Generators Buying Guide


There are many different types of generator for marine applications. For the purpose of this paper we will concentrate on diesel engine-driven generators. Their function is to generate electricity which is used on board to power a variety of domestic appliances. In simple terms a generator consists of a diesel engine close-coupled to an alternator, some switch equipment and for the elimination of noise, a cocoon. Generators for boats are mainly designed to operate as single phase machines i.e. they produce 220 volt at 50Hz (or 120 volt at 60Hz if for the U.S.). There are also 3-phase generators which are more suitable for large, ocean-going ships.


To understand the workings of a generator, some basic definitions have to be understood. The most important ones being voltage, current, frequency and watts. The information contained in the glossary explains the relationship between the various terms used when discussing generators. There are additionally terms such as;

Electric load

A term used to describe consumers such as kettles, battery chargers etc. in a more technical manner, using amperes as a reference point.

Example: Kettle with 2 KW = 230 volt x 8.7 ampere = 2000 watts = 2 KW.

Starting current 

An often misunderstood term. We accept that there are different types of electrical consumers and that they can be resistive, capacitive or inductive loads, we also accept that their starting behaviour is different. Whilst pure resistive consumers such as kettles, light bulbs etc. have no starting current, other devices, in particular air conditioner compressors have very high starting currents. In many cases these are 3-5times the ampere rating as printed on the manufacturers plate. These starting currents are responsible for some generators working better than others and the question to ask is always the same …


If the answer is “no” or “don’t know” it may be best to forget that particular make altogether, or at very least make sure you investigate all the relevant technical details properly first.  ” such equipment is not suitable for power generation at all and should not even be on the market ”


With normal AC electricity on board owners can use a variety of domestic appliances. For many boat owners, it is still a novel experience having unlimited 220 volt electric power on board. It means that no longer do they have to worry about flat batteries. They will be able to use ordinary appliances such as cookers, microwaves, refrigerators, freezers, ice-makers, water-makers, power tools, computers, immersion heaters, fan heaters, electric radiators, power showers, air conditioning, electric pumps, radio/TV etc. In short, they can have all the comforts and conveniences of home. The table in the section “Noise and Vibration” shows the average power requirements for typical consumers and what they mean, and it shows how to calculate the electrical demand on board.


Very often too little consideration is given to noise. At HFL we strongly recommend the use of acoustic enclosures for generators. The argument that the engine compartment is already sound-insulated does not mean that it offers adequate protection from the generator noise when on a mooring or at night. Also when cruising at sea, most of the noise from the generator is drowned out by main propulsion engine noise. However, if the generator is used when the boat is moored, the noise can be extremely disturbing, particularly when the unit is running 24 hours a day in order to power air conditioning. When looking at this issue, the noise of the exhaust system must also be taken into account as it is a significant element contributing to the total noise output. Simple solutions will not do and at HFL we have designed and manufactured acoustic enclosures which, through the utilisation of GRP in combination with foam achieve considerable reductions in noise. Furthermore by equipping the generator plant with internal & external resilient mountings, the noise and vibration transmitted to the hull structure is virtually reduced to zero. Airborne noise is drastically reduced by the application of modern design techniques, utilising GRP material in combination with foam. These reductions in noise are achievable at reasonable cost without significant increases in weight.



During the process of selecting a suitable generator for your boat, you will come across a variety of what appear to be contradictory statements. The decision will have to be made whether a diesel or petrol engine is to be used. Assuming that the craft is already diesel-powered, it would be logical to stay with the same fuel for both the main drive engine and auxiliary generator. Experience has also shown that diesel engines are much safer than petrol engines. Diesel power below 2.5KW is however, relatively expensive in terms of initial outlay. Generally, the prime mover will be a water-cooled, 4-stroke diesel engine with a wet exhaust system. This means that the seawater which is used for cooling the engine is afterwards injected into the exhaust system where it mixes with the exhaust gas. Such systems are called wet exhaust systems, they are safe and do not present any real fire hazard on board. The generator should also be equipped with electric start, although hand-start may be available on single cylinder sets as an optional extra. Most importantly, the whole generator set itself should be manufactured by a reputable company in order to ensure a world-wide back-up service and spare parts supply. There are different ways of cooling an engine and the alternator. Conventionally, the engine is either directly cooled by using seawater or alternatively by using a heat exchanger, in which case the engine cooling circuit is filled with anti-freeze thus preventing freezing and corrosion. The external cooling circuit in turn cools the inner circuit and the cooling water is ultimately injected into the exhaust system, creating a wet exhaust.


