I’ve noticed lots of questions and discussion recently about how fast to run your boat, specifically what is the best speed to run a cruising motorboat or sailboat or traditional trawler for efficiency and proper care of their diesel engine? I've noticed this question has been a favorite among boat owners since we started PMM back in the mid-’90s. And while lots of things have changed, I believe the basics still apply.
The older trawler-style boats from the ‘70s and ‘80 were fairly straightforward, powered by low-horsepower diesel engines that drive the boat at a comfortable speed while providing tremendous economy. That was the reason for the great interest in the trawler-style cruiser that created this niche. Slow, comfortable, safe, and economical.
Displacement and semi-displacement hull shapes are most comfortable if their engines are pushing the boats at steady, quiet, single-digit speeds, where there is minimum noise, motion, and fuel burn.
Many engine seminars attended over the years confirm a basic truth, no matter if you have 135hp Ford Lehmans or a turbocharged Deere or even a high-speed Yanmar. Diesel engines like to run under load, and are best kept at near 70 percent load most of the time. Some situations make that difficult, such as running a canal or protected waterway that requires slower speeds to minimize wakes and shoreline damage. But with introduction of electronic engine controls, the ECMs monitor and adjust the engine parameters to keep that from being an issue, unlike large, older mechanical diesels that create black exhaust smoke because the prop is overloading the engine or carbon is building up from lower operating temperatures and under-loaded operation for long periods of time.
However, let's get back to the question of finding that sweet spot for your boat that balances hull speed, engine performance, and fuel consumption to provide the best, most efficient running speed in open waters. When you find that rpm and hull speed, you can expect the boat to run almost forever, comfortably and without stressing systems from speeds that are too fast or too slow.
Despite whatever formula you may have heard about maximum hull speed, it is way simpler and more accurate to think in terms of finding an operating range that feels good to crew, the engines, and the way the boat handles.
Take a lovely motoryacht, such as a Fleming 55. While engine choices varied over the years, let’s say for this example that at 11 knots, the two engines burned 14 GPH (total). By slowing the boat down to 9 knots, the fuel burn drops to 6 GPH. (Obviously many factors come into play here, but this is close enough for this discussion.) The Fleming hull needs more horsepower to get up to speed, but the efficiency goes out the window when you crank the boat up to 17-18 knots, with fuel burn out of sight. Yes, the boat can make that speed, but it is neither economical nor efficient. The same is true for most boats, with the exception of high-speed planing boats that are designed to live at that upper speed range.
To best understand this relationship between hull speed, engine speed, and load, and fuel burn, it is very helpful to create a speed and fuel consumption curve for your boat (and not from builder literature). Many factors come into play, such as whether the props are sized properly for the boat and engine, how clean the bottom is, sea conditions, and how the boat is loaded. But to build a basic curve, you just need a clean bottom and running gear, and some deep, flat open water.
I was lucky to spend lots of time over the years with the late Bob Smith of American Diesel, and I've watched him go through this more than once. What you do is simple: Run the boat along a straight course, recording boat speed at incremental increases of engine rpms, giving the boat time to settle at the new speed. Also note the fuel burn if your display shows that information. (Not to worry if you have older engines, as all engine manufacturers provide tables of fuel consumption for each engine speed.) Keep recording increasing engine speeds at intervals up to wide open throttle (WOT), although that becomes less relevant for this purpose but does prove your props are matched to allow the engines to reach WOT. For many diesel engines, WOT is around 2,500 rpm, while for the higher turning engines such as Yanmars, that might be much higher, say 3,500 rpm. Keep an eye on engine temperature and oil pressure.
Bob Smith always insisted that one must run this course in both directions to mitigate tide and current for averaged performance numbers which will be used to create the curve. And very importantly, be aware of how the boat feels at these different speeds, as I guarantee you will notice a specific speed zone where it all seems to come together in harmony and unison.
You then take this information and develop a chart from these numbers, showing rpm and boat speed and fuel burn, or you can label them on graph paper, with dots representing the various engine speeds you measured and what GPS-sourced boat speeds they represent. Then include a fuel burn curve to match the rpm. However you list this information, chart or graph, you will see a point where the fuel drastically increases as attempts at higher speed push the engine, and it starts working harder.
Sample Speed/Fuel Burn Performance Curve
Engine Speed (RPM) Boat Speed (NMPH) Fuel Burn (GPH)
600 3.1 0.3
1,000 5.3 0.7
1,500 7.9 1.9
2,000 9.8 4.9
2,500 16 8.4
3,000 19 14
3,500 WOT 23 22
In the above example set of numbers, one can conclude that 16 knots is a reasonable cruising speed for this boat and engine, below maximum speed but sufficiently fast with reasonable fuel economy. It might appear that the next slower rpm is even more economical, but it may be too low to properly load the engine for long periods. I would also add that the boat might feel very much in the groove at one of these mid-engine speeds, more than just loafing along under low load but not so fast as to begin to make the engine work hull harder, create a bigger wake, the boat no longer gliding efficiently through the water.
Obviously, one ideally would take measurements at closer rpm intervals, but I think you get the point. And it is important to do this on your boat, as it is equipped and loaded, not based on marketing literature tweaked in perfect conditions.
By going through this effort, you will find that the best operating speed is just below hull speed for reasonable speed and good economy. (If you have real-time fuel consumption data at the helm, using a Flo-Scan unit or the electronic engine data, it is easy to notice that specific rpm where a small increase in speed begins to shoot up the fuel consumption way out of proportion to the throttle and speed increase.)
It is also worth mentioning that when creating this curve for a cruising sailboat or full displacement trawler, these numbers will be much closer together, as the maximum hull speed range is far lower than my example. Even so, there still will be a point where you notice a definite increase in fuel burn that is not in linear proportion (and not justifiable) to the slight increase in speed it results in. And the hull just digs in as you attempt to go faster.
Even before you have all these numbers all formally documented, I'm willing to bet you will have identified a comfortable speed where the engine is running along smoothly, perhaps near its maximum torque, isn’t making lots of noise in protest, the boat is happily running along without large bow waves, and fuel burn is within reason. That is your sweet spot, and all number-crunching aside, it has been fairly obvious on every boat I’ve owned, until you get into high speed planing boats and power cats.
There are time, of course, where you need to make tracks to get into harbor before a thunderstorm, and the hell with fuel burn and the huge bow wave thrown by your trawler. Again, watch those gauges, though.
Knowing the right cruising speed for your boat is just one aspect of the skill set of an experienced cruiser, and takes into account the factors that work together without drama or uncertainty. It's all about staying in the groove.