green lights when operating at the correct speed. If conditions change and we end up getting better economy than predicted, one light is switched from green to orange to indicate that more speed will still allow the destination to be reached with the intended reserves. If conditions worsen and the fuel economy falls below the goal, then the left light changes from green to red to indicate that the boat needs to be slowed to improve the fuel economy. We essentially just “drive the lights” and verify the computations and electronic fuel level indications using the sight gauges on each engine room check.
Nothing is more relaxing when aiming to finish a run with 200 gallons remaining and arriving with exactly the expected fuel left. It’s nice to see the systems operating as expected. The summary data from this trip are:
- Fuel consumed: 2365.6 gallons
- Fuel left on arrival: 221 gallons
- Total actual distance: 3023.2
- Overall fuel economy: 1.28nm/g
- Overall speed: 6.6 kts
- Total trip hours: 461.3 (19 days, 5.3 hours)
- Gallons per hour: 5.13
- Average RPM: 1681