With the growth in the number of berths, shore supplies of electricity are more readily available and boatbuilders are responding by installing shore power connections and shore power circuits into boats as a standard feature. There are obvious benefits in terms of bringing all the comforts of home on board, but there are considerable dangers if the system is poorly installed.
Electricity and water should not mix, and if they do, water will always win. With battery powered systems, if water gets to any connections or fittings, then the worst that can happen is that you have an inoperative system and a flat battery. With shore supply voltages the risks from water are much greater. The voltages used can be lethal, so a sound installation and care in its use are vital.
The concept of having mains voltage on board opens up the possibility of enjoying all the comforts of home - freezers, electric cookers, electric kettles, microwave ovens, washing machines, water heaters and television. Some of these can be enjoyed by using 12 or 24 volt systems (refrigerators, television and microwave ovens, for example) but unless the battery is being charged they can cause quite a heavy drain, which can exhaust the battery fairly rapidly. For equipment with heavier power requirements such as cookers and water heaters, there is no question of using the battery even when it is being charged because neither the battery nor the charging system would cope.
The advantage of shore power lies mainly in its higher voltage. The wattage of the equipment is the determining factor; the following gives some idea of the wattage of various items of equipment:
Boat Cookers 6000 watts
Boat Microwave ovens 1000 watts
Boat refrigerators 500 watts
Water heaters 3000 watts
Hair dryers 1000 watts
Watts = Amps x Volts, so if we have a 3000 watt water heater operating on 240 volts, this will require 12.5 amps. On 110 volts it will need just over twice that amount, but if you tried to operate the water heater on 24 volts you would need a massive 125 amps-a load which would drain the battery in under an hour and which would require heavy duty wiring the size of the starter motor cables. Using 240 volts the load is acceptable and the cable size reasonable, so if you want the full home comforts on board, then either 240 or 115 volts is the route to follow. There are two main ways of getting this voltage on board.
Shore power supply
This the easiest method because all you need is a suitable cable connected to a power point on the shore. The snag with such a system is, of course, that you can't take it with you when you go to sea, but for many people this is acceptable; they simply fit dual voltage refrigerators and microwave ovens to overcome the lack of high voltage at sea, and temporarily forgo the use of high power users like the cooker and water heater. On powerboats you can retain the use of the water heater by having a water tank which can be heated by either the shore electricity supply or from the engine cooling system. Shore power is fine so long as you operate from the same berth all the time or know that you can find a plug-in point when cruising. There is now increasing standardisation of marina power points, so that connection is easier when away from home, but going abroad can bring its problems with different sockets and different voltages.
The cable linking the boat to the shore needs to be rugged to stand up to the treatment and exposure it will receive and the current it will have to carry. It is possible to use a domestic wandering lead if it has suitable electric plugs and sockets, but this casual approach to high voltage electricity will get you into trouble sooner or later. It also means having a hatch or porthole open to bring the cable on board, so you won't want to leave the lead connected when the boat is unattended. The open hatch or port will also be a nuisance in rain and possibly a danger if water runs down the lead and into the connection box.
There is a strong move towards the standardisation of power supply sockets both on boats and at the shore connection so that you can plug wherever you are. However, different countries still have different approaches to plugs and sockets although the US 60 amp push and twist plug with flat pins is becoming the standard. In Britain BS 4343/ 16 amps and BS 4343/32 amps are still common and are quite suitable for the lower rated power supplies which are often all that is available at marinas. The US type plugs and sockets can handle higher ratings, and -though designed for 110 volts are adequate for 240 volt supplies.
Shore power cables should never have live pins. This means that the end of the cable you connect into your boat will be a socket, with the fixture on board being a plug. This may seem back to front, but the last thing you want is live pins on a plug exposed where you might touch them. Even though they are protected from rain, the connections at each end of shore supply cables should be waterproof to reduce the chance of corrosion and the risk of water running down the cable on to connection.
The shore power cable should be adequate, and well protected, for the current it has to carry. For a 16 amp current a 2.5 sq mm wire is adequate and for 32 amps, 4 sq mm. Heavy duty insulation will protect the cable where it might be trodden on on the marina pontoons. The cable should be long enough so that with movement of the boat at the berth it is not inadvertently used as a mooring line. You can make it just the right size for your regular berth, but if you cruise a lot then a longer cable, say up to 25 metres, will enable you to connect up at most marinas where a supply is available. To cope with different types of connection you should take alternative plugs with you for the shore end of the cable you connect into your boat will be a socket, with the directly into the shore end of your cable.
The location of the shore power connection on board is important. This should not be inside because you don't want to leave hatches or ports open, but even though it is a waterproof connection it will benefit from being protected. On some boats it is fitted in a small locker in the cockpit, or in the transom where stern-to mooring is common. Otherwise it is located in a semi-protected position in the cockpit. If the connection is in the open, then it should be angled downwards so that water will not run down the cable and into the connection.
