Range and Energy Capacity: 30 kW / 40 hp Power Class

Consideration of petrol and electric outboards in terms of range and energy capacity

The future of recreational boating is electric. More and more people are thinking about making the switch. In this article, we compare 30 kW / 40 hp combustion and electric motors in terms of energy demand, weight and efficiency. The focus is on the typical use case for private boaters with a maximum of 25 nautical miles. For a practical comparison, we use the actual consumption of various outboards based on the measurements of Boote Magazine. Aspects such as noise level and environmental aspects are covered in further articles. Learn more about the potentials and challenges of electric mobility on the water.

First Things First – Use Case

Before we dive into the comparison of combustion and electric motors, let’s look at the typical use case for private leisure boaters in the 30 kW / 40 hp power class. For most trips, 25 nautical miles is more than enough to explore idyllic bays, beaches and fishing spots. Longer distances are usually covered with larger boats and more power.

Maximum range for a typical day trip: 25 nm

Consumption Measurement Boote-Magazin

In order to enable a well-founded comparison with real reference, we take our data from the consumption measurements of the renowned Boote magazine (Engines: 40 hp outboards – The subtle differences, german). These measurements were carried out specifically for 40 hp combustion engines and provide us with important information about the energy requirements of these engines in practice.

The test team of Boote-Magazin used a self-built boat with a length of 4.8m, a width of 1.88m and a displacement without engine of around 280 kg. An economic planing speed of 16.2 kn (30 km/h) was selected. The following data and engines (4-stroke engines) from the consumption measurement are used:

Table: Data basis from the article „Engines: 40 hp outboards – The subtle differences“ by Boote Magazin
Honda Mercury Tohatsu Yamaha Average
Weight [kg] 98 98 97 98 97,8
Top-speed test boat [kn] 31,0 31,3 31,1 30,8 31,1
Consumption eco-speed [l/nm] 0,32 0,35 0,35 0,32 0,34
Consumption top-speed [l/nm] 0,52 0,43 0,44 0,44 0,46

From this data, the consumption per hour and the consumption for the 25 nm are calculated:

Table: Calculation of consumption for one hour and for the 25 nm day
Honda Mercury Tohatsu Yamaha Average
Consumption eco-speed [l/hour] 5,3 5,7 5,7 5,3 5,5
Consumption top-speed [l/hour] 16,1 13,3 13,8 13,7 14,2
Consumption eco-speed [l/25nm] 8,1 8,8 8,8 8,1 8,4
Consumption top-speed [l/25nm] 13,0 10,6 11,1 11,1 11,5

As displayed in the table, the 25nm day requires 8.4 l at eco-speed, or 11.4 l at top speed. Typically, a tank on boats of this size holds 25 l of petrol. At Eco-Speed, one tank would not even be enough for three days.

With a typical 25 l tank, one fill will not last 3 days at Eco-Speed.

Energy at the Propeller Shaft

With the average consumption data determined for the various outboards and taking into account the efficiency of the engine as well as the mechanical efficiency of the power transmission (outboard), we can calculate the energy that actually arrives at the propeller. The following table shows the derived values:

Table: Energy arriving at the propeller based on 25 nm consumption
Eco speed Top speed Unit
Fuel tank capacity 8,4 11,5 l
Calorific value petrol 8,7 8,7 kWh/l
Energy capacity fuel tank 73,5 100% 99,7 100% kWh
Engine efficiency 30,0% 25,5%
Energy capacity motor shaft 22,1 30% 25,4 26% kWh
Mechanical efficiency outboard 95,0% 95,0%
Energy capacity propeller shaft 21,0 28% 24,2 24% kWh

In the above derivation, we used the most favourable values for the internal combustion engine in each case. The typical range of the calorific value of petrol is between 8.5 kWh/l and 8.7 kWh/l. The efficiency of the internal combustion engine is between 25% and 30%. The efficiency of the engine at top-speed is also validated by the power rating of the engine. If you use the consumption for one hour at Top-speed instead of the consumption for the 25 nm as in the table above, you get exactly the 30 kW at the propeller shaft that the engines are rated for. The mechanical efficiency of 95% is largely made up of the bevel gear (typically 92% to 96%), the impeller for cooling and bearings.

Daily demand of the 25 nm is 
21 kWh at eco speed and 24.2 kWh at top speed.

Furthermore, the percentage that finally arrives at the propeller shaft is given in the above table for the two speeds.

At best, at least 72 % of the energy is wasted and 
28 % actually reaches the propeller shaft.

