Insights, 15 May 2023

Range and Energy Capacity: 45 kW / 60 hp Power Class

Consideration of petrol and electric outboards in terms of range and energy capacity. In the 45 kW / 60 hp power class, it is becoming apparent that a large proportion of boat engines will switch to electric propulsion systems in the near future. In this article, we will address the central aspects of this development and compare gasoline and electric motors in terms of energy consumption, weight, range and efficiency. The focus is on inland lakes and coastal waters, where the maximum necessary range is 45 nautical miles.

For a practical comparison, we use the measurement data from Boote-Magazine to make well-founded comparisons and derivations. In contrast to the first article on the 30 kW / 40 HP power class, we will also consider noise development in this contribution. Environmen-tal aspects and the Total Cost of Ownership (TCO) will be covered in a later article to focus on the technical aspects and facts.

Highlighting the Use Cases in Terms of Range

Outboard engines with 45 kW / 60 HP are typically used on boats up to a length of 5.5 me-ters, with a maximum of 6 meters. These boats are suitable for rivers, lakes and calm coastal regions. The upper limit of pure driving time in cruising mode for sports and leisure boats is about 3 hours per day. The cruising speed is limited by noise development (>85 dB continu-ous noise already poses a risk of hearing damage) and economical travel at 16 knots (30 km/h). This results in a maximum necessary daily range of 45 nautical miles.

Biggest influence on pure driving time: comfort, noise and consumption

For this 45 nm maximum daily range, we have examined several examples on lakes and riv-ers and determined the range. Longer travel times, as well as shorter travel times, are usual-ly handled by larger and faster boats. The following table shows a selection of the necessary range for various use cases on different bodies of water:

For lakes such as the Müritzsee in Germany, the Attersee and Wörthersee in Austria, the Lake Lucerne in Switzerland, or the Iseo and Trasimeno Lakes in Italy, the 45 nm range is already sufficient for a round trip with intermediate stops.

Maximum range for a typical day: 45 nm

The large lakes, such as Lake Geneva, Lake Constance, or Lake Garda, are more comparable to coastal cruises in terms of range. If the use case is limited to a specific region within these large lakes, the necessary maximum range is reduced to below 45 nm. This demonstrates that electric boats in the 45 kW power class are also suitable for a wide variety of use cases on larger bodies of water, as long as the trips are well planned and restricted to specific regions.

Sufficient range for lakes and coastal areas: 45 nm up to 125 km²

Consumption Measurement Boote-Magazin

For a practical and well-founded comparison of the two engine types, we again refer to the consumption measurements from the renowned Boote Magazine (60 hp engine class comparison – six-pack, German). In the test, a total of six four-stroke gasoline engines were compared. The test boat was a 5.01-meter-long RIB with a maximum engine power of 60 hp and an 80-liter built-in tank. When fully fueled, the boat weighs around 635 kg.

According to the test team, the cruising speed of the boat is between 16 and 21 knots. For longer trips, we have defined the cruising speed (eco-speed, eco) at which the 85 dB limit was not or only slightly exceeded. Thus, the cruising speed is 16 knots.

The following data from the Boote Magazine article „60 hp engine class comparison – six-pack“ are relevant for our consideration:

As can be seen in the table above, we average 85 dB during cruising speed. These are comparable to heavy city traffic, lawn mowers, or kitchen mixers. The 95 dB at top speed (top) can only be maintained for a limited time and are comparable to riding a motorcycle without a helmet, electric tools such as drills or grinders, or a passing train. For prolonged use, hearing protection is already required.

Maximal tolerable noise level for cruising: 85 dB

To calculate the fuel consumption, we will use the data from the table above to determine the consumption per hour and the consumption for 45 nm:

As seen in the table, for a 45nm day, 20.1 liters are needed at eco-speed and 26.7 liters at top-speed. With the 80-liter tank of the test boat, this means that a full tank lasts for 4 days at eco-speed or 3 days at top-speed.

One full tank of 80 litres with eco-speed: lasts up to 4 days

Energy at the Propeller Shaft

Based on the average consumption data of the different outboard motors, taking into account the efficiency of the engine and the mechanical efficiency of the power transmission (outboard), the energy that actually arrives at the propeller was calculated. The following table shows the derived values:

In the derivation above, the most favorable values for the combustion engine were used. The typical range of the heating value of gasoline is between 8.5 kWh/l and 8.7 kWh/l. The efficiency of the combustion engine is between 25% and 30%. The engine efficiency at top speed is 26.1% and is also confirmed by the engine’s power rating. Using the consumption per hour at top speed, instead of the consumption for the 45 nm, results in exactly 44.1 kW of power at the propeller shaft, which the engines deliver. The mechanical efficiency of 95% is mainly composed of the bevel gear transmission (typically 92% to 96%), the impeller for cooling, and ball bearings.

