Alternative motorisation systems
The electric motorisation on boats is a recurring question. The numerous comments posted on the Excess Lab site confirm the growing interest in this trend.
Although sailboats are, by definition, naturally propelled vessels, the demand for alternative propulsion is undeniable. The reduction of CO2 emission, as well as noise and odours, are major assets in order to enhance the comfort of our cruises.
Therefore, it seems worthwhile to take stock of the available technologies, or those in the process of development. We will aim to deal with this subject using a pragmatic approach, quantifying the advantages and drawbacks, and avoiding preconceived ideas or shortcuts. Basically, to make the state of the art survey as objective as possible.
In order to do so, we will interview François Charrier, Motorisation Project Manager for the Beneteau Group.
François, we often refer to electric propulsion. What is it precisely?
FC: It can be a 100% electric motorisation system, or a hybrid one (combination of two energy sources). To this day, the most accessible hybrid technologies combine a combustion engine with an electric motor. In fact, one finds very similar concepts to those of automobile technology...
Then, regarding hybrid propulsion, two concepts can be distinguished for boats:
The series hybrid system, where the two sources are not mechanically linked (example : a generator that powers an electric motor and its batteries).
The parallel hybrid, where the two energy sources are mechanically linked (example: a combustion engine coupled to an electric motor).
Advantages and disadvantages of the series hybrid system
- The components of the chain can be freely positioned in the boat.
- It is possible to use 230VAC with the generator during navigation.
- It is possible to use 230VAC with the batteries, and without noise, at anchor.
- No mechanical inverter on the transmission.
- Simple system
- Less costly than the parallel Hybrid solution.
- Propulsion power limited by generator capacity.
- A lot of energy is lost through transfers between all intermediate elements.
- Heavier than a standard propulsion system.
Advantages and drawbacks of the parallel hybrid system
- Compact: the electric motor is coupled to the combustion engine.
- All the engine power is transferred to the propeller.
- Safety: two independent propulsion solutions.
- It is possible to recharge the batteries when using the combustion engine.
- It is possible to use 230VAC with the batteries, and without noise, at anchor.
- Less costly than the series Hybrid system.
- Complexity of the electrical/mechanical installation.
- High complexity of maintenance and use.
- Heavier than a standard propulsion system.
What factors contribute to the choice of propulsion system for a specific type of boat?
FC: There are many factors that can influence the choice of technology, but one needs to fully understand the capacities of each solution:
The 100% electric system is limited by its battery capacity. Therefore, it is more suitable for occasional use (day sailing, harbour manoeuvres, lake cruising, etc.). For longer all-electric cruises, higher battery capacity is required, so this system soon becomes unaffordable. In the future, if the cost of batteries continues to decrease, 100% electric systems will become more and more autonomous...
With the series hybrid, once the batteries are empty, the power available depends on the generator power. Therefore, it is suitable for our cruising catamarans, but the available power must be consumed carefully.
The size of the battery pack and the generator power must be appropriate for the intended use. For trips over a few days, a larger battery pack is recommended. For longer trips, generator power is privileged.
In contrast, with a parallel hybrid system, all the power of a combustion engine remains available, so its capacity is comparable to that of a standard combustion engine. However, the electric mode provides less power than a series hybrid system, so it is more suitable for sailing at low speed in open spaces (e.g. anchoring). To this day, the parallel hybrid system remains the most complex and expensive solution.
I understand that for an Excess, hybrid solutions are the best choice. How much electric motor or generator power would be required?
FC: In fact, you need to look at the system as a whole. An electric motor develops a lot of torque at low rpm, so it is not always relevant to compare its power to that of a combustion engine. For example, a 10KW electric motor corresponds literally to 13.4HP, but can be compared to a 15 or 20HP combustion engine, if you consider the torque that it provides.
On the other hand, a combustion engine is never used at full power. At cruising speed, it is at around 40% of max power. It occasionally runs at between 50 and 60% for short periods. In the range between 70% and 100% power, the speed increases very little (+5 to 10%) but the consumption doubles.
With combustion engines, boats are often "over-powered" to improve comfort (cruising at 1800 rpm instead of 3200 rpm, so less noise and vibrations) and to decrease consumption.
With an electric motor, the approach is different because the motor can run at full power without increasing consumption or affecting comfort. The engine should, therefore, be sized according to the cruising speed, +10-20% for top speed. This is also one of the reasons why the power in kW is lower than that of a combustion engine.
In the case of a series hybrid installation, the units are sized for propulsion, but also for other uses onboard (air conditioning, etc.). The electric motor is sized solely according to the power required for propulsion.
With a parallel hybrid installation, the combustion engine is generally comparable to that of a "standard" boat, in order to provide sufficient autonomy. The electric motor is complementary and silent at low speed, while immediately effective at low rpm.
In any case, the motorisation of the boat is linked to its resistance curve, and when observing the typical curve of a cruising catamaran, it is noticeable that doubling the power can double the weight, consumption and cost, but it does not double the speed!
Yes, we can see that over-motorisation is not a very viable solution! Concretely speaking, is electric propulsion much more environmentally- friendly than a standard diesel engine?
