Diesel Generator

Marine diesel generators on board yachts. You should know that!


On-board power for the sailing yacht

Hardly any sailor wants to do without electricity on board his yacht today. If the port is left and the plug is pulled out of the shore power socket, the question of the on-board power supply arises. A reliable on-board power source means comfort and safety. The autopilot, the navigation instruments, the marine radio, the position lights or the refrigerator require a continuous energy supply. The battery bank usually supplies this current.

However, the energy stored in the battery bank is finite and needs to be replenished from time to time. In the port, as mentioned, this task is carried out by a charger that is supplied via the shore power line. But where does the energy come from on the go, outside the port, at anchorage or at sea?

Where does the energy come from on the go?

Most sailing yachts have a built-in motor as a slack pusher and for safe maneuvering in port. It stands to reason that the alternator on the main engine is used to charge the battery bank while the engine is running.

However, this approach is only suitable to a limited extent, as the main engine generates noise on the one hand and, on the other hand, the load is very low due to the low power of the alternator in relation to the energy consumption of the engine. This results in unnecessary wear on the engine and high fuel consumption. Exhaust emissions are another factor.

However, this charging method can be expanded with solar or wind energy, for example. In this way, depending on the dimensioning, the consumers can be wholly or partially supplied with electricity. These are undoubtedly good systems, but they also have the catch that they only work depending on the weather. For solar energy we need daylight, for wind energy as the name suggests – wind!

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Wind generator and solar modules on a cruising yacht

There are also tow generators. In turn, they only work when the yacht is in motion – they are useless at anchor. So there is not just one perfect system that covers everything.

The generator as a convenient alternative

Many sailors want the same level of comfort on board as at home, but it is not uncommon for the existing battery banks to be insufficiently dimensioned. This is a challenge, especially when very large consumers, such as a freezer compartment, air conditioning, stove with induction field, TV, washing machine or heater are used. Then a very large battery bank is required or another reliable power source has to be found, for example a diesel generator.

The advantage of the diesel generator is that it does not depend on the weather or the journey of the yacht, but can be started quickly and easily at any time if necessary and then immediately supplies electricity – provided there is enough fuel on board. And that with a very good ratio between energy consumption and energy gain – very different from the effectiveness of the alternator.

Field of application and purpose of the generator

If a generator is to be installed on board, a few parameters must be clarified beforehand in order to select the right generator. The size of the yacht and the number of electrical consumers play an important role. In general, the total consumption on board should not be underestimated. Even smaller boats usually have more than 50 different circuits.

For targeted advice from a specialist, it is helpful if the crew draws up a list beforehand in which all consumers on board are listed with their power consumption and daily usage time – a so-called energy balance. The energy balance also records the voltage with which each consumer must be supplied. On blue water yachts this is usually 12 or 24 volts or 230 volts and in individual cases 48 or 400 volts.

Another important point are the climatic conditions. At outside temperatures in northern Germany, a refrigerator consumes significantly less energy than in the tropics, where the temperature difference between the inside and the environment of the refrigerator is significantly higher. So this also has an influence on the energy demand. In addition, in the tropics, air conditioning systems are often used on board larger yachts, which have a comparatively very high energy requirement.

Note: Air conditioning systems, compressors and pumps have very high starting currents and should be checked particularly when determining the energy requirement.

And of course the period without shore power also plays a role. Anyone who spends months at sea and at anchorage because there is no marina nearby – for example in the South Pacific – needs a system that suits this life on board and is accordingly reliable.

The creation of an energy balance offers the necessary clarity. It then gives the required size of the battery bank and, as a result, the required power of the generator.

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Which generators are there?

There are essentially two types of generator used on board yachts – the battery charging generator (direct current) or the alternating current generator. The two generator types work differently, but both have the advantage that they can also be used to cook electrically, so that a gas cooker is no longer required on board.

The battery charging generator

If there are only a few 230 volt consumers on board or if they are only operated from time to time, the battery charger generator is of interest. Depending on the voltage of the battery bank, this type of generator generates 12, 24 or 48 volts and thus charges the battery bank. The system should be designed in such a way that the batteries in the port can alternatively be charged via shore power.

Consumers that require 230 volts are supplied from the battery bank via an inverter that generates the alternating voltage. Larger consumers can also be supplied at short notice if capacity permits. The battery charger generator is usually powered by a small diesel engine and can charge the battery to around 93 percent of its capacity.

