Fuel Consumption and Curves

Fuel consumption in inboard engines on large powerboats varies based on several factors, including engine type, horsepower rating, boat hull design, and operating speed. Understanding how fuel consumption changes with engine RPM (revolutions per minute) is crucial for efficient boating, especially when planning long journeys or optimizing fuel use.

Understanding the Fuel Curve

The fuel curve of an inboard engine represents the relationship between engine RPM and fuel consumption. Generally, as the RPM increases, fuel consumption rises. However, this increase is not linear; it tends to rise steeply at higher RPMs due to the increased load and resistance on the engine.

• Lower RPMs (Idle to Moderate Speed): At lower RPMs, fuel consumption increases gradually. Engines operate efficiently in this range, using less fuel relative to the speed and distance covered.
   
• Mid-Range RPMs (Cruising Speed): This range, often between 2,000 to 2,800 RPM for diesel engines, is where the engine operates most efficiently. Fuel consumption is balanced with speed, providing optimal performance and fuel economy. For gasoline engines this range is higher in RPM at 3000 to 3600 RPM
   
• High RPMs (High Speed or Full Throttle): At higher RPMs, fuel consumption increases significantly. The engine consumes much more fuel due to higher load demands and increased water resistance. This steep rise in fuel consumption can lead to inefficient operation, especially if maintained for extended periods.

Example: Diesel Engine on a 40-Foot Powerboat

A typical diesel engine on a 40-foot powerboat might be rated between 300 and 600 horsepower (hp), depending on the boat’s design and intended use. Here’s an example of fuel consumption rates at various RPMs for a 400-hp diesel engine:

1,000 RPM: At this low idle speed, the engine consumes about 2 to 3 gallons per hour (GPH). The boat is likely moving slowly, and fuel efficiency is high in terms of fuel used per hour but low in terms of distance covered per gallon.

1,500 RPM: Fuel consumption increases to around 3 to 6 GPH. The boat starts to move at a more noticeable pace, but it’s still at an economical cruising speed.

2,000 RPM: The engine might consume about 5 to 9 GPH. At this speed, the boat is generally cruising efficiently, balancing speed and fuel consumption. This RPM is often considered the “sweet spot” for many diesel engines, where the boat covers more distance per gallon of fuel.

2,500 RPM: Fuel consumption rises to around 10 to 14 GPH. The boat is moving faster, but fuel efficiency begins to decrease as the fuel consumption increases more rapidly than the speed.

3,000 RPM: At this high RPM, the engine could consume between 15 to 20 GPH. This rate is much higher due to the engine operating under maximum load and increased drag on the hull. While the boat is moving at a high speed, it is not fuel-efficient, and operating at this RPM for long periods is costly in terms of fuel use.

Here are the fuel curves for various Yanmar diesel engines from 400 hp to 640 HP. Read the estimated fuel consumption from the curve. For example, the 6LY 400 operating at 2500 RPM uses about 10.5 US gallons per hour = 40 liters per hour. The consumption rate also depends on the load on the engine. Other factors such as propeller pitch and if you are drawing electrical charge will slightly change the rate.

Efficient Cruising for Fuel Savings

A skipper is well-advised to identify the optimal fuel consumption rate for their boat to reach their destination using the least amount of fuel. Operating at a lower RPM, where the engine is most fuel-efficient, will typically consume less fuel overall, even if it means traveling for a longer period.

Since different hull designs determine the speed of the vessel for each engine RPM setting, you will have to determine the most efficient cruising speed yourself for your boat. However, this is relatively simple.  For example, suppose you have a vessel that travels at 15 knots of speed at 2500 rpmw with a 6LY400 engine shown above. To travel 15 nautical miles @15 knots, it will take you 1 hour. From reading the curve above and at 2500 RPM you consume 40 liters per hour. Since your trip will take 1 hour then you will consume 40 liters of fuel.

To find the most efficient operational speed for your boat, take several readings of boat speed at various RPM settings. Below is an example table from a fictitious boat. The fictitious owner went out on his boat and measured the boat speed at 1800, 2000, 2200, 2500, 2700, and 3000 RPM. Then he applied those numbers along with the predicted fuel consumption rate from the engine curve. For example, he determined that at 2200 RPM his boat consumes 28 litres of fuel per hour from the curve. On the water, he measured his boat speed at 12 knots at 2200 rpm. If he was to travel 10 nautical miles it would take 0.83 hours. At 28 liters per hour, he would consume 23 liters of fuel in 0.83 hours.

As it turns out, from observation of the table of various speeds, 2200 RPM is the most fuel-efficient operating speed. For perspective, had the owner put down the hammer to 3000 RPM, he would have saved 19 minutes in getting to his anchorage 10 nm away but at a cost of approximately $16 (and subsequent reduced engine life cost).

Conclusion

Understanding fuel consumption rates and how they relate to engine RPM is essential for efficient powerboat operation. By finding and maintaining the best cruising speed, skippers can optimize fuel usage, reduce operational costs, and extend the range of their journeys. Using the fuel curve to identify efficient RPM ranges and avoiding high RPMs except in emergencies can help boaters achieve the best balance between speed, efficiency, and fuel consumption.