KNOW-HOW - Generators & Inverters

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Having mains power on a boat is delightfully convenient and can be financially prudent. But a long extension cord will only get you so far. NORMAN HOLTZHAUSEN explains

KNOW-HOW - Generators & Inverters
KNOW-HOW — Generators & Inverters

Having a reliable supply of mains power onboard is all about the options. Domestic appliances can be installed rather than the considerably more expensive low-voltage marine units. And given the rate at which technology changes, you are likely to upgrade long before you experience any issues with the un-marinised versions.

Domestic power is always alternating current (AC) and does not have a positive or negative terminal. Instead, the polarity is swapped (or alternated) some 50 times per second (50Hz). This occurs in a manner referred to as a sine wave, with the current switching smoothly between positive and negative. By contrast, a battery provides direct current (DC), where the positive terminal is always positive and does not change.

Producing alternating current on a boat can be achieved in two ways — installing a suitable inverter with a larger battery bank or opting for a stand-alone generator.

Inverters are a quiet, clean option for providing mains voltage from a battery power source. Apart from a slight buzzing (and sometimes the hum of a cooling fan), the inverter is silent and produces no fumes. Most modern batteries also produce no fumes, although older lead-acid types do produce hydrogen gas, which needs to be vented. A good inverter and battery system should operate almost maintenance-free for years.

It requires some clever electronics to convert DC from a battery into AC, and inverters are rated according to the type of sine wave they produce as well as their maximum output (in watts). The three types of sine wave are known as square wave, modified sine wave, and true sine wave. The smoother the wave the more expensive the inverter will be and the same wattage in pure sine wave could cost three or four times more than a square wave unit. Most inverters will be either modified sine wave or pure sine wave. Square wave units are seldom seen these days.

Some electronic devices require a pure sine wave, especially high-fidelity items like Plasma TVs, stereos and DVD players. Some computers will also only work properly with a sine wave supply. Luckily, most laptop computers come with their own filters and these will generally work with all wave types. However, you may hear their power supply "buzzing" more than usual as it works to smooth out the incoming voltage.

Appliances like washing machines will work adequately with modified sine waves, since electric motors are less fussy about their power. As long as you get a unit rated at the right power output, one of the cheaper inverter types will suffice. Power tools, cellphone chargers and most items that have a power ‘brick’ will also work with a modified sine wave, but you will need to check individual items to be sure.

All inverters generate heat, especially if they are operating at close to maximum capacity. They should therefore be installed in a location with good ventilation and cooling, and of course it should always remain perfectly dry and splash-free. Often, they are installed close to a battery, which does carry a slight risk. Batteries can release explosive gases when being charged.

The second part of this power solution is the battery. It is important that you only use deep-cycle type batteries, as starting batteries will have a greatly reduced lifespan when used with an inverter. Batteries may be connected in parallel to gain more overall power, but if so, then they should all be of the same type and capacity. Lithium-ion batteries, as used in cellphones, are starting to become available in larger capacities and are the ultimate in battery choice. But their cost is still prohibitive for most users.

When deciding on the size of an inverter and the supporting battery bank to install, you first need to estimate your total power requirements. Look at the power requirements of the devices you wish to use, usually listed on a small sticker on the back. Add-up the wattage of all devices that you think you will have turned on simultaneously, then provide a generous margin of error, perhaps 30 to 50 per cent, and that is the size inverter to buy. When in doubt buy the next biggest model.

Note also that most inverters will have a "surge" rating. Electric motors draw up to twice their normal current for a second or two when they start up, and an inverter with a generous surge rating will be able to cope with this.

Next you need to estimate the battery power requirements. Deep-cycle batteries are rated in ampere-hours, so we need to convert the total wattage into the number of amps. From high-school science we can calculate the amperage required (watts divided by volts), so an 115W bulb at 230V draws half an amp at 230V.

However, an inverter will need to draw more than 8amp to power the same bulb (100W divided by 12V. If you have a 24V supply then it will take half as many amps). The inverter itself uses some power, so probably around 10amp will be used to run this bulb. Thus a battery with a 60 ampere-hour (amp/h) capacity would run this light for six hours. If you fitted six such bulbs in the boat you would only get one hour’s light before the battery was exhausted.

And this is the main disadvantage of an inverter and battery combination — there is only a finite amount of power. After that has been used up you have no more power until you recharge the batteries. When planning your boat’s electrics it makes sense to use lower-power devices (LED bulbs, low-energy fluorescents and so on) to maximise battery life.

A genset combines a small petrol or diesel engine with an alternator. Although the vessel’s main engine also has an alternator (which is used to recharge the starting batteries), it is inefficient to run a big engine just to produce electrical power. A genset matches a small engine with a bigger alternator, so that fuel used is converted into electrical energy more efficiently.

The advantage of a genset is that it can produce power continuously, as long as you have fuel available. The disadvantage is that the engine in the unit also makes a noise. However, gensets have become smaller, quieter and cheaper and in a moderate-sized launch or yacht the genset can often be located in a spot where the noise is unobtrusive. A sound shield also helps.

Another advantage of a genset is that considerably larger total-power output is available. Inverters and batteries start to hit some practical limitations in large sizes, while gensets scale up to very large outputs. Also smaller and quieter diesel gensets are now available, so these have become a viable option for boats where petrol may be considered a safety hazard.

Gensets can produce DC or AC, and on a yacht it is common to combine a DC genset with a battery and inverter. The inverter gives continuous mains power, with the genset being started to charge the batteries as required. On a launch a genset may be used as the sole source of AC power, or again in a combination with batteries and an alternator.

Portable generators are generally not suitable for marine use. Apart from their poor sound and vibration, dampening their exhaust is usually straight out the side of the unit. A marine genset on the other hand is specifically designed for permanent installation in a confined space.
These are generally plumbed in, with a water intake for cooling and exhaust pipes, and run off an external fuel tank. A genset will also run a lot more quietly than a portable unit.

AC gensets do not have the same issues with sine wave shape as an inverter, since they generally use an alternator to generate the current. An alternator produces AC directly in the correct smooth waveform. When used for charging a battery the AC output has to be rectified to DC to charge the battery. However, for mains power it merely has to be stepped up and regulated to produce the correct voltage to operate 230V devices.

There are numerous brands of marine gensets now available, and most makers of marine engines also provide a genset option, especially when paired wit air(con). If you are cruising it may make sense to choose the same brand as your main engine, to simplify servicing in remote locations. On the other hand one of the specialist genset manufacturers may have a model that more perfectly suits your requirements of output, space and price.

1). A state-of-the-art inverter like this one from Victron is expensive but worth the money on a larger boat with significant power demands.

2.) The type of sine wave produced is important. As a minimum it’s probably sensible to select a Modified Sine Wave inverter.

3). A well organised battery bank. Personally, I would like to see it better ventilated.

4). A plumbed-in diesel generator is common on most cruising launches. This looks like a good setup but perhaps a location farther from the living space to keep the noise down could have been considered.


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