2.2 Driving time and range (Battery capacity)

The capacity of your battery is very important in determining the range of your electric drive. The required capacity is calculated based on your boat weight and the desired driving time. Therefore, consider exactly how long the pure driving time should be maximum with you.

We calculate for the economic speed with a power requirement of 0.5 kW per ton of boat weight. So if your boat weighs 1000 kg and you would like to drive for 6 hours, we would recommend a battery with at least 3 kWh of stored energy.

This energy must be effectively available, so we also take into account the maximum depth of discharge specified by the manufacturer. If an AGM battery can only be discharged to 80%, the recommended battery would need to have at least (3/0.8) 3.75 kWh of rated capacity. Taking into account the Peukert effect at 6 hours discharge, one would even need 4.7 kWh nominal capacity in the AGM battery. (3,75/0,8)

However, if you want to drive a few km/h faster and consume 1 kW per hour, the same battery will only last for 3 hours. Although you drive a little faster, the possible range decreases drastically. Here is an example calculation for a 6 m long sailboat:

3 kWh capacity / 0.5 kW per hour = 6 hours of driving time

6 hours of driving time * 6 km/h = 36 km range

3 kW capacity / 1 kW per hour = 3 hours of driving time

3 hours of driving time * 8 km/h = 24 km range

We assume energy consumption at optimum speed and usually take guide values. For this reason, we recommend that you personally still include a small buffer. You can do this most easily by specifying a higher driving time.

It is very important to us that we do not arbitrarily recommend a battery that is too large and that you may not even need. After all, the battery makes up a large portion of the total price of an e-drive system.

If you want to achieve planing speed with your boat, then you have a high power requirement in addition to the high power requirement. Very high currents flow, which require a minimum size of the battery. As a guideline, you should have the same amount of stored energy as the motor has as maximum power. So a 6 kW electric motor should be operated with a 6 kWh battery.

For small boats with very large engines, you can also work with less battery capacity. It should be noted only that the battery system can withstand the maximum discharge current over a longer period.

The range of electric boats depends on several factors: Your boat, the usable battery capacity, the electric motor and the power consumption, which depends significantly on the driving speed.

We do not want to take a closer look at other environmental factors such as current, strong headwind, temperature or precipitation, but they can still have a significant impact on your range. It very much depends on your area.

You can calculate your range using the travel time and speed (s=v*t). The driving time can be determined by the current drawn and the usable capacity of the battery system. With 20 amps of current drawn and an effective capacity of 100 Ah, you can expect a drive time of 5 hours.

On the basis of the speed, the range can then be determined. At a speed of 8 km/h, this would be a range of 40 km! Experience values from the boat are very important here, otherwise we can calculate in advance only an approximate expected driving time.

The power consumption is largely determined by your driving style. Here is a mathematical principle: to double the speed of their boat, you need eight times the power. For this reason, the motor consumes very little power at low speeds.

For example, about 0.5 kW per ton of boat weight is enough to move a boat at 70% of hull speed. If you are even more leisurely on the road, your range increases but again. In most areas, no more than 12 km/h are allowed anyway.

To reach 95% of the hull speed, already circa 2 kW per ton of boat weight are necessary. From this you can see that especially with an electric motor it is worthwhile to pay attention to the driving. So you can achieve a much higher range with the same battery capacity.

If you want to drive fast, then the motor draws current up to the specified maximum power. Usually, the driving time is then a maximum of one hour, depending on how large your battery is. For example, a 6 kW motor with a 6 kWh battery. Under full load, moreover, not only the engine is heavily loaded, but also the battery system, since very high currents flow.

Independent on your power consumption, you should make sure that you always observe the maximum depth of discharge of your battery system and do not fall below it. More information on this topic can be found under the category "Battery" in our guide and also in the manufacturer specifications of the battery system.