A new practice has evolved with water-cooled alternators. The seawater is first used to remove the heat produced during the process of generating electricity. This is conventionally done by air. One can however, imagine that large quantities of air being pushed through an alternator and afterwards through the sound cover will make additional noise. This is therefore contrary to our objective of obtaining a generator which operates as quietly as possible. Combined cooling water circuits are therefore the solution to the problem and this is very successfully demonstrated in the case of the HFL MARRINER generators. The generator, or more accurately the alternator, should be of the brushless synchronous type. In simple terms, a synchronous-type alternator will maintain voltage under inductive load. An asynchronous alternator will suffer voltage loss and ultimately the collapse of the voltage altogether, and without voltage, current cannot flow in any electrical system. As pointed out earlier, recent years have seen the introduction of water-cooled alternators to the market. Again the same design principle should be applied, i.e. only synchronous-type alternators should be considered for on-board power generation. A recent case study between two different makes of generators revealed that although the same engine was used in both cases, one 4KVA generator was able to start a l6,000 BTU domestic airconditioner without trouble where as the alternative suffered a voltage collapse as the compressor-starting current switched on. In this case the voltage dropped from 220 volts down to 19 volts! Such equipment is not suitable for power generation on pleasure boats at all.


Since space on board is at a premium, the generator should be as small and as light as possible. Dimensions and weight displayed in literature are not always up to date due to technical amendments to the build spec, and it is therefore always sensible to ask the manufacturer specific questions concerning this data. Sometimes it is advisable to obtain dimensional sketches beforehand in order to check that the proposed installation will be feasible. The total weight should include the generator plus sound enclosure plus all accessories.


Manufacturers quote consumption in either g/hp/hr (grams per hp per hr) or flat ratings such as 3 ltrs/hr. This is normally the consumption at full load per hour. It is important not to confuse these two figures. If you draw only 2KW of electric power, then the fuel consumption is similar irrespective, whether you use a 1, 2 or 3 cylinder engine.


Too much time is spent on this subject. There is basically an argument in favour of each, and the right thing to do is to decide the issue with regards to available space and money to be spent. Slow-speed machines only partially extract the maximum power from a given engine. It is therefore understandable that the noise levels maybe lower and that the life expectation with regards to cylinder wear etc. could be greater. In case of the marine generators on board boats, however, this is not so cut-and-dried since these generators do not run excessively long hours anyway. At approx. 500-1000 hours per year, high-speed generators offer sufficient longevity and provide the added advantage of better cost-effectiveness, significantly smaller dimensions and lower weight. Since the introduction of water-cooled alternators, these high-speed generators are even quieter than slow-speed machines. High-speed machines offer a better starting current capability since the mass moment of inertia is much higher than that of equivalent slow-speed machines. Again this is important when operating electric motors and in particular compressor motors.


Experience has shown that most faults on the equipment which occur during the first six months of operation are down to poor or wrong installation of the generator in the first place. Attention should be paid to the installation instructions, better still if an installation data sheet is obtained and the installation planned prior to carrying out the work. This is particularly important with regards to length and dimensions of fuel lines, seawater and exhaust pipes, electric cables, circuit breakers, battery sizes, starter cables etc.

Wrongly dimensioned pipework and cables can cause trouble forevermore. Fuel systems which do not allow for proper bleeding can lead to the total destruction of the generator. Much care and attention has to be paid to the exhaust system. A variety of alternative installations can be considered. The most common are wet exhaust systems consisting of a water-lock and a rubber muffler. There are also dry exhaust systems whereby the cooling water is not injected into the exhaust pipe system. Recently, we have seen the introduction of the Exosilent which is a device that splits the wet exhaust back into water and gas (dry exhaust) thus resulting in a very quiet exhaust system both internally and externally. The system also allows the seawater to be discharged above or below the waterline. Please check whether a syphon-break is to be required. Whenever the engine-driven seawater pump is below the waterline, it must be assumed that seawater will enter the cooling circuitry and subsequently the exhaust system and gradually fill the same up with seawater. Eventually the level will rise to the point that seawater will enter the exhaust ports and contaminate the engine oil. It is therefore also important that the waterlock is sufficiently sized. It acts as a protector of the engine in two ways, firstly, through prevention of back-flow of water from outside and secondly, in the event of the engine not firing immediately, this is because when the starter motor turns over the engine, the seawater pump is operational and will continue to fill up the exhaust system. It would go too far to explain in greater detail the electrical installation and it is suggested to consult the relevant literature on this subject.


It has been said that a piece of equipment is only as good as the back-up service. This is perfectly true. Service must be available world-wide and spares should be available at very short notice. Most reputable manufacturers entertain such organisations and in most cases these are set up along a global network. This is an important point as without proper backup and the availability of service personnel, it is almost impossible to maintain equipment in the long run.


As pointed out above, the correct installation of the equipment is crucial for trouble-free operation of the generator. In fact, it is just as important as the correct operation and correct use of the equipment. It is important to know what to do, how to apply load and how to remove electric load. It is important to know what to do in case of problems and how to consult the trouble-shooting charts. Preventative maintenance is just as important as service school participation or an introductory course concerning the new equipment.