Powerboating - Part Five

Relaxing in airconditioned comfort on a boat gently swaying at anchor is only a few boaties' idea of nirvana. Most, of course, go boating to experience less manufactured environments. The fresh salt air, balmy evenings sipping a cool treat on the back deck - that sort of thing.

That said, anybody who's had to endure sleepless sweaty night after sweaty night down below will vouch for the comfort mod cons can bring to powerboating.

It's a different sort of AC which helps make boating such a utopian pastime in the 'Noughties. Aircon's great, but we couldn't have it without AC - alternating current electricity First up, however, don't be lulled into a false sense of security on its safety. AC power (110V or 240V) is deadly - an absolute killer! Thus if you have any queries on AC electricity, seek professional advice.

That said, in emergency situations, it's important the boat-owner has at least a rudimentary understanding of the boat's entire electrical system. So, boil the billy - it's time for some theory...

THE ALTERNATOR
Alternating current cannot be stored in a battery like DC (direct current), it must be generated by mechanical means (ie. via an alternator or generator). Both terms are interchangeable, although it's more correct to say 'AC generator', but let's not be pedantic.

In a previous discussion on engine charging of batteries the generation of AC was briefly mentioned, since that's the method used on alternators. Remember it was discussed that when electric current flows in a coil of wire, a magnetic field is built up with a north pole and a south pole. If this magnetic field were caused to rotate, and if this field passed over another coil while doing that, then a current flow would be induced in the second coil.

Fig One shows four coils wound on a rotor and when DC electricity flows through them, magnetic poles will be built up (as shown and represented by the dashed blue lines). The strength of these magnetic fields is determined by the amount of electricity that the regulator allows to flow from the battery, through the coils and back to the battery. Each of the coils is wired in series, with one end connected to the positive slip-ring and the other to the negative slip-ring.

The stator consists of two coils connected in series. As the rotor rotates, the magnetic field also rotates and cuts the stator coils, inducing in them a flow of electricity that goes first in one direction as the north field passes and then in the opposite as the south field moves across in turn.

Fig Two depicts a sine wave. This graph reveals the action of an electron in one of the stator coils. It starts from zero, moves to maximum positive and then slows to zero again. As the opposite magnetic field sweeps across the coil, the electron now moves in the opposite direction to maximum negative and then back to zero. There are billions of electrons all doing the same thing and one complete period of the events described is called a cycle.

This wave form occurs over and over in nature; in fact, tides obey its mathematical laws just as surely as our alternator. It is of interest, particularly regarding inverters, because designers of electrical appliances base equipment on the expectation of supply. And when it isn't, there can be some strange results. (More on sine waves later. Now, back to our alternator).

If the speed of rotation of the rotor was once per second (or 60rpm), then the frequency of the cycles (or full sine waves) would be one per second. This is called one Hertz, in honour of one of the early physicists who did a lot of the maths associated with waves.

If the speed of rotation is increased 50 times to 3600rpm, then the output frequency will be 50Hz (the same as the frequency of the domestic supply). This frequency is of extreme importance when generating our own electricity and the tolerance above or below this figure is small.

Thus the speed of rotation of the alternator must be fixed accurately, and this is monitored with the aid of a frequency meter. Boaters who operate their own alternator onboard should have one fitted. (More on this later).

Fig Three shows the addition of another pair of magnetic poles. It is now a four-pole machine due to the altered design of the rotor, whereas it was only a two-pole machine prior to being redesigned.

The difference now is that for one rotation there will be two complete sine waves (or cycles) and to obtain 50Hz frequency the alternator need only be rotated at 1800rpm, half the speed of the two-pole machine. (This is also important later).

Subsequently, this relationship between speed, number of poles and frequency is written in a formula: f=NP/60 (where f is frequency in Hz (cycles per second); N is the speed of rotation; and P is the number of pairs of poles, and this is divided by 60 to bring it to minutes).

The last concern in regard to the output of the alternator is the voltage. This is determined by two things - the speed in which the magnetic field cuts the coils; and the strength of that magnetic field.

THE REGULATOR
If the speed of rotation is fixed, as it must be for constant frequency, then the only way to control the voltage output is to control the amount of DC current flowing in the field coils.

And this is exactly what the regulator does. A sensing circuit from the output of the stator coils feeds information to the regulator as to the voltage level. As additional load is placed on the alternator, the voltage starts to drop because now more electrical pressure is required by the additional load. The regulator senses this and increases the feed current to the rotating field coils, which increases the strength of the magnetic fields, and this restores the voltage to its former level.

At the same time, the extra load has caused the speed of rotation to decrease, so the governor on the driving engine opens up slightly to bring the speed of the engine up to the correct level. In this way, most small generating units maintain voltage and frequency within design limits.

