With cars, everyone started talking about range anxiety. Which proved out to less of an issue, especially here in the Netherlands, then we thought it would be. For boats, it seems everyone needs to take care of charging themselves. So if you harbour supports (fast) charging, you are in luck. Otherwise, well, don’t go too far.

So, How far?

But how far can you go with an electrical boat? There are many factors to take into account. We use these ourselves too, to figure how much power and battery we will we need. We’ll discuss these here.

To determine the power, first some basic information regarding electric sailing. These are some basic numbers to follow for the required electrical power for full displacement boats.

• 2 – 4 kW (5 – 8hp) Up to 6m length or 2 tons of water displacement
• 4 – 7.5 kW (10 – 15hp) Up to 8m length or 3 tons water displacement
• 10 kW (20hp) Up to 10m length or 7.5 tons water displacement
• 15 kW (30hp) Up to 12m length or 10 tons water displacement

Speed

The same speed as a boat with a diesel or petrol engine is of course possible. It must be taken into account that non-planing boats have a maximum speed equal to the hull speed. The closer to that hull speed, the more energy is consumed or wasted. Sailing above hull speed requires much more power, and therefore consumption. A more economical cruising speed is slightly lower. Below is an indication for the hull speed and economical cruising speed:

Boat length at the waterline m	Hull speed km/h	Economic cruising speed km/h	Required power for economi cruising kW
4	9	7	0.75
6	11	8	1
8	12.5	9	2
10	14	10	4

To achieve the economical cruising speed, less power is required. Below is an indication:

Another general rule that is often used is:

• To achieve the maximum speed, a power of 2.5x the weight of the ship must be taken into account, i.e. 2.5kW per ton
• The cruising speed of a ship is around 1 kW per tonne.
  So e.g. With a sloop of 7.5 meters in length and 2 tons of water displacement (with load), you could theoretically achieve the economical cruising speed with a 2 Kw engine.

Required battery capacity

Based on such a 7 meter boat, the 4.3Kw engines could be chosen to achieve the hull speed and some excess capacity. Assuming 2Kw for the economic speed of 9km/h, it also consumes 2Kw per hour.

In practice, sailing at full throttle all day does not happen. After all, in theory the hull speed is usually achieved at 1/2 or 3/4 power. In terms of sailing time, hardly anyone spends the whole day behind the wheel of their white boat. It is therefore important how many engine hours are actually made in a day. As an indication, here are some typical usage situations assuming a normal cruising speed:

Sailing area Use Engine hours per day
On the lake, in the city Small trips within a lake area or in a city, distance covered up to 20 km. 2-3 hours
Region Day trips within a region, distance up to 40 km. 3-4 hours
Rural Tours during holidays for longer distances, up to 60 km. per day 5-6 hours
National Long sailing days, to cover longer distances, up to 80 km. Or use it for boat rental from early in the morning until late at night. 7-8 hours
To calculate the required battery capacity, we assume the 48V models and specifically the 4.3Kw motor for convenience.
(The calculation below can also be performed for the other powers).

To get 48V you need at least 4 12V batteries connected in series.

You can use the wattage of a battery as a general calculation tool for the operating time in hours.

As an example: A battery of 260Ah 12V = 260ah x 12V = 3,120 Watt x 4 (pieces to obtain 48V) is 12,480 Watt total power. However, the Aquamot batteries can be discharged up to 80%, so 9,984 W is available.

Assuming that 9 p/h km can be achieved on 1/2 power (2Kw) of an engine.
So in this example, the 4.3Kw motor with 4x260AH batteries can run for approximately 7 hours at 1/3 power and 4.6 hours at 1/2 power (= 9,984W of battery power divided by consumption of 2,150W) and 3 hours at 3/3. 4 power and 2.3 hours at full power.

To double the sailing time you must double the battery capacity. This is possible by connecting the batteries in series-parallel (calculation is indicative and depends on various factors such as wind, waves, hull resistance, temperature, etc. etc.). Series-parallel switching is therefore also possible with batteries with lower power to distribute the weight or to achieve a specific sailing time.