A good battery charging generator recognizes when the voltage of the battery bank falls below a certain value and then automatically starts the charging process. As a result, with such a system, the crew can be sure that they always have enough energy on board and that the battery is protected from deep discharge. Especially with continuously operated devices such as a refrigerator or an autopilot.

The battery charging generator should be able to generate the maximum charging current suitable for the battery capacity in order to avoid long generator run times. As a rule, charging times of 40 to 60 minutes per day are sufficient for a battery charging generator.

The alternator

The alternator generates 230 or 400 volts and supplies large electrical consumers on board with electricity. Of course, such a generator can also be used to charge the battery bank. To do this, the shore power charger, which is already on board, is simply connected to the generator’s 230-volt output.

It should be noted that, as a rule, simple battery chargers are no longer installed on long-haul yachts, but instead modern combination inverters that combine the inverter and charger in one unit and thus form a power management function. They combine the energy from the generator, the battery bank and the shore power connection and make this available as required.

In the system consisting of an alternator and a combination inverter, the generator takes over the continuous energy supply for the largest 230-volt consumers. If only little energy is required for a 230-volt consumer, it can alternatively be supplied with energy from the battery bank by the combi inverter. If only DC consumers (12, 24 or 48 volts) need to be supplied, they can take this energy directly from the battery bank. If, on the other hand, there is excess energy from the generator, this can be used to charge the battery bank thanks to the combination function.

The combi inverter can use the shore power connection in the port and tap the generator en route to charge the battery bank. You can switch between the two energy sources either manually or with a so-called transfer switch.

In short: If the alternator is used together with a combi inverter, the generator is always optimally utilized.

The alternator is available in two different types. On the one hand as a generator with constant speed and on the other hand as a so-called inverter generator with a flexible speed that is continuously adapted to the energy demand during operation.

Constant speed alternator

Until the 2000s, the alternator was only available with constant or regulated speed. This means that he always gave off the same amount of energy, regardless of what was needed. Therefore, the energy consumption on board had to be calculated very precisely before the generator was purchased in order to operate the unit optimally.

In technical jargon, one speaks of the “light load problem” in this regard. This means that it can happen that significantly less energy is consumed than the generator produces. It can be compared to a tap. Due to the fixed speed of the generator, the same amount of water always comes out of the tap, regardless of what is being taken out of the bucket (battery bank) into which it is running. If the ratio is unfavorable, the bucket overflows or empties. If the power requirement is too low, a load (dummy load) must then be switched on, which consumes the excess energy.

This is not a problem for consumers with a constant power requirement – for example when operating the autopilot, the refrigerator, the air conditioning system or the diving compressor. It becomes difficult when the energy requirement fluctuates.

Variable speed alternator

If the energy requirement on board the yacht fluctuates significantly, a generator with variable speed is the better solution because the amount of energy produced can be varied depending on the speed. These generators are called inverter generators.

An inverter generator offers remarkable fuel savings compared to a traditional fixed speed generator. The electrical load is supplied with a constant output voltage via the inverter; the speed of the diesel engine can be regulated depending on the needs of the consumers switched on. As a result, the engine always runs in the optimum power range and, in comparison, needs less fuel.

Depending on the load, savings of up to 30 percent can be achieved, which also benefits the environment. This also means that there are no light load problems because only as much energy is produced as is actually needed.

Important attributes of a generator


The lowest possible noise pollution is part of the comfort on board. This also applies to the generator and this should be taken into account when purchasing. Generators that are housed in a so-called sound insulation capsule are considered to be particularly quiet. Depending on the performance, this is made of GRP or stainless steel.

If the generator also has water cooling, this capsule is hermetically sealed. This is not possible with air cooling. Units with sound insulation capsules and water cooling are therefore particularly quiet.

Water cooling

When purchasing an alternator, I would make sure that the electrical winding is water-cooled. If the diesel engine and generator are water-cooled, the units can be selected to be significantly smaller, as they do not require any ambient air for cooling. Space is usually limited, especially on yachts.

In addition, the water cooling ensures that the temperatures in the sound insulation capsule remain in the range of acceptable values ​​even under tropical conditions, if the best possible sound insulation is required at the same time. The water used for cooling is usually colder than the ambient air and is more efficient.

