Fitting out a new cruising boat - Part Two, power generation

8:32 AM Mon 30 Mar 2009 GMT

'Lavezzi - the question of power generation' Multihull Solutions

So, after much research, many discussions and a reasonable amount of anguish, you have bought the new cruising boat, and now you have to fit it out.

Peter Salisbury here gives the new owner a starting check list of how to decide what to do next, and this week he considers the question of power generation on the yacht, in all its forms.

This is Part Two of the series. For Part 1, click here

Power generation

What you get
The new boat will be most often equipped with a 12 volt (occasionally 24 volt) electrical system. This will consist of two battery banks, one for motor starting and the other to supply all the other electrical equipment. Both batteries will be charged by alternators mounted on the engines. The boat may also be fitted with a shore power connection for use with mains power in the marina.

How much power do you need?
The power requirements of cruising boats vary widely. A boat connected to shore power all week and used for overnight trips on the weekend will probably be fine with the standard equipment. Boats that spend long periods away from shore need a well designed power management system.

The starting point in determining power requirements is to think about how the boat will be used.

Some possible scenarios to consider are:
. Anchoring out overnight with several guest on board
. Sailing day and night on long ocean passages
. Living aboard and anchoring out every night for weeks on end
. Running the engines for several hours each day while moving from place to place

You need to carefully consider what equipment you are likely to use in each situation and how long you are likely to use it.

Some items that use a lot of power are:
Fridge or freezer - perhaps 50 AmpHours (AH) per 24 hours
Standard anchor light - 20 AH per night (unless you use a LED light)
Household microwave 10 minutes - 20 AH
(An AmpHour is a current of 1 Amp supplied for one hour.)
Air conditioning for a few hours each day - at least 100 AH and very likely much more.

The design of your system involves deciding what types of equipment you intend to use and estimating haw long they are likely to be used for on average each day. The consumption figures of all equipment are then added up to determine your likely total power usage.
As an example a conservatively run 12 metre live aboard cruising boat might use 100 - 120 AH per day. Add a watermaker and a few other bits and pieces and this could easily reach 200 - 250 AH per day. Adding a washing machine, extra refrigeration or even air conditioning will push the power requirement
up even further.

For owners not conversant with electricity it is well worthwhile to think about planned usage and then work through the power requirement calculations with an experienced person.

Keeping track of usage
In any situation that involves the boat being away from shore power for more than a few days at a time it is important to install a good quality battery monitor (that measures the total AH into and out of the batteries) so that the skipper can readily check the state of charge of the battery. A car's fuel gauge is a good analogy.

Where will you get the power from?

In practice the power requirements will come from a range of sources. All have benefits and disadvantages and none should be relied upon in isolation.

shore power - the easy solution - .. .

Shore power
Shore power is the most reliable and largest capacity source, but you cannot be away from it for more than a few days. It is ideal for a boat that spends its week days at the marina or on a private pontoon and then cruises for one or two days each weekend.

When the boat is used this way it only needs a moderately sized battery charger. It must, however be a three stage charger to avoid over charging the battery. An automotive charger is never sufficient. The charger should have a minimum amp rating about one twentieth the amp hour capacity of the battery. Ie a 400 AH battery should have a minimum of a 20 Amp charger.

Main motors
The main motors are fitted as standard with reasonably sized alternators that will supply your power needs while they are running.
There are a couple of problems with the standard alternators. The first arises because the voltage regulators fitted are not particularly sophisticated and will not charge at maximum rate for very long. For example a 70 Amp alternator might only deliver a maximum of 50 Amps for ten minutes or so and then gradually decline to 15 Amps even though the batteries are nowhere near charged. This means the motors end up running for a long time at low load, and consequently:
. A diesel's life expectancy is compromised if it is run for long hours at low load
. Engine hours will build quickly
. There will be excessive fuel usage
. They are noisy while running, particularly in an anchorage

The second problem arises if you do not run the motors for long enough each day to completely recharge the batteries. In this instance the batteries will gradually decline over many days or weeks, to the point where they are permanently damaged.
Because of these limitations, if the main motors are to be used as the primary power generation source they should be fitted with larger alternators and a three stage regulator specifically designed for battery charging. This is relatively expensive and does not solve all of the problems.

marine solar panels - getting smarter all the time - .. .

