Remember when we used to hang around the beach barbecue telling tall tales about the last storm or the big one that got away? Now, we find ourselves comparing fuel additives and rpm/fuel consumption rates, or swapping wild stories about where we found the cheapest diesel. Suddenly a small improvement in efficiency can mean a significant improvement in the thickness of your wallet. To keep our tanks full longer, there are a few big things we can do and many small things.
Slowing down is the biggest fuel saver. We all know that, but what some of us don’t know is that a small reduction in speed can make for a dramatic reduction in fuel consumption. Traditional displacement yachts have a maximum hull speed of roughly 1.34 times the square root of the waterline length. For example, a vessel with a 64-foot waterline has a hull speed of around 10.7 knots, a 49-foot waterline can reach about 9.4 knots, and a 36-foot waterline yields about 8 knots. However, to get that last knot or so of speed requires ever increasing amounts of power.
The venerable Skene’s Elements of Yacht Design by Francis S. Kinney has a nice diagram illustrating a resistance curve for displacement hulls at various speeds. The curve begins with a gradual slope until a speed of around 1.10 or 1.15 times the square root of the waterline length. At that point, the curve becomes a skyrocket, shooting up at a very steep angle. This indicates that ever-larger increases in power are required to make ever-smaller gains in speed. According to the diagram, at a speed ratio of 1 (1.0 times the square of the waterline length) there will be a resistance of about 12 pounds for every 2,240 pounds of hull displacement. Up that speed ratio to 1.34 and the resistance shoots up to about 54 pounds for every 2,240 pounds of displacement — an increase in resistance of 440 percent for an increase in speed of only around 25 percent. The last 25 percent increase in speed requires a huge increase in horsepower and therefore fuel consumption. In other words, that 49-foot waterline boat will require a lot of horsepower to go its hull speed of 9.4 knots, but slow it down around 25 percent, or to 7 knots, and you’ll save a lot of fuel.
This curve generally applies to most displacement type of power craft, including sailboats, trawlers, and commercial vessels. Robert P. Beebe described this phenomena in detail in his classic book Voyaging Under Power. With many formulas, graphs, and equations, he details exactly how fuel consumption increases with speed. The bottom line is that for high fuel efficiency, long-range yachts need to travel at speed ratios of around 1.0 to 1.2. Real-world testing has confirmed these facts. According to the manufacturer, the Nordhavn 55 has a potential range of around 3,000 miles at its cruising speed of 8.25 knots, but if you accelerate to its hull speed of about 9.5 knots, range decreases to 1,500 miles. A 15 percent increase in speed cuts your range in half!
Similar results will be found with most displacement yachts, and even with planing or semi-planing types that are operated at and below hull speed. A rough rule of thumb is that reducing your top speed by about 25 percent will cut your fuel consumption to around half it’s maximum, or less. Maybe it’s time that boat owners started thinking in terms of miles per gallon, as we do when driving on land. A 25 percent decrease from maximum hull speed will yield approximately a 50 percent increase in miles per gallon. But, the benefit begins to taper off as you slow up more and more, and of course you are traveling less miles each hour at slower speeds, so your increase in range is not as dramatic as the decrease in fuel consumption.
Beebe argued that a practical speed ratio was 1.2 times the square root of the waterline length, because that allowed a good speed with the possibility of decreasing fuel consumption further, if needed, by reducing speed to a 1.0 ratio. At speed ratios below 1.0 the decrease in fuel consumption is much more gradual and you may encounter a situation where your engine is running inefficiently too close to idle. Beebe also argued that you should plan on being able to reach your destination by traveling at a speed ratio of 1.2, with a modest reserve. If you encountered adverse conditions, you could always slow up to increase your range substantially. Beebe was most worried about range under power, but these same techniques can be used to increase the range of your bank account.
Is synthetic the real deal?
Slowing down is the big saver, but there are many things that can be done to gain a little here and there. The obvious ones are to make sure your bottom is clean (the boat bottom that is), the air cleaner is clean, and the oil has been changed on schedule. Here’s where things get controversial. Some companies, notably Amsoil, claim that using a full synthetic oil can result in fuel savings of 4 to 5 percent or more. Search the Amsoil Web page for more information and tests using fleets of diesel trucks. Of course, some marine engine companies do not recommend synthetic oil and warranties may be voided if you use a non-approved product. However, many boat owners do use full synthetic oils and blends and are happy with the results.
One negative factor is that the cost of synthetic oil is greater, which you may want to offset by longer drain intervals, but then again, you may not want to stretch the drain interval considering the above-mentioned warranty implications and the harsh marine environment. For example, Shell Rotella T Synthetic 5W-40 oil, at around $17 per gallon, is currently around twice the price of the non-synthetic version. Assuming a 4 percent savings in fuel, that would mean a savings of four gallons in every 100 burned. If your trawler burns four gallons per hour that means around 800 gallons burned between 200-hour oil changes. If your engine requires four gallons of oil in that time, the cost of synthetic would mean roughly an additional $34 for one change, with a fuel savings of about $160 in that same time span (diesel at $5 per gallon). So, at a glance it appears that synthetic oil has the potential to save some significant money, plus oil manufacturers claim that synthetics provide better wear characteristics. Of course, doubling the drain interval would result in greater savings. One thing to keep in mind is that synthetic oil may be difficult or impossible to find in many remote areas outside the U.S., and even if you do locate your brand it may be a different formulation. I have had difficulty just finding any suitable diesel engine oil when in the southwest Caribbean, and when I did find it the cost was higher than in the U.S. If you are locked into a particular oil, bring plenty of it.
