Powerboating - Part Three

Right now I'm at anchor on the Broadwater in sunny Queensland. The beer's cold, the sun is warm, and my dog is starting to work on her afternoon nap. Another couple of beers and I'll follow her example. If you haven't guessed by now, I enjoy living on my boat, and one of the things that makes it good is when all the onboard systems work, such as keeping the beer cold, etc.

Which started me thinking about electricity, for without it, I certainly wouldn't be enjoying this fabulous afternoon. Electricity is the absolute lifeblood of the modern powerboat, and an understanding of the physical laws that apply to it will certainly help keep the arteries healthy.

THE BASICS
A ship's electrical system is generally composed of two voltages, 12V DC and 240V AC. In complexity it can rival a small aircraft and failure can lead to inconvenience at best, and serious trouble or even the loss of the boat if things get out of hand.

The most basic of terms, electricity is the flow of electrons, which are minute particles of the atom. If we have an area where there is an excess of electrons (ie. negative pole), they will flow to an area where there is a deficiency of electrons (ie. positive pole). To make it possible for this flow to occur (ie: a circuit) we connect a conductor between these poles, and the electrons will flow until a balance is achieved between the two poles.

Now the type of material that the connection is made of will significantly affect flow. If it is a copper wire, the electrons bounce along quite happily (the copper offering very little opposition or resistance). If the wire is composed of stainless steel, the resistance to the flow is quite remarkable, and the effect of this resistance is to heat the stainless steel wire by some degree.

Materials which electrons flow through easily are called conductors, while those offering maximum opposition are termed insulators.

The flow of electrons is called current and is measured in amperes (amps). Thus two amps of current is twice the flow of one amp. The amount of difference in electrons between the two poles is really the difference in electrical pressure and is measured in volts, so 12V has a higher pressure than 10V. The resistance in the wire to the flow of current is calculated in ohms, and once again, a wire with a resistance of 10 ohms has twice the resistance to the flow of current as a wire of five ohms.

All this can be expressed in a formula known as Ohm's law, and it is such a useful little devil in sorting out electrical gremlins that we will be referring to it constantly.

Simply stated, George Simon Ohm found that the amount of current flowing in a circuit depended on how much voltage was pushing it, divided by how much resistance was holding it back ie. I = V/R (Where I is the current in amps, V is the voltage in volts, and R is the resistance in ohms.)

When the stainless steel wire heated due to its resistance, work was being done. The rate of that work is called power and is measured in watts ie. P = V x I. (Where P is in watts, V is the voltage in volts, and I is the current in amps).

DC & BATTERIES
For the moment we will confine our discussion to DC (Direct Current) systems. In a DC system the flow of current is always in one direction (ie. negative to positive). The voltage can be anything, but the standard for pleasureboats around the world has been pretty much set at 12V - the same as the automotive industry. The heart of this system is the battery.

The most common type of battery used on contemporary boats is the lead-acid. This is a box into which a group of cells is arranged and connected (more detail later).

The final connections emerge generally from the top of the box as two posts of lead (positive and negative). These should be marked accordingly: red with a (+) sign for positive; and green or blue with a (-) sign for negative. If not, the positive is always slightly the larger of the two.

This battery is a continual source of current, flowing from the positive post to the negative post via the circuit. This process will continue until the battery has depleted itself, whereupon it must be recharged.

All of this begs the question: How much electricity is in a battery? Manufacturers rate batteries in 'Ampere hours', and a battery with a rating of 100 amp hours could in theory supply a 100 amps for one hour or one amp for 100 hours, or any combination in between.

I say in theory because a lot of factors work against this - a good rule of thumb is to take 70% of the manufacturer's rating as the true capacity of the battery.

CIRCUITS
We have a battery and a circuit. Also in the circuit are a switch and a light.

When the switch is closed (on), current flows from the negative along the wire, through the switch and light and into the positive post. Let us assume that the resistance of the wire and switch is zero, and the resistance of the light is 12 ohms. According to Ohm's law the current flowing in the circuit is:
I = V/R
I = 12/12
I = 1 amp
So in this circuit a current of one amp is flowing.

