Range and Operating Costs

When we started design work on this project, one of the first things we had to do was establish a desired speed range for which the hull would be optimized. From a cruising standpoint this is a tradeoff between operating costs, range under power, and completing a passage quickly. Speed is a major safety factor when it comes to managing weather, so within reason, we wanted the option to cruise as quickly as possible between destinations – with the ability to maintain Beowulf’s average speed of 270 to 300 nautical miles per day when required.

What surprised us when we went through this comparison is that under all scenarios, the operating costs of the Unsailboat were substantially less than those for Beowulf, when maintenance, repair and replacement, and fuel were considered. This is true at US$35 per barrel oil and at US$60 per barrel.

The biggest variable with the Unsailboat is speed. The faster we go, the lower the “mileage” (just like with a car), and the more per nautical mile it costs to run the boat. But since these costs are always going to be less than what we’re used to under sail, the issue becomes optimizing the hull for a speed at which we think we’ll run most of the time.

We normally use a 1200 nautical mile passage as an average when evaluating performance factors. This distance covers most of the cruising passages: East Coast of the US to the West Indies, New Zealand to the tropics, Southern California to mainland Mexico.

With our 1200-mile passage, there is about an eight-hour time differential between 10 and 11 knots. Is this worth a substantial increase in cost? Not to us. However, if running a little faster would get us through the pass into the lagoon before dark, then we’d speed up. And where weather risks are an issue, the extra speed is used to reduce our exposure time. Our best estimate during the design cycle was that we’d probably run at 11.5 knots based on noise and comfort levels. So our hull shape was optimized for an 11.5.

With the boat in the water we’ve found that in smooth water, and when plugging uphill in head seas we seem to average about 11 knots through the water. With a bit of breeze and sea behind us, we speed up to 11.5 to 12 knots. All of our passages so far have had a weather element involved, so we’ve pushed a little harder than might have otherwise been the case.

When you consider the overall costs of owning a yacht, fuel itself is a small part of the total. Insurance, dockage, not to mention the cost of capital (if you allow for this) add up to a significantly greater total than the fuel bill. Where fuel does come into account is when we look at how far we can go on a single fill up.

Industry norm is to calculate range at half load, in smooth water, without any allowance for generating power or using stabilizers. That’s not very realistic. In the following sections you will find data based on actual passaging, starting with full tanks, and then refilling to see what was burned. The data is quite interesting.

Pago Pago, American Samoa

When we first looked at doing an Unsailboat, the cost of running a boat like this was much on our minds. Not only were we concerned with the costs in general, but this had an impact on the speed for which we would optimize the hull shape. What we found, with oil at US $35 per barrel, was that we’d be spending a little over one third the amount per mile with the new boat that we’d spent with the 78′ (24m) ketch Beowulf – when all expenses were considered.

So here we are in Pago, Pago, American Samoa, and oil is currently going for US $60 per barrel. We’re in Pago Pago because this is the cheapest place in the Pacific, except for Panama, to purchase diesel. There is a large fishing fleet here, which consumes huge quantities of fuel, so the fueling is efficient, there is good quality control, and the price on the 14th of September, 2005 was US $2.15/US gallon (57 cents per liter).

In the last three months we have consumed 1,696 US gallons (6419 liters) of diesel for powering the boat, making electricity, heating domestic hot water, and creating hydraulic power for the stabilizers. This works out to 6.82 gallons (25.8L) per hour and 1.83 nautical miles per gallon at 11.5 knots average boat speed (we ran at 11.4 coming up from Fiji, 11.8 knots powering around Fiji, and 10.2 knots uphill to Pago Pago from Fiji). On a nautical mile basis, this comes out to US $1.27. To this we need to add the cost of oil changes (roughly three cents/mile for oil and filters), the cost of an overhaul on the engines at 20,000 hours (4.5 cents per mile), transmission overhaul at the same time (2.7 cents per mile, and general fudge factor of another ten cents per mile to cover incidental maintenance. This adds up to US$1.44 per nautical mile.

You might argue we should use the cost of fuel in New Zealand, or an average of New Zealand and Pago Pago. New Zealand diesel came aboard at US$2.33 per US gallon (duty free) in early July.

The cost could go up or down depending on the price of diesel the next time we have to fill up. It is also affected by the weather we encounter. Headwinds significantly increase fuel consumption, while tail winds reduce it. We also have the ability to impact cost by reducing or increasing average speeds. And, given the huge fuel capacity of this boat – 3,600 US Gallons (13,600 liters) – we usually have the option of waiting for cheap places to fill up.

