1.2 Power and speed of electric boat motors

In the following video and article, you will find information about the performance in displacement boats (motorboats and sailboats), as well as gliders, semi-gliders and planes.

Power depends on your boat and how you drive it. For streamlined sailboats or multihulls, often only less power is needed for good maneuverability than for a monohull steel boat. If you are only looking for an auxiliary engine (slack pusher) for mooring and unmooring, less power is already sufficient. On the other hand, significantly more power is needed for planing boats and semi-planing boats, but here the calculation is somewhat more complex.

The term minimum power is the power specific to each boat that it needs to drive with good maneuverability. This is mainly influenced by the weight. The heavier a boat is, the greater the power of the engine must necessarily be. Other factors are current and weather influences, as well as the hull shape.

The maximum speed depends mainly on your boat and not really on the engine. An electric motor brings her boat without problems to the same speed as a combustion engine. 

But at high speeds, the energy requirement is also very high. If you have a boat with a hull shape suitable for planing and have mounted a sufficiently powerful engine, you can of course achieve planing speed with your boat.

There are three types of boats: displacement, semi-displacement, and planing. Displacement boats require the least amount of power. For example, about 0.5 kW per ton of boat weight is sufficient to bring a boat up to 70% of its hull speed. Up to a speed of about 80% of the hull speed, the power requirement is relatively low. Then the power consumption increases rapidly without the boat becoming significantly faster.

To double the speed, about eight times the power is required. For this reason, it is recommended, especially with electric motors, to pay attention to an economic speed in order to achieve a longer driving time. This speed is often even higher than the regional speed limit. If you push the throttle too far, your displacement boat will only go slightly faster but consume exponentially more power. You can see this relationship well in the following graph:

Generally, hull forms of boats are distinguished between displacer and glider. There is also the intermediate form of a semi-glider and, more recently, the aviator.

For displacement boats, we recommend about 2 kW of power per ton of boat weight to ensure good maneuverability of your boat. If you are traveling on the coast or on waters with strong currents, we recommend 3 kW. If you only use the motor as an auxiliary motor, you can also use a smaller motor. However, we recommend at least 1 kW of power per ton of boat weight.

A displacer is a boat that permanently displaces exactly as much water as it weighs. No displacer can exceed the hull speed, i.e. overtake the wave generated by the bow while driving with the stern. The hull speed depends solely on the waterline length of the boat. The longer it is, the higher the theoretical hull speed.

Basically, every boat behaves like a displacer at rest or at slow speed. However, a planing boat is able to lift out of the water and glide on it due to its hull shape at increasing speed. For planing, a smooth, elongated hull and a wide stern are advantageous. The hull speed is far exceeded because after lifting out of the water, the water resistance also falls away. The maximum speed of a planing boat is limited not only by the drive power but also by the air resistance.

The weight of the boat is the main factor relevant to the sizing of the engine power. If a boat is to achieve planing, it is even more important to know the approximate weight of his boat. If you want to know it exactly, you can also use a suitable scale. For example, a crane scale can be used for this purpose.

The weight of your boat when loaded should include any weight due to equipment and people. An important reference point is the empty weight of your boat. Based on the maximum number of people, then add the weight of people to it (guideline = 75 kg per person). You should also take into account other components such as the engine, batteries and anchor. Finally, estimate the weight of the rest of the equipment on your boat. You can also count on additional security.

Significantly more engine power is required for planing than for displacement. It is important that your boat has a hull shape that is suitable for planing. In addition, your boat should be as light as possible, or have little payload, as the power also depends heavily on the weight of the boat. Since the transition to planing depends heavily on the hull shape of the boat and we cannot take this into account very well in the calculation, this is only a guideline.

The value is set very low and is calculated on the basis of the power required to reach 2.8 times the hull speed, where a boat normally transitions to the planing state. More conservative guidelines are 50-70 hp per ton of boat weight required for planing. We would like to give you an initial overview of the motors and manufacturers in this area and are happy to advise you on your individual project.

The transition to planing mode occurs at about 2.8 times the hull speed, which of course varies depending on the boat. Therefore, boats with a lower hull speed also enter planing mode more quickly. The current world record for the fastest electric boat is 142.6 km/h. Of course, this was achieved with a boat specially converted for this purpose and on a test track. But electric boats that can reach almost 50 knots, or 92.6 km/h, are also already being produced in series. As you can see, even with an electric motor you can go faster than the police allow. However, for longer boat trips, a moderate speed is recommended. In addition, of course, still apply the speed limits on the waters

In addition, there are also semi-glider hulls, which are a hybrid between displacers and gliders. Semi-gliders are able to "ride up" on your bow wave and partially overtake it. This allows them to reach higher speeds of 2 to 2.5 times the hull speed. However, they cannot fully transition into glide. The idea behind the concept of semi-planing hulls is that higher speeds are achieved with the same amount of energy input.

If you have a planing boat, then you should either go in displacement or planing. Semi-planing is very uneconomical because a high bow wave piles up, giving your boat a lot of resistance. If your boat is not suitable for planing, or if the engine power is not high enough, you will not be able to overcome this bow wave and will push it in front of you.

With half-planing, a lot of energy is lost and your boat will only get marginally faster. With electric boats, the power consumption increases sharply, your range or travel time decreases extremely. In addition, with this large wave, you disturb other boaters and water sports enthusiasts and can damage shoreline fortifications.

The newest category in this section of hull shapes are "fliers". These boats have foils (skids) that hold and stabilize the boat in the water. The boat lifts out of the water with increasing speed and is held in the water only by the skids. The boat does not keep tipping on the water as in planing, but flies over larger waves. Therefore, it looks like the boat is flying.

The engine usually sits under water and provides propulsion. For the transition phase into flying, a lot of power is needed for a short time. In the case of the Candela 7 (circa 1,700 kg loaded), about 50 kW is needed for a short time to get it to fly. Subsequently, only 5 kW per hour is needed for the flight phase to bring the boat to a cruising speed of 22 knots (30 knots maximum). This makes the boat about 10 times more efficient than a conventional planing boat. Foiling, of course, has been around for some time on sailboats and surfboards. Thanks to the latest technology and software, foiling will also be available on larger boats and ferries in the future, fundamentally changing mobility on the water. The best known are the boats of Candela from Sweden.