Now remember when we mentioned power? Because of the higher resistance of the lamp, as the electrons flow through this, work is being done, and we can calculate this work, the units being watts.
P = V x I
P = 12 x 1
P = 12 watts
The 12W of power being dissipated over the filament of the light will cause it to heat up and glow with visible light.

Suppose we want more light. Simple, add another.

As we can see, the two lights are connected in series with one another, and when we close the switch, how disappointing to find that the lights barely glow at all. Why?

Well first, let's see how much current is flowing in the circuit. The total resistance of the two lights is 24 ohms (12 + 12), therefore the current flowing is:
I = V/R
I = 12/24
I = 0.5 amp
Now if only half an amp is flowing because the resistance of the circuit has doubled we should be able to work out the power each light is dissipating:
P = V x I
P = 12 x 0.5
P = 6 watts
But this six watts is being dissipated over the two lights, so therefore each light is only using three watts, barely enough to make them glow at all.

We can prove this by working out the voltage drop across each light. Because the total voltage drop in the circuit equals the applied voltage (ie. 12V), and because the resistances of the lights is equal, then six volts is being dropped across each light. If we take just one light:
P = V x I
P = 6 x 0.5
P = 3 watts
From all this dry academic stuff we can draw a few conclusions.

In a series circuit:

But what if we really want more light? Well we just have to connect them in a different way. Straight away we can see that the voltage drop across each branch or leg of the circuit is the same as the applied (ie. 12V), so therefore the current through A-B must be the same as B-C if the resistances of the globes are the same, which they are. For those interested, I will allow them to use Ohm's law to prove the following.

In a parallel circuit:

CONDUCTORS

We mentioned earlier that materials that offer little resistance to the flow of electricity are called conductors, and copper is one of the best when things such as price and useability are considered.

However, even copper has some resistance and if we force more of them down the wire than it can comfortably accommodate, then our old friend 'power' will make its presence felt and the wire will start to heat up.

Just as a four-lane freeway can handle more cars than a single-lane road, a conductor with a larger cross-sectional area can pass more electricity without heating up than a smaller one.

Another factor also comes into play, voltage drop, and this is a real enemy in a boat's wiring. Imagine a copper wire that has a resistance of 0.01 ohm per foot. Not much you might say but let us see what the voltage drop across this resistance would be if we were passing 10 amps through with an applied voltage of 12V. Back to 'Uncle George's' law:
V = R x I
V = 0.01 x 10
V = 0.1 volt

We can see that for every foot of this wire we use to pass 10 amps through, we will lose 0.1 volt, or to put it another way, for every 10ft of wire, we lose a volt.

Now we only had 12V to begin with, and if this wire is 30ft long by the time it wriggles its way from the switchboard to, say, a deck-wash pump mounted in the bow and back to the board we now only have nine volts to apply to the pump.

The pump will probably operate, but way under what the designer anticipated, delivering a stream of water that wouldn't threaten an ant, let alone wash down a muddy anchor. Of course, the pump will get the blame (pumps can be blamed for almost anything on a boat), but the true villain is voltage drop.

Thus it is important to size all conductors of electricity to suit the task they are expected to perform. A table of conductor sizes will appear with the next article to help you select the correct size.

A word of caution. If the conductor is going to be bunched up with a lot of other cables, if it is going to run through hot engineroom spaces, then go the next size up for safety's sake. In the long-term this will keep voltage drop to a minimum and allow a lot of pumps to get on with their job.

MORE ON VOLTAGE DROP
The reason we are devoting some time to this subject is because it is probably the major source of electrical problems in a boat.

The arrows point to six connection points in the circuit. If each one of these were poorly made or, more importantly, had corroded over time, such that each now offered one ohm's resistance to the flow of two amps that the light requires, then the opposition would be such that the lamp would not glow at all. (And, of course, it would get the blame.)

It cannot be emphasised too strongly that bad connections are the cause of nearly all electrical failures on a boat.

Moreover, in high-current situations, such as the starter circuit or anchor winch circuit, high-resistance joints can cause so much power to be dissipated over them that a fire can be the result.