Bottom line, this is still significantly less costly than what we lived with under sail.

Fuel Burn – Hawaii to California

As we’ve discussed previously, there are all sorts of ways to look at fuel consumption and range under power. In our own calculations, we started out with smooth water figures, and then added in allowances for adverse weather and other power consumers. The data presented in the preceding section, based on our fuel stop in Samoa, was from a series of passages which had only modest negative impact from weather. As we’ve just filled our tanks again (which were last topped off in Kona, Hawaii) we now have some real-world numbers for strongly adverse conditions – a worst-case scenario, if you will.

The trip back from Hawaii was under the Eastern Pacific high, with headwinds for all but the last day and a half. In addition to the headwinds, we had a northwesterly swell crossing the northeasterly wind waves, creating a worst-case scenario in terms of rough water drag. This in turn caused the stabilizers to work harder, so they used a lot of hydraulic power. With the stabilizers working to keep the boat on an even keel (which they did wonderfully), the engine had to work harder to overcome the induced drag of the stabilizer fins. Add to the above windage drag from the breeze, which averaged about 18 knots for most of the trip. Finally, we were running the air conditioning for half the passage, with the heater going for the last few days, in addition to the normal electrical requirements of the boat.

Our speed through the water varied with conditions, but averaged right on 11 knots for most of the passage. The last day, with the barn close by, we sped up to 12.5 knots.

Based on hours run from the engine hour meters, and a 100% refill in California, fuel consumption for this leg was 7.6 US gallons (29 liters) per hour. Keep in mind that we kept the boat at full load for all but the last day (by replacing fuel with fresh water from the watermaker) to maintain maximum comfort.

Pacific NW through Alaska

And now for the latest. The following is after changing to a new set of more efficient propellers, and is based on refilling our fuel tanks at Kovich and Williams on Lake Union in the center of Seattle.

Just under 4000 nautical miles have passed under the keel since we last topped off the tanks in June. Most of these miles have been in protected water ways with smooth conditions. What breeze we have experienced has tended to be on the nose, but light – typically in the 10-knot range. There were two short jaunts outside in fresh following winds. Our speed averaged 11.3 knots 90% or more of the time. There have been a few periods where we ran at 12 to 12.5 knots and a few hours where we slowed down to 10.5.

Subtracting fuel for the genset (28 hours) and the diesel heater (operating for 50 days) leaves us with an average fuel burn of 6.4 US gallons (24.5 liters) per hour. Included in this is the consumption of the hydraulic system (for stabilizers – lower than normal due to smooth water) – and power generation to charge batteries and do laundry.

When we’re offshore, in big seas, we tend to keep the boat heavy, replacing consumed diesel with fresh water from the water maker. This summer we’ve tended to carry a couple of tons of extra fresh water, so we could take full baths and long showers, but overall we’ve been lighter than we would be offshore.

All of which has led to a figure of about 1.76 nautical miles per gallon on average, for a range of 6300 miles at 11.3 knots (1900 RPM). This is an improvement of around 10% from what we tested during sea trials, indicating the new props are taking us in the right direction. It is also better than what we saw on our passage from New Zealand to Samoa. However, it is hard to compare the figures from that period, as they include a lot of miles with the wind and waves behind us, a few with wind and sea on the nose, and more air conditioning and stabilizer loads (which do not, in any event, create large power needs).

When we started out this summer we had expected to spend most of our passaging time at 10 to 10.5 knots. However, we naturally found ourselves running faster. Somehow the extra speed always seemed to be called on to get us to a really cool spot with plenty of daylight so we could put the dinghy in the water and explore, or in time for a quiet supper at anchor. We’re still working on learning to go slow – we could significantly enhance range and reduce cost by slowing down just 10% – but after all these years of sailing fast, and with Wind Horse no louder at 11 knots+ cruising speed than we were accustomed to with Beowulf at the same under sail, we’re finding it hard to back off.

You can easily calculate horsepower from fuel consumption. In this case, burning 3.2 US gallons per hour per engine equates to about 60 horsepower per engine at 1900 RPM. This is about half of what the engine is capable of delivering at this RPM, so there is plenty of power left for adverse conditions, yet enough load to keep the engines warm (and happy).