Motoring at an “optimum” RPM might not always produce the greatest benefit

By STEVEN J. HENKIND and MICHAEL RYAN

Most sailors understand that by paying attention to sail trim, they can make their boats go faster under sail. But many sailors don’t realize that by paying attention to engine RPM, they can make their boats go farther under power. Because the engine is an integral part of the cruising experience, it’s worth taking time to understand how to improve your motoring performance.

Three critical components for optimal use of your engine are to:

• Understand your boat’s fuel-consumption characteristics.

• Use those characteristics for voyage planning.

• Monitor and manage fuel consumption while under way.

FUEL-CONSUMPTION CHARACTERISTICS

In general, the higher an engine’s RPM, the faster a boat will go. For example, Michael Ryan’s Fiona Rois, a Cape Dory 45, will make 6 knots at 1,500 RPM and 7.25 knots at 2,200 RPM; obviously, different boats have different performance characteristics. Note that increased RPM is not matched by a proportionate speed increase; throttling up from 1,500 to 2,200 is a 47 percent increase in RPM, but the resulting increase in speed is just 21 percent.

Moreover, while speed increases at a lower rate than the increase in RPM, fuel consumption rises at a greater rate than the increase in RPM. For example, Fiona Rois consumes 1.0 gallon per hour at 1,500 RPM and 1.8 gallons per hour at 2,200 RPM. In other words, it takes 80 percent more fuel to motor at 7.25 knots than to motor at 6 knots, but the increase in boatspeed is just 21 percent. Simple arithmetic tells you that while lowering a sailboat engine’s RPM will decrease its speed, it also can significantly increase its range under power (see table).

Some engine manufacturers recommend in their operator manuals a specific “cruising RPM”; generally, the suggested RPM is fairly high and has been selected to maximize engine torque without considering fuel economy. What’s more, fuel-consumption curves are rarely found in these manuals. As a result, use of the engine is often thought of as an all-or-nothing proposition with the maximum range under power being a fixed distance. This is not the case.

There are a number of ways to determine your vessel’s fuel-consumption characteristics. Perhaps the easiest way is to fill up the fuel tank, motor for several hours at a fixed RPM, and then measure how much fuel is required to top off the tank. For example, if you motor at 2,000 RPM for 6 hours, burn 8 gallons of fuel, and travel a distance of 36 miles, your consumption rate is 8÷6 = 1.3 gallons per hour at 2,000 RPM and your average speed is 36÷6 = 6.0 knots at 2,000 RPM. You should, at a minimum, determine the fuel consumption and speed of your boat at its cruising RPM and at a lower, economy RPM. Try to do your tests on a relatively calm day, because wind and waves can affect your boatspeed/fuel-efficiency data. A foul bottom or currents can also affect the results.

A word of caution: Running at very low RPM for extended periods (many hours/days) can cause carbon fouling of the engine. To avoid this problem, a good rule of thumb is to run at an RPM that is at least 50 percent of the engine’s maximum RPM.

VOYAGE PLANNING

When planning for a passage, you should consider weather patterns, currents, navigational hazards, your desired departure and arrival dates, and other relevant factors. To help simplify the planning, break the voyage into logical segments: harbor departure, shipping channel, and open-water segments are typical examples. Then assign appropriate objectives to each segment; rapidly clearing the shipping channel is a good example. Once you’ve established the segments and the objectives, you can then work out the desired speed/RPM/fuel consumption for each segment.
Don’t forget that, in addition to propulsion, fuel is needed to run the engine in order to charge the batteries and operate the refrigeration unit. You should also make sure that there’s some fuel remaining for emergencies. While the powerboat credo of “one-third to get there, one-third to get back, and one-third in reserve” may not apply to sailboats, a prudent sailor will always plan to keep some fuel in reserve.

MONITORING FUEL CONSUMPTION

While under way, keep a written record of fuel consumption. At a minimum, you should record the fuel level, engine hours, and RPM in the log at the end of every watch. Keeping such a record and paying attention to the trends allows you to avoid unpleasant surprises (such as running out of fuel) and adapt your voyage plan if circumstances change.

While motoring, try to also use the sails; they provide drive and thus decrease engine load and fuel consumption. Even if the sails don’t provide much drive, they can stabilize the boat’s motion, which will help to increase fuel efficiency and possibly make the ride more comfortable. A good rule of thumb is to keep a sail up as long as it is not flogging or backwinded. In general, it is more effective to motorsail with the mainsail than with the genoa, and visibility will be improved as well.

While motorsailing, you should decrease the RPM periodically in order to determine how much drive the engine is actually producing; you may be able to maintain the desired minimum boatspeed at the reduced RPM (thus saving fuel) or even shut down the engine entirely. In addition, when fuel conservation is an issue, unnecessary electrical gear and accessories should be shut off as these consume energy, which ultimately must be produced by the engine.

Note that the fuel you can actually use from your tank may be significantly less than its total capacity. Depending on tank shape, as much as 10 to 20 percent of the fuel may not be usable in rough conditions; heeling or pitching might cause the fuel pickup to draw in air, which would stall the engine. What’s more, as the fuel level decreases, the likelihood of picking up sediment from the bottom of the tank, and thus clogging the fuel filters, increases. You could supplement your fuel capacity by carrying jerry jugs of fuel, but they can be difficult to store and to use unless weather conditions are favorable.

In summary, decreasing the engine’s RPM and moving at a slower speed will add time to your passage. It will also save fuel and can significantly increase your potential motoring range. As an added bonus, you may also wind up spending less time and money at the fuel dock.

PASSAGE PLAN IN REAL TIME

A recent passage from Bermuda to Maine on Fiona Rois provides a good example of real-life voyage planning. The predeparture forecast called for several days of calms or very light wind followed by a frontal passage with gale-force conditions in the Gulf Stream. Because the Gulf Stream is no place to be during a gale, we decided we wanted to be well north of the Stream before the winds built. We calculated that we would accomplish that objective if we powered at normal cruising RPM of 2,200, but we’d run out of fuel 250 miles before we reached the Maine coast.

After considering several alternatives, we devised a passage plan that dealt with the issues involved: By leaving Bermuda a day early, we could cross the Stream before the front arrived, and by running at a reduced RPM of 1,500, we could significantly increase our motoring range. The plan worked out exactly as intended. As an additional bonus, our fuel conservation during the initial portion of the passage enabled us to increase the engine RPM in the Gulf of Maine in order to avoid a line of strong thunderstorms. In contrast, other boats that motored at normal cruising RPM ran low on fuel, and boats that left Bermuda a day later experienced rough conditions in the Stream. Our passage was a success because of careful planning and because we had monitored and managed our fuel consumption while under way.

Steven Henkind, M.D., Ph.D., is a marine-safety consultant based in Larchmont, New York. He has served as captain, navigator, and crew on sailing vessels up to 295 feet and holds a 200-ton U.S. Coast Guard license. Michael Ryan, Ph.D., is a retired professor who sails Fiona Rois, his Cape Dory 45, out of Bass Harbor, Maine.