CIRCUIT PROTECTION
A short circuit is when the high resistance of the load is bypassed by a low-resistance conductor.

The load, in this case the light globe, has been shorted out by the wires touching behind the base. With little resistance left in the circuit to limit the flow of current, massive amounts of electricity will flow, causing the wire to melt (our old friend power at work) and possibly cause a fire.

What we really need is a protective device in the line to stop this from happening, and this is achieved by either a fuse or a circuit-breaker.

A fuse is a piece of fine wire that will melt when a certain amount of current, determined by the manufacturer, flows through it. Once the fuse melts, the circuit is broken and everything is safe. Never use a fuse of higher capacity than specified!

ELECTRICAL CONNECTIONS
Good electrical connections are vitally important in the marine environment due to the enemy - salt air. If your boat is riddled with wires where the ends have been bared and the copper strands just pushed into a screw terminal and tightened down, then trouble is headed your way.

These will corrode over time, and either the wire will break off at that point or a high-resistance joint will result. Use only crimp connectors. Invest in a good set of crimping pliers. Spray each connection with WD-40 or similar.

BATTERIES
Firstly, let me say that I'm a fan of lead-acid batteries. Lead-acid batteries can supply phenomenal amounts of current, there is none of this memory nonsense one strikes with power tool and mobile phone batteries, and they give a good service life.

The battery in a car is the most neglected thing on earth, but it's madness to treat your boat batteries in the same fashion. They require inspection at least every month, every week is even better.

Basically, batteries work as follows. If two electrodes made of lead are immersed in a bath of sulfuric acid and connected to a source of electricity, a chemical reaction will take place. Lead peroxide will form on the positive plate and it will have a deficiency of electrons. Oxygen will form at the plate.

The negative plate will have an excess of electrons and will also liberate hydrogen gas from the water.

When the cell (this is what we call this arrangement) is fully charged no more lead oxide can form at the positive plate. The potential (or electrical difference between the two plates) will be around two volts. If we now connect a load, in this case a two-volt light globe between the electrodes, the excess electrons will flow from the negative plate to the positive. Work will be done on the filament of the globe, (power) causing it to glow, and the electrons will return to the positive plate.

At the same time lead oxide (PbO2) will combine with the acid to form lead sulfate (PbSO4), and once this has all been converted the cell will be discharged.

The specific gravity of the acid has gone from 1.26 times the density of water to roughly 1.18 times as the sulfate ions leave the acid. This is a good check on the state of charge in the cell, and in fact is what we measure when we use a hydrometer.

The charge and discharge reactions are time related, and as a consequence the surface area of the plates is important, since most of the chemical activity takes place there.

In a battery many plates of each polarity are interleaved and assembled into groups of cells with an output voltage of two volts. These cells are then connected in series to produce 12V at the master terminals. The plates themselves are constructed like a lattice, with the active material forced into this grid as a paste. Positive plates have lead peroxide paste and are dark brown in colour. Negative plates are pasted with grey spongy lead, this being a manufacturing method of increasing the surface area of the plate.

A porous, nonconducting separator (plastic sheet) is used to make sure the plates cannot touch each other.

This battery of cells is then encased in an acid-proof box with the familiar vent caps on the top.

This method of construction results in a reasonably robust product, however the battery is susceptible to jarring and hard knocks which tends to dislodge the active material from the grid. This is generally the biggest cause of early battery failure, as some have had the daylights beaten out of them in the journey from the manufacturer to the warehouse to the wholesaler to the retailer and eventually you. There is little the consumer can do, except hope that someone else gets the dud.

PURCHASING BATTERIES
Batteries that have to supply high current for short periods (eg. engine cranking) have very thin plates of the maximum number that can fit in the case.

This high surface area allows huge current flows for the three to five seconds it takes for the engine to fire, without damaging the internal structure of the battery.

The massive heat generated by overcranking a reluctant motor will bring about the rapid demise of a battery. Also, due to the vigour of the chemical reaction caused by the high current demand, the active material is mechanically eroded from the lattice due to the fragile construction of the grid.

