How To - Fuel Efficiency

Most boat owners believe they intuitively understand how to get the most miles out of a fuel tank. Or at least they think they do. The optimum combination of power and trim is actually a surprisingly complex equation with variables too numerous for the average punter’s grey matter to cope with. Even factors such as the sea state and load position all play a part. For us mere mortals, who prefer to concentrate on the task at hand, namely not sinking the boat and catching a few fish, help is here.

Regular readers will be aware that any new common rail diesel inboard or outboard engine of the post-carburettor era is likely to feature some sort of computer, quietly completing a myriad of tasks that will keep you buzzing across the ocean issue free. Almost all will include a control mechanism that ensures exactly the correct quantity of fuel is delivered to the engine for a given set of load parameters. Much of this data can be monitored by plugging a communication cable into the back of almost any modern navigation system. The most up-to-date version of the protocol that makes this process possible is called NMEA 2000.

What it means to a boat owner is that no matter which supplier a particular product comes from it will be able to interface with all of the other equipment on the vessel. That means any new NMEA-capable multifunction navigation machine can receive the important data (revs, temperature, oil pressure, tilt level and fuel usage) from an inboard or outboard engine and display it.

The development and utilisation of this information is quite varied between the multitudes of manufacturers. At the very simple end of the spectrum data is displayed in its raw form, leaving the skipper to make his or her own rough calculations on the most efficient speed and rev setting to operate any given vessel. At its most comprehensive the instrumentation will make adjustment suggestions that allow the skipper to ‘hyper-mile’ at the optimum level.

Modern marine diesel engines

Even in its most basic format the real-time display of this data is very useful. Modern common rail diesel engines are staggeringly fuel efficient when run at the optimum revs for the vessel’s hull speed, but like all engines, regardless of vintage, they become thirsty beasts driven outside those parameters.

To give a recent example, while operating a pair of 1100hp MTUs in a 51ft Riviera, I observed that the vessel travelled at about 8.2kts at 780 revs, using a total of 21lt/h (yes, that’s only 10.5lt/h per engine to propel a more than 30-tonne vessel at an effective lure-trolling speed). Push the engines up to 840rpm and the vessel’s speed increases to about 8.7kts, with the fuel burn increasing to around 36lt/h. That is a 70 per cent increase in fuel consumption for only half a knot increase in speed.

The reason is complicated, but in layman’s terms the most efficient hull speed for any vessel is linked to its waterline length. This defines the hull speed and that cannot be exceeded without the vessel attempting to plane, which is effectively like pushing up a slope, rapidly increasing the load on the engine and burning more fuel.

The hull-speed for that 51ft Riviera must be very close to 9kts, so as we speed towards that speed, the fuel usage increases at an exponential rate until it starts to plane.

At these slower off-the-plane speeds this real-time display of the raw data is more than enough technology to allow a competent skipper to run a vessel efficiently and effectively. However, vessel trim both longitudinally and laterally should be taken into consideration to get the best possible result. A good rule of thumb is to keep the loading as low and as near to the centreline (fore and aft as well as side-to-side) as possible.

To be honest correct vessel loading is basic seamanship and should be considered a safety issue long before it becomes a fuel efficiency one. Using trim-tabs to allow for the three big blokes all sitting on the same side of your eight-metre trailerboat is lazy and just plain dangerous, something you will discover in no-uncertain terms if you ever broach crossing a bar.

Fuel economy

Travelling at speed on a vessel with genuine power is one of the great pleasures of boating. I love blasting out to the marlin grounds with a couple of thousand horsepower at my fingertips, and Jimi Hendrix blasting through the bridge. It is almost a shame to have your fuel usage displayed so bluntly, encouraging you to pull the throttle back in the name of economics.

More basic fuel management technology is not quite as useful at speed as it is off-the-plane. This is where advanced fuel optimisation technology comes into its own. One system I have used extensively is Mercury’s SmartCraft ECO-Screen. To keep it simple for those, like me, who just care that it works rather than how it works, the ECO-Screen takes speed data from the GPS, and fuel usage and motor trim data from the outboard to recommend power and trim adjustments to the skipper that should give the optimum mile per litre settings for the conditions. What is particularly cool is the way the display changes from amber to green when all the parameters are aligned.

It is quite an eye-opener to see just how fast the optimum speed on the plane actually is. Trimmed out to a flutter by virtue of the ECO-Screen, the vessel I have used this technology on the most (a workboat of the company’s powered by a 150hp Mercury OptiMax) is optimised at around 3600 revs, achieving 24kts. You can cover plenty of miles at that speed, happy in the knowledge that you are making the most of your hard-earned fuel dollars at the same time.

This technology is certain to see fast uptake by all the major manufacturers and there is at least one multiplatform system that can be retrofitted available in Australia today (see www.floscan.com). How far this hardware progresses is anyone’s guess. I have no doubt that systems capable of automatically adjusting a vessel’s parameters are in use at the heavy commercial end of the industry today so it is only a matter of time before it becomes available recreationally.

How to troll faster?

Magazines and online forums endlessly debate the topic of trolling speed while gamefishing. Eight to 8.5kts seems to be the most favoured speed range but I am not convinced. My experience suggests that it’s when the engines are humming along nicely but not under excessive load; in short, a good speed that is pleasant on the ear is when the fish will bite. Often this “sweet spot” is well below 8kts, particularly on smaller vessels.

For trailerboat fishermen this is good news, as those short hulls tend to be inefficient at speeds between about seven and 16kts. My advice is to be patient and try slowing down to a more comfortable and economic pace for a couple of days. You will probably catch as many fish, maybe even more, and you will burn considerably less fuel. If you are worried that your lures will not be active enough, run noisier lures and have pitch-baits ready for action.

Caterpillar's on NMEA engine-monitoring displays (above) coupled to a pair of beautiful 1000hp C18 marine diesel engines (below).
Drawing depicts a typical NMEA network on a modern boat (image published by Wikimedia Commons).
Mercury's SmartCraft ECO-Screen in the "green" zone.

From Trade-a-Boat Issue 427, May-June, 2012. Story by Joe Fish; photos from Trade-a-Boat files.