Yacht Generators
A more flexible way of producing the higher shore power voltages is to have a generator on board. With marine gen sets you are completely independent and can have the use of electrically powered equipment both at sea and in harbour. To avoid having to run the generator all the time, a common arrangement is to have an installation which can be connected to the shore supply when in harbour and which can be powered by the generator when at sea or away from your home port where a shore supply may not be available. A changeover switch enables the onboard circuits to be fed from either power source and prevents feed back from one to the other.
Modern generators are mainly diesel powered generators, although the guiding rule here should be to use the same fuel as the main engine. Diesel generators tend to be noisier and produce more vibration than their petrol counterparts, but are more economical and last longer. However, noise and vibration can be critical with marine gen sets, because while you may be happy to put up with engine noise at sea, the noise of a generator running whilst stationary can disturb a quiet anchorage or upset your neighbours in a marina.
The problem of noise and vibration means that particular care has to be taken when installing boat generators. Locating it as far as possible from the accommodation will help, but the engine compartment is often under the saloon or the cockpit, which are the prime social areas on board. The normal engine compartment insulation will not absorb the noise, so it is common practice to enclose the generator in a sound box to help damp out the noise. This box can turn the generator into quite a large package, so unless it has been designed in to the boat it can be very difficult to find adequate space for a retrofit. The generator will need servicing just like the main engines, so leave space for access to all parts.
Marine generators should be flexibly mounted to reduce vibration, which involves introducing flexible sections into all the connections such as fuel lines and exhaust. The exhaust is a difficult area for boat generators; it is generally taken outside the boat to reduce the onboard noise, but it can prove offensive to neighbours who have to suffer it. Silencers can help keep the noise levels down. Another alternative is to have the option of switching the exhaust to alternative outlets so that it can be directed away from neighbouring craft.
Generators tend to be taken for granted, particularly when they are shut up in a soundproof box. Monitoring of their operation tends to be casual, so it is a sensible precaution to fit them with alarms governing water temperature, oil pressure and rpm, so that you will receive warning before the engine has major problems. Keep the generator self-contained as far as possible, including a separate fuel feed from the tank and its own starter battery. The installation should be up to the same standards as the main engines. Since the generator could be left running when there is no-one on board, an automatic shut down system should be considered if any of the monitored parameters such as pressure or temperature changes, so as to avoid expensive damage to the unit.
The size of the generator is decided in much the same way as for the DC battery system. Add up the wattage of all the equipment on board and this will give you the top consumption. Remember that there isn't a battery in the system to compensate for any temporary extra loading. Generators are normally rated in kilowatts (kW), with one kilowatt being 1000 watts. You are unlikely to have every device switched on at once, so you can probably accept a generator with a rating less than the total, but not too small otherwise you will have to go round switching equipment off before you can switch something else on, or the generator will shut down because it is overloaded.
Another reason for having a higher rated generator is that it will be better equipped to handle changes of loading. When you switch on new equipment, particularly with a high wattage, there can be a drop in voltage as the generator struggles to cope with the extra load. This is bound to be more of a problem with a smaller generator working near to capacity than a larger one. This extra loading can also be a problem where the unit being switched on has a powerful electric motor. With air conditioners, for example, when the motor cuts in, the initial loading can be two or three times the normal running load, and this extra consumption could be enough to make the generator cut out if it is working to near capacity.
As far as generator controls are concerned there are two main options, manual or automatic start. With manual start you have to activate the starter switch just like starting the main engines. With automatic start, sensors detect when a switch has been closed and automatically respond by starting the generator. This adds to the convenience of the system, but could result in the generator starting at night when a light is switched on, to the annoyance of everyone on board. Manual starting is to be preferred in that you will check the output voltage and running of the unit at start up.
All modern generators produce AC, and all high voltage equipment used on board will operate on AC. This is the same power as used at home. Much of the equipment like hair dryers, television sets, microwave ovens and refrigerators will be identical to your home appliances. AC is current which switches back and forth and it does this at a certain number of cycles per second, normally 60. This cycling rate is carefully controlled on shore at the generating station, but on board it is unlikely that the generator will be able to produce the same accurate number of cycles per second, which explains why some equipment like electric clocks and some types of tape and record player will not operate satisfactorily on board when a generator is providing the power. This matching of the cycles of AC is one of the reasons why shore power and on board generated power cannot be mixed in the same circuits. It is almost impossible to get the match perfect, and major damage could result.
Another point to watch with alternating current is the way the current flow switches from one way to the other. All shore supplies and marine gen sets will have what is termed a sine wave curve, which means that there is a gradual switch in the flow of each cycle. From a peak in one direction, the flow diminishes and then steadily builds up in the other direction. The alternative is the square wave flow, where there is a sudden transition from one direction to the other. This square wave current is produced by some convertors of DC to AC, which we will examine later. Some equipment will not operate happily on square wave AC, mainly equipment such as microwave ovens, television sets, computers and electronically controlled battery chargers.