Do additives add up?
Most marine engine specialists seem to agree that it is not a good idea to add anything to your engine oil — buy a good quality oil and change it per the manufacturer’s recommendations. However, start discussing fuel additives and you end up with a lot of different opinions, and probably an argument or two. The ongoing trend in diesel fuel in the U.S. is to lower sulfur levels to 15 parts per million (ppm) in order to meet EPA regulations and to allow for the production of cleaner marine engines using advanced pollution control devices. This product is known as ultra-low sulfur diesel (ULSD). Starting in 2007 sulfur levels were lowered to a maximum of 500 ppm. ULSD is already out there and will be the only fuel available in the U.S. on land after 2010 and on the water after 2012, with every exception expiring by 2014. This transition has already occurred in California. ULSD may very well be in your boat’s tanks already — check labeling at the fuel pump the next time you fill up. Possible issues with ULSD include lower energy content, less stability, and problems with gelling at low temperatures. Fuel manufacturers are putting in additives that will replace some of the qualities lost with the reduction in sulfur, but there are reports of the new fuels causing maintenance problems with older diesel engines.
A disadvantage of the new fuels is that the removal of the sulfur reduces the energy content of the fuel, resulting in up to a 1 percent reduction in mileage for truckers. However, this has yet to be documented in real-world testing on marine engines. This may be a case where there will be a clear cut need for proper additives, especially with older engines, as there was when lead was removed from gasoline. What is also clear is that the cleaner fuel will and already does cost more, and therefore we need to concentrate further on reducing fuel consumption. Another problem emerging for marine engine owners is the possibility of biodiesel being added to your marine fuel. At this time it appears that boat owners should not look to biodiesel as a fuel cost saver without doing a lot of research.
My own non-professional take on fuel additives, based on many thousands of miles of powering over the course of around 30 years of cruising, is that the majority of us don’t need to worry about biocide additives. Filter your fuel going in your tanks, if possible, filter it going out, and change those filters religiously. The major cause of bacterial growth is water in the fuel — check your water separator. I always try to purchase fuel from the busiest location, not necessarily the cheapest. If in any doubt, pump a little fuel into a glass jar, prior to pumping a lot into your tanks, to see what it looks like. I do use and recommend fuel stabilizer products, especially during layup periods.
What about additives to save on fuel consumption? There does not seem to be a definitive answer. Companies producing additives claim anywhere from 1 to 7 percent decreases in consumption, which in some cases is documented with long-term studies, usually of truck fleets. I have seen claims of reductions in fuel consumption of up to 17 percent. Who knows if you will be able to duplicate these results on your boat. I would toss out the higher figures, simply because if that were true, everybody would be flocking to those additives. Also, the lower numbers may be achievable, but it is hard to conduct scientific, controlled tests that can measure such small savings accurately. Imagine trying to do this in the marine environment. Maybe a company like Nordhavn can organize a rally with enough identical yachts sailing over an identical route to come up with some meaningful figures. Until then, I will remain skeptical of fuel saving claims that cannot be documented. Most engine manufacturers say that the use of fuel additives is not necessary nor desirable. My own philosophy is to realize that if it sounds too good to be true, it probably is.
Go with the flow
Another example of low-hanging fruit is careful voyage planning to maximize the use of favorable currents and to minimize running time.
When I recently voyaged from Isla Mujeres, Mexico, back to the Florida Keys, I spent days waiting for several tropical systems to clear out. During that time I also studied the Naval Oceanographic Office charts (https://oceanography.navy.mil/legacy/web/cgi-bin/graphic.pl/metoc/ 40/145/0-0-17/0) showing the Yucatan and Gulf Stream currents. A huge loop current extends from the Yucatan well up into the Gulf of Mexico, before descending back down toward Cuba. We aimed to get into the favorable current as quickly as possible, where we gained as much as 3 knots over the ground! And we avoided heading straight into the bad current on the other side of the loop. When you’re getting a boost of 3 knots you can cover a lot of additional territory chasing the current and still come out ahead. Similarly, when heading north from ports on the east coast of Florida, it pays to head offshore at a right angle until well out into the Gulf Stream in order to pick up 2 knots or more of favorable push. If your cruising speed is 7 knots, a gain of 2 knots is about a 29 percent reduction in fuel costs, which is a lot better than any fuel additive.
I am also skeptical of most mechanical and electrical fixes that are not already incorporated by the engine manufacturer. If these products are so good, why wouldn’t the engine manufacturer adopt them?
In summary, go for the big obvious gains when trying to save fuel and money.