Daily demand of the 45 nm day: 49,9 kWh at eco speed, 57,5 kWh at top speed.

Additionally, the table above shows the percentage of energy that ultimately reaches the propeller shaft for both speeds.

Energy wastage during operation: 72% to 75%

By dividing the necessary energy at the propeller shaft by the 45 nm daily range, we get the energy consumption per nautical mile at the propeller shaft. With this value, the efficiency of boats with different propulsion systems can be compared.

Energy demand per nautical mile at propeller shaft: allows comparison between boats with different propulsion systems

Battery Capacity Calculation

With the calculation of the necessary energy at the propeller shaft, we can determine the required battery capacity. This capacity is crucial for achieving the same range and performance as with the combustion outboard engine. As an electric outboard, we will use the Torqeedo Deep Blue 50 R. The following table shows the derivation:

The basis for the derivation is the necessary energy at the propeller shaft from the table „Energy arriving at the propeller based on 45 nm consumption“. The mechanical efficiency is similar to one of the combustion engines. By using an electric pump instead of a cooling water impeller, the efficiency increases. Here we use the specifications from Torqeedo and derivations based on their information. Torqeedo specifies the efficiency for the motor and controller at 92%. It should be noted again that there are already systems like those from Molabo that operate at a system efficiency of 95% (motor: 97%, controller: 98%).

Battery capacity required for 45 nm day trip: 56 kWh at eco-speed, 65 kWh at top speed

As before, the table above shows the percentage of the energy that arrives at the propeller shaft.

Efficiency of Torqeedo’s electric motors: 89% of battery capacity reaches the propeller shaft

Weight Comparison of Propulsion Systems

After determining the energy consumption for the day trip for both combustion and electric outboard motors and defining the required battery capacities, it’s time to compare the weights of the two propulsion systems.

As the table shows, the weight of the electric drive is more than three times higher. This is due to the battery and the comparatively heavy Torqeedo outboard motor.

For the combustion outboard motor, an average weight was used, with an additional 10 kg for steering and wiring. The full fuel tank with 80 liters of gasoline has a weight of approximately 75 kg (80 * 0.75 kg/l gasoline + 15 kg tank). The weight of the electric motor, including electronics and wiring, is provided directly by Torqeedo, and the hypothetical battery of 7.5 kg per kWh is derived from the specified data and multiplied by the required energy capacity.

Electric propulsion systems‘ weight: significantly heavier than internal combustion engines due to battery weight.

Torqeedo offers 38 kWh batteries for its Deep Blue outboard motors. With two batteries, the energy capacity is covered, but the weight increases by another 82 kg. If only one 38 kWh battery weighing 284 kg is used, the range in Eco-Speed is reduced to 32 nm and in Top-Speed to 27 nm.

Electric propulsion systems‘ limitations: not yet able to cover all application scenarios effectively.

Conclusion and Outlook

After analyzing various aspects of combustion and electric outboard engines in the 45 kW / 60 hp power class, the following findings emerge:

1. Electric motors have higher efficiency, which means that less energy is wasted and the energy requirement is drastically reduced for the same performance.
2. The weight of the batteries for a range of 45 nm is relatively high, with more than three times the weight. This means that electric drives cannot yet meaningfully cover all use cases.
3. The measured noise level of 85 dB in Eco-Speed is already perceived as loud, and the 95 dB is already in a damaging range within a short time.
4. Aspects such as cost, vibrations, odor development and environmental factors were not addressed in this article but are also important factors in decision-making.

The same approaches to reducing weight in electric drives apply here as in the article „Range and Energy Capacity: 30 kW / 40 hp Power Class„:

• On the technical side, the energy density of batteries is improving, reducing the weight of batteries over time. Additionally, propulsion systems with higher system efficiency can be used, which increases the range per kWh.
• Boats specifically designed for electric drives have higher overall efficiency due to the different requirements electric drives have on the hull. As more suitable hulls increase efficiency, this positively affects the range per kWh. Another possibility from the boat side is to reduce the weight of the boat, which also increases efficiency and reduces energy demand per nautical mile.
• The propeller has a significant influence on overall efficiency. With 40% to 50% efficiency in design speed, it destroys more than half of the energy arriving at the propeller shaft. The use of new technologies like Hydro Impulse, with its 80% efficiency across the entire speed range, provides a noticeable extension of the range. A special consideration with Hydro Impulse will follow in further articles.

One Last Word

Refueling and charging in this area follow the same principles as before. Refueling typically takes place at the end of the day. Depending on the operation at the gas station, this can take some time. With electric drives in this power class, it is still done with an additional manual step when mooring and unmooring.