FC: That is an excellent question. In fact, it entirely depends on the programme and the use of the boat:
From a purely technical point of view, in the case of a series hybrid solution, diesel fuel is used to power a combustion engine and then it converts the energy into electricity via an alternator, charges the batteries via a charger, and converts this electrical power back into mechanical power via an electric motor, before transmitting it to the propeller via an inverter... This whole sequence leads to significant efficiency loss.
In contrast, on a "standard boat", the fuel energy is converted directly into kinetic energy, with less loss. Therefore, in principle, the "ecological" balance is better...
As a matter of fact, hybrid solutions become very advantageous if you charge the batteries with other sources (shore power, solar panels, wind turbines, etc.). This is often the case on our cruising yachts.
One cannot consider electric propulsion as THE sole and absolute ecological solution; it is a major alternative, but a global vision of the energy on board is necessary, and above all, one must take into account the navigation programme. This is exactly the same intellectual approach that can be observed in the automobile industry: Everyone can understand that an electric car is ideal in the city, that a hybrid one provides a higher range, and that a diesel car is still unbeatable on long motorway journeys...
You mentioned charging the batteries with other sources than the generator. Could one imagine a boat running solely on electric AND solar energy, for example?
FC: Anything can be imagined; that's what we're here for!
But it is important to keep in mind the values to be considered here. Take for example, a "small'" 10kw motor, and 0.6m² solar panels (120W). Concretely, at full speed for 1 hour, the motor will consume as much as the production of 83 solar panels (57m²), while aiming for 100% utopian solar efficiency...
All solutions need to be considered, but technology "shortcuts" often clash with reality principles...!
In short, why switch to electric today, rather than yesterday or tomorrow?
FC: Previously, this technology was not really available, lead batteries were too heavy and provided very poor performance, lithium batteries were too expensive or dangerous, and electric motors were rare and not marinised...Also, our customer demand was just beginning...
Nowadays, lithium-iron-phosphate batteries provide an energy storage system that is cost-effective, easy to install, reliable, and above all, very efficient. Electric motors have also improved greatly, regarding both brushless and conventional technology. Finally (and most importantly), users seem to be ready, and are leading us towards these innovations.
Why not tomorrow? Why wait until tomorrow? Switching to hybrids today means anticipating the future. Besides propulsion, the energy production on board must aim to be less polluting and more simple. Nowadays, a combustion engine is used for propulsion and a generator for on-board comfort. Tomorrow, there will be an energy source that will charge batteries and power consumers (propulsion, air conditioning, refrigeration, lighting, electronics, etc.). The series hybrid meets this new configuration. Currently, the main energy source is the generator. But if tomorrow there is a less polluting form of energy production technology that becomes reliable (hydrogen or other), it will just have to be substituted for the generator without changing the rest of the system's configuration.
To sum up
It is understandable that, according to the sailing programme, hybrid-electric propulsion does not necessarily provide a significant gain in terms of environmental impact. If we now try to list the other advantages or drawbacks of this technology, for our ideal Excess...
- - Silence
- - Versatility
- - Flexibility
For short cruises, "comfortable" navigation providing 2 or 3 days of full autonomy and silence.
For long distance cruising, "reasonable" navigation in hybrid mode. The user benefits from the advantages of electric propulsion, and the hybrid system enables the navigator to apprehend more severe weather conditions.
- Navigation under sail for longer periods.
Indeed, in light wind, an electric motor can create apparent wind to enjoy the sensation of sailing longer... Our owners are already aware of energy consumption on board, but when they go on a long cruise without wind, they may prefer to use the engine alone because motor-sailing with a combustion engine is not very pleasant. In the future, with an electric motor, they will be able to enjoy the sensation of sailing longer (it's very Excess!) but mostly, consume less electricity for the boat propulsion.
In this case, the hybrid configuration triples: Thermal-Electric-Velic, with even more enhanced litre/mile performance!
- Globalised energy
Electric propulsion can be considered as one form of energy on board, among others... Reconsidering propulsion means reconsidering the whole energy management system. As the battery pack and charging capacities are increased, entire on-board comfort can (and should) be reconsidered more globally (but this could be the content of a new Excess Lab topic!)
- Skipper awareness
One of the effects of electric motorisation, which is often observed in the automobile sector, is the user's awareness of its consumption. Thanks to the engine control interface, the navigator can immediately view its range according to the boat speed. This can encourage them to sail in a more energy-efficient manner, and help them to better understand the influence of waves, current or wind on the efficiency of their propulsion.
- The weight
Although this notion is mostly linked to technical choices and the desire for autonomy, one can observe that a boat equipped with hybrid propulsion is generally heavier than its diesel equivalent. The difference is less significant with fully electric vehicles, but the weight of the battery pack often counteracts the gain in fuel and engine weight.
A sailboat electric propulsion runs at high voltages (from 40 to 350V DC depending on the model). Therefore, the electrical risk is non-negligible, and you need to be fully trained and authorised before you can "get your hands on" the motor.
Be it all-electric, series or parallel hybrid, these high-end solutions are - to this day - more expensive than thermal propulsion. It is yet to be seen how much the owner is ready to spend on silence, comfort and the other numerous advantages of these technologies...