Important: At high water temperatures and a high salt content in the sea water, the risk of electrolysis increases. The metal components of the generator can then be attacked by galvanic corrosion. Even a very small current can cause devastating decomposition. It is therefore advisable to purchase a generator with galvanically isolated dual-circuit cooling. In these systems, the engine and generator are cooled with fresh water, whereas seawater only comes into contact with the heat exchanger, which should be manufactured in such a way that it is not susceptible to electrolysis.

The water cooling also ensures that the resulting heat is dissipated directly via the cooling water system. The room temperatures at the installation location of the generator – usually in the engine room – remain constant and there is no risk of other devices being heated up.

The quality of the energy

In the case of alternators, it should also be ensured that the generator produces a clean sine wave – that is, a high quality current. If the wave of the alternating voltage is not sinusoidal, the connected consumers can be damaged and fail or break. Inverter technology offers another advantage here, as it has very good voltage and frequency stability and is therefore particularly suitable for sensitive electronics.

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The practical implementation on board

Combination of energy supply systems

If a generator is on board, it usually plays an important role in the energy supply because it is usually the largest supplier of energy on board. Continuous operation around the clock is usually not necessary – especially when the energy requirement is low or when electricity is generated in parallel from other energy sources, such as the alternator, solar modules, towing or wind generators.

Such a combination of energy suppliers ensures that the generator is not used at low loads and helps reduce fuel consumption. However, if there is a high demand for energy or if the other systems on board do not meet the demand, the generator can quickly deliver large amounts of energy at the push of a button. This increases safety and is extremely comfortable – especially on cruising yachts.

Parallel and multiple connections of generators

Depending on the manufacturer, it is possible to combine several generators with each other and switch them in parallel to increase the output power. That allows for a couple of interesting options. For example, redundant systems can be created in this way. In terms of weight, it can also be of interest to distribute the weight on the hulls of a multihull yacht.

As a rule, the alternators with constant speed must have the same power. With the inverter generators, however, it is possible to connect several generators of different sizes with the help of special parallel inverters. This achieves the highest level of security and redundancy. For example, a system could be set up that consists of two 20-kilowatt generators and one 5-kilowatt generator. The two large generators run during the day, when a lot of energy is usually required, and the small one at night. This guarantees a constant power supply on board at all times.

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Electric propulsion system on board

Sustainability and environmental awareness have become important considerations for sailors, and yachts with electric drives are slowly increasing. If there is an electric drive, it can be operated as a generator while sailing and charge the battery bank. A fixed propeller is required for this.

When the yacht sails, the propeller rotates in the flowing water and the static electric drive can be used “the other way around” to generate electricity. Most of the time this only works when sailing at a minimum speed of 6 to 7 knots. The battery can be charged with around 10 percent of the actual power of the electric motor. This is a nice side effect, but in most cases not enough in the long run.

In order to charge the battery efficiently and completely over the long term, a diesel generator should be integrated into the drive system. Especially when longer sailing trips are part of everyday life on board and no shore power connection is available on the way.

Such a solution has a single electrical system that can supply both the propulsion system and the electrical consumers on board with energy. The battery bank and the generator become the central power source for both systems. If the focus is primarily on supplying the electrical devices on board, an alternator is generally the better solution.

If, on the other hand, the focus is on driving, a battery charger generator is recommended. If there is a shortage in the energy supply of the consumers on board when the boat is slow, the battery charger generator can be used temporarily to close this gap. If no electricity is required for the consumers on board, the generator is fully available for charging the battery bank.

If you do not want to do without the main engine on board, there is of course the option of installing the electric drive system in parallel. When the main engine is running, the parallel electric drive can run “in reverse” and help charge the battery bank. Conversely, the electric drive can be used when maneuvering in the harbor or for slow cruising.

In any case, when purchasing a combined system for consumers and drives, you should use a supplier who coordinates all of the main components of the system and can supply them from “one source”.

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Depending on the area of travel, the duration of the journey and the energy requirement, on-board generators are an important addition, as they can easily deliver larger amounts of energy at the push of a button – either as an alternating current generator or as a battery charger.

If a high-quality system is purchased, the generator offers a reliable solution and is also very well suited for use in areas with high ambient temperatures. Systems with water cooling and sound insulation have the advantage that they are particularly small, light and quiet.

In any case, thorough advice from a specialist should be given before purchasing. A previously prepared energy balance helps to develop a solid, tailor-made solution in such a consultation.

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