Storage batteries which deliver low currents for long periods, such as the house batteries in a boat, can have much thicker plates because they do not generate the internal heat of a starting battery. Thicker plates are structurally more robust.

BATTERY CAPACITY
The old method of rating capacity, now hardly used was to fully charge a battery and allow to stand for 24 hours before connecting it to a load that was 10% of its capacity. If the battery was 100% then it should maintain this rate of discharge for 10 hours. Anything less determined the percentage of capacity left. We always felt that 60-70% of rated capacity was a reasonable expectation for a good battery.

The disadvantage of this system was that it did little to indicate the ability of the battery to supply heavy cranking current. Subsequently a new industry standard was implemented, CCA (Cold Cranking Amps). In this method, the battery is reduced in temperature to -18°C (0°F) and a high-current load imposed on it for 30 seconds. If the battery could supply this and maintain a terminal voltage of 7.2V, then the value of the current is called its CCA.

Marine batteries have a similar method known as MCA (Marine Cranking Amps). The difference in this rating is that the test is carried out at the higher temperature of 0°C (32°F).

An addendum to CCA and MCA is the Reserve Capacity test. This figure indicates the length of time a battery can supply a load of 25 amps at a terminal voltage of 10.5V. This test is carried out at a temperature of 25°C.

For information, a popular-sized battery made by a leading manufacturer for marine applications yields a rating of 500 CCA with a reserve capacity of 110 minutes. Of course larger capacities are available.

Suffice it to say, when you consider a battery's importance to your powerboat it is worthwhile to buy the best available.

Top quality batteries, properly maintained, will not let you down.

CARING FOR THE BATTERIES
Locate the batteries in as cool a place as possible, as higher temperatures make the battery work harder. Keep safety in mind at all times. Batteries vent explosive hydrogen gas (remember the Hindenburg!), so using your cigarette lighter to check the water level just means that you're not going to be around for long.

Check all cells for water at least once a month, and only add distilled water, just enough to cover the plates. The acid never wears out. Keep the tops clean and dry as this assists in the prevention of self discharge.

As far as possible, try not to discharge the battery to below 80% of its capacity, and certainly never below 50%. Forget all those experts telling you that a "good flattening" is what the battery needs.

It doesn't.

Deep discharging of lead-acid cells allows the formation of 'hard sulfate' which is almost impossible to reconvert back during the charging process. This results in loss of capacity of the battery and its early termination.

Measure the specific gravity of the electrolyte in each cell with a hydrometer. Remember that the SG of water at 4°C is 1.000. The electrolyte is more dense (ie. it has a higher SG due to the acid). Fully charged the reading should be 1.260 or above.

If one cell returns a reading below the others, it indicates a potential problem and the battery is letting you know that all is not well. I would advise more regular checking and if the problem persists, take the battery out of the starting circuit at least.

Buy a good quality hydrometer and learn to use it correctly. Some require you to do a temperature correction to arrive at the right reading. If a battery is discharged below 1.150 on your hydrometer, then serious damage will result unless it is charged as soon as possible.

Low-maintenance and maintenance-free batteries are manufactured with sufficient water to last them their anticipated life with correct charging. Generally they cannot be checked with a hydrometer, and are not my idea of good engineering.

Gel batteries have the advantage of being spill-proof (the only advantage that I can see), but I have never seen this as a problem in powerboating applications. They are claimed to be completely maintenance free and have three times the life of the wet-cell types. Possibly, but the prices quoted would turn your hair white.

BATTERY CHARGING
The advent of the automatic battery charger has made life easier for the big boat-owner - and the battery. The more modern of these use switch mode technology which does away with transformers and as a consequence, they are extremely light in weight.

Most of them are designed to be left on all the time to maintain the batteries in top condition. A further advantage is that when on shore power, the battery charger will supply power to the DC circuits rather than drawing it from the battery bank, providing it has the capacity to do this.

Which brings us to the next question, how large a charger should one fit? As a rough guide, the charger should be able to charge all the batteries at one-tenth of their ampere-hour capacity, so in a boat with 300AH batteries, a 30 amp battery charger would be sufficient. This would also supply most of the consumer loads when berthed at the marina.