Solar
Solar panels are reasonably expensive to install, but they cost nothing to run, are silent and will last for a long time - in excess of 20 years. The amount of power they produce depends on:
. The area of panel installed
. The amount of time the panel is exposed to the sun with no shade on any part of it. (A shadow from the boom (or even a rope) can reduce the output by 90%
. The number of hours of full sunlight per day
. The latitude and orientation of the panels

Very roughly. 100W of solar panel might produce 40 - 50 AH on a clear spring day in any really sunny climate - probably a little over half that on average.
A conservatively run cruising boat can get nearly all of its power requirements from 200 - 300 watts of solar in good weather. It is quite feasible to set up a boat to leave on a mooring with a smaller solar array to keep the basic systems in operation and the battery fully charged.
The main disadvantage with solar for a live aboard boat is that a backup power source is required to cover the inevitable cloudy week.

wind generator - DON’T get a noisy one - .. .

Wind
Wind generators can contribute a useful amount of power in windy weather. The amount of power is basically related to the blade diameter, but most generators need 5 - 7 knots of wind before they produce any power. Given that most cruisers select anchorages protected from the wind, they are unlikely to produce anything like their rated output for most of the time.
There are two other problems with wind generators. In strong wind they spin to a high speed before they cut out and so some are quite noisy, even in a marina if the owner does not turn them off. This means asking around and choosing a wind generator that is reputedly not excessively noisy. If you have already obtained one that is extremely noisy, some relief without much loss of power has been obtained by cruisers who 'trim' the blades. Do not underestimate this issue.

Secondly they have to be rigidly mounted in a position where the blades cannot possibly strike anything, particularly crew members in any wind direction. They are probably at their best on a sailing boat on extended ocean passages.

Trailing generator

sailing generator at work, reliable and simple - .. .

This simple and inexpensive mechanism provides excellent power for sailors on longer voyages. A rode of about 100ft with a propeller at the end is trailed behind the boat. As long as the boat is moving, the trailing generator will put in six amps and hour 24/7, regardless of the weather.

Generator
If a large amount of power is required, particularly for air conditioning, a generator is the preferable and often the only solution. With careful design the generator size and running time can be reduced, however there will always be a certain amount of noise (unless you use a "Whispergen") and a constant usage of fuel.

It is reasonably simple to set up a power system using a generator and inverter that will provide continuous AC power without running the generator continuously. In fact the generator running time can be reduced to only a few hours per day. The system operation can easily be made fully automatic.
In addition, with modern inverters it is possible to use the inverter to drive short term peak loads or to supplement the generator to provide large electric motor starting currents. This feature can significantly reduce the size of generator required.

Generators can be AC or DC, diesel or petrol. All have their advantages and disadvantages. The major drawbacks of generators are that they are noisy and that they need a continuous fuel supply. Placing the generator in a soundproof enclosure substantially reduces the noise, but in a quiet anchorage the owner and his neighbours are always aware of its presence.

It is not uncommon for an inexperienced owner to specify a relatively large generator to supply maximum load they can foresee. This leads to the generator being run for very long hours at low load, with the consequent noise, vibration, weight, excessive wear and fuel problems. With proper design a much smaller generator with vastly reduced running time can be used.

Whispergen - new generation - .. .

Whispergen
A relatively new generator option is the Whispergen. It has been developed in New Zealand based on a heat engine first invented in the early 1800s. It operates by using a different system than a normal diesel motor and so is virtually silent, particularly when it is mounted in its enclosure inside the engine compartment.

It is a small DC generator, producing 800 Volt Amps, or about 65 Amps at 12 Volts using only about 0.75 litres of diesel per hour. It also produces a significant amount of heat that can be used for hot water heating. Its control system can be arranged to start the unit whenever the battery state of charge drops or the hot water temperature falls. Thus it provides continuous power along with continuously available hot water - an attractive proposition, but it is relatively new to the marine market and quite expensive. Think in the order of $20,000.

Theoretically the Whispergen could be set up as the main power source for a very well equipped boat, but I have not seen one yet.

