What needs to be considered when buying an electric motor? Electric motors use the latest technology. We give you an overview of the most important topics around your boat. Your team from greenboatsolutions wishes you a lot of fun reading.
We have divided the article into the following sections:
Before we get started, let´s start with a few brief information about boat types and hull shapes:
A basic distinction is made between hulls and displacers. A displacer is a boat that permanently displaces as much water as it weighs. No displacer can exceed the hull speed, i.e. overtake the wave generated by the bow with the stern. At rest and at slow speed, every boat is a displacer. Thanks to the shape of the hull, a glider lifts out of the water at increasing speed. A smooth, elongated hull and a wide stern are ideal. The hull speed is far exceeded since after lifting out of the water, the water-resistance also disappears.
The weight of your boat (displacement) is decisive for the design of the electric motor. In short: the heavier your boat is the more power your engine needs. It is important that you are also prepared for bad weather conditions. Violent currents and strong waves shouldn´t be a problem for you.
If it is difficult for you to estimate the weight of the boat exactly, it is best to start from the "worst-case". So take the people and payload into account. Specify the weight a little higher for safety, in any case, to achieve good maneuverability and marching speed with your boat.
For a displacer, we assume 2 kW per ton of boat weight. You multiply by the weight of the boat (in tons) times 2 and get the corresponding power. We usually calculate 75kg per person and calculate a further 50kg for the additional load.
The empty weight of the boat in the picture below is 1,000kg. In our case, we drive a maximum of 2 adults and add another 50kg for a toddler, food, drinks, and anchors. The total boat weight is 1,200kg. Multiplied by a factor of 2, we get a necessary motor output of 2.4 kW for our electric outboard.
If you have a boating license, you can easily skip the paragraph. If not, you should stick to the maximum value of 15 HP, with which you can drive without a license. 15 HP corresponds to an output of 11 kW of an electric motor.
Depending on the performance, different manufacturers are relevant to you. In the example above, a Minn Kota rear engine or a Torqeedo outboard would fit very well. However, to compare all of the more than 20 different manufacturers, we recommend using our comparison portal!
A gliding boat is able to lift and slide out of the water at increasing speed. The fuselage speed is far exceeded. Significantly higher engine power is required for sliding. In our digital consultant, we calculate an approximate engine power for your gliding boat.
Have you had a burner before? Do I need a stronger engine? Don´t worry: electric motors are more efficient and usually more powerful, especially at lower speeds. Depending on the electric motor, step-less speed regulation is even available.
You can calculate a similar performance for the electric motor. We recommend that you use our digital advice beforehand. Internal combustion engines are often oversized and you spend too much money on the purchase.
Get all the information about the electric motor and the power required? It continues with the speed and then with power consumption. Is it worth driving with an electric motor or not ?!
How fast can I drive with my electric motor? Does it have less thrust than my combustion engine? We clarify the myth of speed:
The speed depends on your individual boat and much less on the engine than you think! So it doesn´t matter whether you have a combustion engine or a clean electric motor on board. An electric motor brings a sliding boat with the corresponding power to the same speed as an internal combustion engine with the same nominal power would do.
If you have a boat with a hull shape suitable for gliding and have a sufficiently powerful motor installed, you can of course also achieve gliding with your boat. However, you should not forget that at high speeds the energy requirement is also very high, but more on that later.
What is the waterline length and what is it used for? With the waterline length, you can give an approximate statement about the expected speed of your boat. The larger this is, the higher the hull speed of your boat is.
This information is of fundamental interest to you, but only plays a minor role in the selection of the motor. For gliding boats, the waterline length is important for the calculation of the minimum power! You can read more about the special case of a gliding boat here.
The transition to gliding takes place at around 2.8 times the hull speed, which of course differs depending on the boat. Therefore, boats with a lower hull speed also get into the glide mode more quickly.
The maximum speed of a glider is limited by the drive power and the air resistance. This increases with increasing speed and eventually leads to the hull lifting off. The boat loses contact with the water, which should definitely be avoided.
The transition of the displacer to gliding can be compared to breaking the sound barrier on an airplane. The stern of the boat overtakes its own bow wave. It is similar to an airplane. In-flight it breaks through the self-generated sound wave.
There are also the hulls of the semi-planes. Semi-planes are able to “hit” their bow waves and partially overtake them. This way they can reach higher speeds of 2 to 2.5 times the trunk speed.
However, you cannot completely switch to gliding. The idea behind the concept of the semi-glider hulls is that higher speeds can be achieved with the same amount of energy.