Battery Capacity Calculation

With the necessary energy at the propeller shaft, we can calculate the necessary battery capacity. This capacity is crucial for the same range and performance as with the combustion engine outboard. As an electric outboard, we use one with an axial flux motor (cf. Emrax 188). The following table shows the derivation:

Table: Derivation of the necessary battery capacity for electric outboards.
Eco speed Top speed Unit
Energy capacity propeller shaft 21,0 87% 24,2 87% kWh
Mechanical efficiency outboard 96% 96%
Energy capacity motor shaft 21,8 90% 25,2 90% kWh
Efficiency electric motor 96,0% 96,0%
Efficiency motor controller 94,0% 94,0%
Energy capacity Battery 24,2 100% 27,9 100% kWh

The basis of the derivation is the needed energy at the propeller shaft from the table „Energy arriving at the propeller based on 25nm consumption“. The mechanical efficiency is close the one of the combustion engine. By using an electric pump instead of a cooling water impeller, the efficiency increases by 1%. The motor achieves an efficiency of 96% in continuous operation at both Eco-Speed and Top-Speed. We assume the efficiency of the controller required for the electric motor to be 94%. The system efficiency (motor + controller) is thus 90%. Meanwhile, there are motors such as those from Molabo that operate with a system efficiency of 95% (motor: 97%, controller: 98%).

The battery capacities required for the 25 nm day trip are 
24.2 kWh for eco-speed and 27.9 kWh for top speed.

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

With electric motors, 
87% of the battery capacity reaches the propeller shaft.

Weight Comparison of Propulsion Systems

After we have determined the energy consumption for the day trip for both combustion and electric outboards and defined the required battery capacities, it is time to compare the weights of the two propulsion systems.

Table: Weight comparison of different propulsion systems.
Petrol outboard Electric outboard Difference Unit
Engine / motor weight 98 45 -53 kg
Fuel tank / batterie weight 24 240 216 kg
Total weight 122 285 164 kg

In the tests of Boote magazine, the average weight of the individual combustion engines was given as 97.8 kg. However, current research shows that the actual weight is about 15% higher (cf. Mercury FourStroke 40 Pro), at around 112 kg. This increase can be attributed to the trend of 40 hp engines being sealed off 60 hp engines. For the electric outboard, the weight of the edyn marine 2035 outboard was used.

Typically, boats with 40 hp outboards have a 25-litre tank. With a weight of 0.75 kg/l for petrol and an additional 5 kg for the plastic tank, the total is just under 25 kg. The batteries for the electric outboard are also from edyn marine. With a weight of 120 kg per 15 kWh, the total weight is 240 kg with an energy density of 125 W/kg.

In this use case, the electric drive with 285 kg is about 164 kg heavier than the combustion engine with 122 kg. The majority of the weight of the electric drive, 240 kg, lies with the batteries.

Electric propulsion systems are noticeably heavier than 
internal combustion engines due to the weight of the battery.

Conclusion and Outlook

After analysing the various aspects of combustion and electric outboards in the 30 kW / 40 hp power class, the following assumptions can be made:

  1. Electric motors have a higher efficiency, which means that less energy is wasted for the same performance and the energy demand is drastically reduced.
  2. The weight of the batteries is still relatively high for a range of 25 nm. With Eco speed with a required battery capacity of 24.2 kWh, the extra weight is covered. The range under Top speed will be difficult to achieve.
  3. Aspects such as noise, vibration, smell and environmental considerations have not been addressed in this paper, but are also important factors in the decision-making process.

We would like to conclude with approaches on how the weight of the electric motors – especially the battery capacity – can be reduced:

  • On the technical side, the energy density of the batteries is improving. This reduces the weight of the batteries over time. On the other hand, propulsion systems with higher system efficiency can be used, which increases the range per kWh.
  • The use of boats specifically designed for electric propulsion have a higher overall efficiency due to different requirements of electric propulsion systems on the hull. If the efficiency is increased by using more suitable hulls, this has a positive effect on the range per kWh. Another possibility on the part of the boats is to reduce the weight of the boat. This also increases efficiency and reduces the energy demand per nautical mile.
  • The propeller has a major influence on the overall efficiency. With 40% to 50% efficiency at design speed, it wastes a good half of the energy that arrives at the propeller shaft. The use of new technologies such as the Hydro Impulse, with its 80% efficiency over the entire speed, brings a noticeable extension of the range. A special consideration with the Hydro Impulse follows in further articles.

One Last Word

When we talk about electric drives, we also have to talk about charging and refuelling. Refuelling is well known: Typically, the boat is refuelled at the end of the day. Depending on the activity at the filling station, this process can take some time, including waiting, mooring, refuelling, paying and casting off again. Electric boats are usually charged overnight (approx. 10 hours). With a 3 kW charger and household electricity, the boat is fully charged again in the morning. Connecting the charging cable is just an additional move when mooring and unmooring the boat, which takes less than two minutes in total.