An alternator has a maximum design load which should not be exceeded. In fact, with most of the modern brushless alternators, this is not possible anyway. The exact reason is beyond the scope of this article, those interested should enrol for an electrical engineering course.

In reality, our basic alternator is pretty much as drawn hereabouts, except that the brushes and slip-rings have practically been dispensed with on small alternators - albeit the principle is the same.

The ratings of an alternator often cause confusion, so read on and it will all be explained later (when detailing alternator installation).

Now, before breaking for billy tea (about time!), the reason this needs to be known is for those who want to fit a cruising alternator to the engine. Exciting stuff!..

SHOREPOWER
Shorepower from the marina is usually supplied from a single-phase 16-amp outlet, which is connected to the boat through an extension cord to the inlet shorepower socket.

AC power does not have polarity as does DC, however, the generating authorities keep the neutral line and the earth line at the same potential by connecting them together at the generator and various substations along the supply line. Thus, it is important that the active from the supply be connected to the active bus on the vessel. The reverse of this is called reverse polarity and is a potentially dangerous situation.

Imagine for a moment a person who decides to use a knife to dislodge a slice of toast stuck in the galley toaster. Obviously he knows this is foolish, because he has turned off the power outlet, but because the boat has been unknowingly connected to reversed polarity, the toasting element is still at full potential with respect to earth.

Serious injury is just a few breadcrumbs away!

The only way to be totally safe is to have all appliances and power outlets double pole switched, since with this system both conductors are opened when the switch is turned off (see Fig Four). This is a requirement of some supply authorities.

Another good idea (and present on many craft) is the installation of a polarity warning system.

When plugging in shorepower from a strange source (eg. a different marina), always have the main circuit-breaker in the off position, then check for reverse polarity by ensuring the reverse lamp is out and the correct lamp is on. If all is well, turn on the main breaker.

If your boat does not have this means of checking polarity then I strongly suggest it be fitted. However, small polarity checkers can be purchased from electrical wholesalers. These are plugged into an outlet and will indicate if the polarity is correct. This is not as good as having the lights permanently wired into the boat's system.

Check that the ship's power cord is in good condition and always turn the power off at the pole before connecting or disconnecting the supply. Never attempt to do this with wet hands and make sure the shorepower inlet is dry before connecting.

BASIC WIRING DIAGRAM
Referring to Fig Five, four possible sources of AC supply are indicated, although many boats will only have the option of possibly two or three.

Each of these sources can be selected in turn with the AC selector switch, but only one at a time. This switch is double pole (ie. it breaks both the active and neutral).

The arrangement of this switch makes it impossible for shorepower to be applied to any of the terminals in the other sources, and the same applies to the generators and inverter when they are selected. This is important from a safety viewpoint.

Also in the shorepower line is an additional double pole circuit-breaker located adjacent to the shorepower inlet. This is a requirement if the switchboard is more than two metres from the shorepower inlet and is there to protect the wiring to the switchboard in the event of a short-circuit.

All AC cables on a boat must be double insulated for safety, and the cross-sectional area of the conductors sufficient to carry the load current (see Fig Six).

Use good quality flexible cable and secure at distances of no more than 300mm. Where the conductors terminate into switches, appliances, etc, use the same crimp ends as for the DC wiring.

It's not good enough to simply bare the wire and insert it under the screw. That's sufficient for household wiring, but not on a boat subjected to motion.

The earth wire must be the correct colour (green/yellow) and must never be used for any other purpose. The protection devices should be circuit-breakers of the correct size for the circuit they are protecting. Fuses are not acceptable in AC marine wiring.

Five AC circuits are considered as the minimum for the average boat, with more added as the equipment list grows. This allows the ability to load shed (or turn off) individual circuits if the available power cannot supply the total demand.

In the average marina, one 16-amp outlet per boat is generally all that is provided and sometimes this means that the hot-water system or airconditioning may have to be turned off while cooking, for example.

If you doubt the safety of your boat's AC wiring, then I urge you to have it inspected by a qualified electrician with experience in correct AC wiring practice.

SHIP'S GENERATORS
Engine-driven AC generators are now much more common on powerboats. They can vary in size from the small carry-on petrol units to large diesel-driven sets permanently sited in the engineroom or machinery spaces.

The rating of these generators can cause confusion as some manufacturers rate their machines in watts and others use volt amps, which is more correct.

The explanation/difference is a bit complex, but I'll attempt to be brief.

The power output of a DC generator is the product of its voltage and current and is measured in watts for smaller machines and kilowatts (1000s of watts) for larger types.

In an AC generator a third element comes into play and this is called power factor. Power factor is a term used to measure the amount of useable power the alternator can output. To explain further, if the output was connected to a heating circuit, all the energy is being converted to heat and we say that the power factor of that circuit is unity (or 1.0), and this is the maximum value it can ever be.