The current world record for the fastest electric boat is 142.6 km / h. The speed was reached with a specially modified boat on a test track. But electric boats are already being produced as standard, which can reach almost 50 knots, 92.6 km / h.
As you can see, you can also drive an electric motor faster than the police allow. However, a moderate speed is recommended for longer boat tours. In addition, of course, the speed specifications on the waters also apply, which we, of course, recommend adhering to.
In order to double the speed, approximately eight times the power is required. For this reason, it is advisable to pay attention to economic speed, especially with electric motors, so that longer driving time is achieved. In most cases, this is even above the speed limit.
The power consumption depends, among other things, on the shape of the hull. A distinction is made between displacement, semi-glider, and glider. The least amount of electricity is required in displacement travel. With houseboats, you can already drive independently.
0.5 kW per ton of boat weight is often sufficient to bring a boat to 70% of the hull speed. Up to about 80% of the fuselage speed, the power requirement is relatively low. In addition, consumption increases significantly.
If you turn the throttle lever over 80% of the hull speed, your displacement boat will only be marginally faster but will use exponentially more electricity. If you have a gliding boat, then you should either go on displacement or gliding. The semi-glider ride is very uneconomical.
What does exponential mean? The third power is required to double the speed. Calculation: 2x2x2 = 8. To accelerate from 10km / h to 20km / h, 8 times as much energy is used.
The principle is the same for combustion engines and electric motors, so care should be taken to ensure that there is enough fuel for the combustion engine and that the electric motor has a sufficiently large battery.
If your boat is not suitable for gliding or the engine power is not sufficient, you cannot overcome the bow wave and push it in front of you. A high bow wave piles up in front of the boat and offers great resistance. A lot of energy is lost and your boat only gets marginally faster.
Electricity consumption also increases sharply and the range / travel time decreases extremely. In addition, the engine and battery system are heavily loaded. This can limit longevity. In addition, with a large wave, you disturb other boaters and water sports enthusiasts and can damage the bank fortifications.
The maximum travel time depends on the capacity of the battery. The longer you want to drive, the more energy you need. With an empty battery, you can only be drawn to the next port by other boats.
To speak from personal experience: most boat owners are very helpful and happy to help. As a precaution, a long rope onboard cannot hurt. However, a boat must first be nearby. Therefore: Better too much capacity than too little.
The travel time depends on your driving style! For maximum consumption, multiply the power of your engine by the driving time. At 4 kW and 2 hours of driving, you would need an 8 kWh battery on board.
Attention: You usually do not drive directly across the lake at full power. Usually, you drive at economic speed. (Again: exceptions confirm the rule)
What is the economic speed? This is more efficient and energy-saving. It is around 70% of the maximum speed. Most of the time, the speed is still significantly faster than is permitted on most waterways and bodies of water in Germany.
Often the entire battery is not drained on a day trip of ~ 8 hours. Most people take anchor during the tour to soak up some sun or go swimming. So always remember to consider the maximum travel time. The battery is the be-all and end-all here.
We always calculate the effective capacity in the calculation. This is the energy that can actually be drawn from the battery system. We, therefore, recommend planning with some certainty when specifying the travel time.
In our digital process, we calculate the necessary capacity for you.
We do this so you won´t be unpleasantly surprised, especially on longer tours. Another tip: It is helpful to lay your planned routes along with ports or piers with power supply. Of course, it makes a huge difference whether you want to take your boat comfortably and relaxed, or if you want to drive reasonably quickly.
It also has an impact on whether you are driving on a displacement or owning a boat with which you can and want to achieve gliding. First and foremost, this depends on your boat, especially the shape of the hull, but secondly, engine power plays a very important role.
Instead of having a pure drive motor on board, many use an electric motor for short distances and with an economical driving style.
If you only use the electric drive as an auxiliary motor, you will need less power. This is often the case if you only want to leave the port or generally only travel for a short time. One engine with a lower engine output is sufficient, for example around 1 kW per ton of boat weight.
With a motor with lower power, you can save money and still get the right electric drive for you. This can be the case with sailing boats, in particular, to drive from the jetty to the lake. Then the wind carries you over the lake. When the weather is calm, you can still use the electric motor and drive back to the landing stage.
Depending on how often you drive, it affects which type of battery we recommend. When used frequently, lithium-ion batteries are the most suitable type of battery. This is mainly due to longevity. Lithium batteries are significantly lighter and therefore the best choice for gliding boats.
For smaller to medium-sized battery systems and with a normal driving frequency, we usually recommend AGM batteries. AGM batteries are significantly cheaper. They are also lighter in weight. However, a detailed comparison of the battery types can be found in Chapter 3 of the guide.
Do you have questions?