When selecting your battery charger, make sure it can service at least two banks of batteries, the house and the starting.

If a generator is fitted and it has its own starting battery, then the capability to service this third bank would be required. Most of the quality marine chargers on the market today have this feature.

Unfortunately, marine automatic chargers are not cheap, but I consider them essential, particularly if extended living aboard is envisaged. They will maintain the batteries in first-class condition without fear of overcharging.

If the budget does not extend to this, a standard automotive type can be fitted and turned on and off manually. I lived with this system for years, turning on the chargers when I arrived home and turning them off as I went to bed, and it worked well. Because they were only single output, I needed two, one for each bank.

It was still a lot cheaper than the fully-automatic one I now have, but not nearly as convenient.

ENGINE CHARGING
Batteries are charged when underway, by the alternator, in the same fashion as in an automobile. The alternator is a relatively simple device based on the principles of electromagnetism. If a coil of wire, as in Fig Nine, is wrapped round an iron former and the wire is then connected to a source of electricity, current will flow in the coil of wire. This will make the iron magnetic with a north and south pole.

The lines of magnetic flux, although invisible, will be as shown by the red dashed lines. The strength of these lines of magnetic force are dependent on the amount of current flowing in the coil. This configuration of coil and former (ie. armature) is the principle of the solenoid.

If we then arrange this armature so that it can rotate, and have another coil in close proximity so that the magnetic lines cut across this coil (ie. stator), then a current flow will be induced in it.

This current goes first in one direction as the north lines cut the coil, and then in the other as the south lines do their work. If we were to draw a graph of this we would end up with a curve that went from nothing to maximum positive, back to nothing, rise to maximum negative in the opposite direction and back to zero again.

This would be called one cycle, and if it took one second for this to occur, it would be one cycle per second, as it was called in my youth. Now, because the world has decided to honour the people who did a lot of the early investigation into electricity, it is called a Hertz (Hz). How often it happens per second is the frequency. How high the value of the maximum value is called the amplitude; in this case it represents the output voltage of the coil.

We can control this by controlling the strength of the magnetic field.

Imagine a runner on the starting blocks. The starter's gun fires and off he runs, up to a maximum speed then slows down and stops. Now he turns around and runs up to maximum speed in the opposite direction then slows down and stops at the same spot from which he started. A graphical representation of this run would look exactly like the current output from the stator. In fact, this graph occurs over and over in nature, and is called a sine wave.

Suffice it to say, at this point the alternator generates a flow of electrical current that flows back and forth between its output terminals. If we connected a lamp across these terminals, it would glow and darken. However, if the frequency of the output was high enough, the lamp would appear to glow all the time. This is what happens in household electricity.

For our purposes here, what we need is direct current (DC) and happily there is a simple device (ie. diode) that will rectify the situation for us. A diode is like a one-way valve that will only let the flow of current through in one direction, and when connected across the output terminals.

Note now that one-half of the output has been blocked by the diode and only the positive half-cycle has got through. DC at last, even though it's pretty lumpy! Also, a machine like this is inefficient, since half of its potential output is wasted. This can be corrected by designing a full-wave rectifier, and for that we need four diodes arranged as shown. The DC output is now a lot smoother and is sufficient to charge a battery.

About the only thing we are left to concern ourselves with is to control the output voltage of the alternator. Remember we said that the amplitude of the output wave form could by regulated by controlling the strength of the magnetic field, and this is exactly what the regulator on an alternator does. The regulator determines the output voltage of the alternator, which in turn allows it to supply more or less current as the load demands.

Modern alternators are extremely robust machines and seldom give trouble. Inspect the belt drive for correct tension when the engine is serviced, and importantly, the belt itself, as it generally also drives the freshwater cooling pump on the engine. Always carry a spare belt!

That's definitely enough for the time being. I hope the DC side of your boat's electrical system is a little bit clearer. Next month we will be looking at the practical side of wiring, and I hope I can help with a few tips.