If, however, the generator is connected to a circuit consisting of magnetic coils, such as an electric motor, then some of the energy generated is used to produce magnetic fields. In a system operating at a frequency of 50Hz, these fields are being produced and collapsed at a rate of 50 times per second. When the field collapses, the energy used to produce it is returned to the generator and the nett effect is zero. The correct term for this magnetic effect is called inductance, and this has an important bearing on AC generators.

The actual useable power will be reduced by whatever the power factor of the motor is: typically this will be 0.8, and this is 20% of the machine's output. Thus if you purchase a generator with a rated output of 1000W, it will only be able to deliver this if the power factor of the circuit it is connected to is 1; for all other applications it will be something less.

To overcome confusion, ratings of alternators are generally stated in Va (volt amps) or kVa (kilovolt amps). This means that the actual power output of the machine will have to be multiplied by the power factor of the circuit before the true figure can be arrived at.

All of this was once only of interest to an electrical engineer, but since the availability of small AC generators to the public at large, the advertising spin doctors have rubbed snake oil all over the whole subject, and it is common to see alternators have their output stated in watts at a power factor of one.

To further cloud the issue, the term 'maximum output power' is often bandied about, but the figure you should be interested in, often buried in the fine print, is the 'continuous output power'. This is often a lot less.

A final word on inductance. Some designs of electric motors are called induction motors because they use this principle in their operation, however one drawback to this is that at the instant of starting they require a huge amount of power.

Just why this is so is beyond the scope of our discussion here, but accept it as a fact. The amount of starting power is roughly three times the power required to operate it in the running condition, and it is here that it affects us. If we have a 1000W motor, it will need 3000W to start! If our AC generator is a 2kVa machine it won't have a prayer of getting the motor away - in fact, a 3kVa unit will struggle, particularly if the motor is directly connected to the load.

These motors are widely used in refrigeration and airconditioning equipment found on boats and we must take this into account when calculating what size generator to fit to the vessel.

CALCULATING THE LOAD
As this is not an article on electrical engineering, we will keep it simple. Power is measured in watts and this unit normally appears on the nameplate of most appliances. If it doesn't, then the figure will be given in amps and we all remember that multiplying amps by voltage gives the answer in watts (eg. a toaster drawing 3 amps, multiplied by 240V, has a power rating of 720W).

So, armed with all this technical information, write down all the appliances on your boat that need 240V electricity, and alongside place the power requirement in watts. I have listed below some of the common ones with average power draws.

Your list may have some or all of these, or possibly more. When it is finished, decide the most important items that must be run on a daily basis. Using my list as an example, it would be the eutectic refrigerator, the hot water and the battery charger.

When I add these loads together the total is 3200W. (The battery charger will seldom draw the stated load unless the batteries are dead flat, but we must allow for it in the calculations).

All the other items can be used if the process of load shedding is applied. This means that if the hair dryer is in use, then the hot-water system can be turned off for that period and so on. This allows the generator to be kept as small as possible with all the benefits of cost, weight, etc.

A word of warning at this point...

I repeat, some loads, mainly those involving electric motors, are inductive, and this technical word means that they have high starting currents, often as much as three times the running load. Thus the refrigerator, although only needing 1000W to run, will need 3000W to start and a generator with less capacity than this would not suit our application.

Even then, the refrigerator must be started first before the heating (or resistive) loads can be applied. The smallest generator I would recommend for this application would be 3.5kVa.

Next instalment, we'll have a look at the options for fitting a small AC generator to a typical powerboat, at least some practical stuff at last, and away from all this dry theory. Speaking of dry...

Play it safe...

When around AC circuits, safety must be uppermost in the mind at all times.

If you are working on the wiring it is not good enough to just turn off the supply from the shore. Unplug the cord and bring it aboard. If the power is coming from the ship's generator, stop it, and most lethal of all, if the AC power is supplied from the inverter, remove its DC input.

Electricity kills by interrupting the normal beating of the human heart, causing it to quiver (or fibrillate) rather than pump. Often the victim will stop breathing and under these two conditions they are not long for this world. Assistance must be quick and effective.

Never work alone and make sure everyone (including yourself) knows exactly what to do in the event of an electrocution.

Step One: DO NOT TOUCH THE VICTIM UNTIL ALL POWER HAS BEEN TURNED OFF! One person electrocuted is already one too many.

Step Two: Place person on their back and ascertain if they are breathing and if the heart is beating. If not, commence CPR immediately.

Step Three: Call for assistance if possible and keep CPR going until victim recovers or help arrives.

We don't want to scare anybody. Commonsense is all that is needed. And repeat, don't work alone.