How will you store it - the main battery
Batteries are easily and permanently damaged by deeply discharging them. Flatten any battery a few times and the capacity and life will be severely compromised - hence the recommendation to install and use a battery monitor. It is not possible to assess the state of charge with any degree of accuracy with a voltmeter. A half charged battery exhibits essentially the same voltage as a fully charged one and a battery showing a noticeably low voltage is already virtually flat. A deep cycle battery should never be discharged below 40%, preferably 50%.

Deep cycle batteries are the only ones that should be considered for the house bank. Never use automotive batteries for anything but the starter battery and even then only the best quality should be considered. Marine batteries are more sturdily built to handle the harsh marine environment - hence their higher cost.

Wet cell batteries are the most efficient in terms of life/cost considerations, but they require constant observation and maintenance. Poor maintenance can result in irreparably damaged batteries in a very short time. They also produce hydrogen gas while charging so certain ventilation requirements must be met.

Gel and AGM batteries are maintenance free and do not gas. They cost more than wet cell batteries, but will last for a similar time to well maintained wet cells. Batteries are one of those devices for which the term "you get what you pay for" could have been written and there are a wide range to choose from.

A new boat will probably come equipped with a moderately sized bank of wet cell batteries. Since it is not possible to mix batteries it is probably best to adjust the size of the bank if necessary, by the addition of extra cells of the same type. When the time for replacement comes, then consider alternative types.

The house battery should be designed to suit the loads they are intended to carry. The system designer will take into account:
. The number of AmpHours expected to be taken from the battery between charges
. The maximum current expected to be taken from the battery at any one time
. The current rating of the charging source
. The length of time it is desired to run the generator or motor for each charge, if necessary.

If the battery is too small for the task:
. The time between recharges will be irritatingly short. This is a particular problem when a boat
that is normally charged with shore power is taken away for a few days.
. It will not accept a high charging current ie. The battery charger Amp rating should not be more
than about one third of the battery capacity.
. They are likely to be quite deeply discharged, which will significantly reduce their service life
. They may be damaged if the load in Amps exceeds about 25% of the AH capacity for an extended time. (such as a large inverter running at high power)
Generally the house bank should have a capacity of at least four times the expected usage between charges - bigger is always better from an electrical point of view. The disadvantages of a large battery bank are cost, weight and space and maintenance.

12 volts vs 240 volts
Traditionally boats have used 12V power systems, with any 240V requirements being met by use of a generator. Modern inverters have evolved to the point where it is realistic to use 12V or 240V appliances, or any mixture of the two.
All of the instruments and communications equipment on a boat will be 12V, but high power devices such as watermakers, microwaves or TVs can be either. High power devices running on 12V draw very large currents and consequently must be connected with heavy wiring. 240V appliances are relatively cheap and convenient.
With an appropriately sized inverter and battery the boat can be set up to provide all of the 12V and 240V requirements from the main battery. Any of the range of charging devices can then be used to keep the battery charged. The important thing is to ensure that the system is balanced; there is sufficient power coming in to replace all of the power going out.

Inverters
Inverters are electronic devices that convert DC power from a battery into 240V AC power that will operate domestic appliances. They are available in two types: Modified sinewave or pure sinewave.

Modified sinewave inverters are much cheaper but tend to produce a lot of high frequency noise that can interfere with radio signals, their output is not smooth and may not operate some equipment properly, particularly computers. They are useful for applications where power is only required infrequently to drive specific types of loads, for example a mobile phone charger. True Sinewave inverters produce an output that exactly matches the normal AC mains power.

Even though they cost considerably more, they are the only type to consider for a marine power system. They will operate any equipment that draws less than their rated output and will cause minimal interference.
The size of the inverter is determined by the maximum expected AC load. Some examples of AC loads are:
Toaster ~ 750 Watts
TV ~ a few hundred Watts
Kettle ~ 1800 - 2400 Watts (occasionally 1200 Watts)
Microwave ~ 1400 Watts
Hair drier ~ 2400 Watts
Small marine hot water service 750 Watts (but you need to run it for at least an hour to heat up)
Air conditioning 4000 Watts or more.
(However, by careful investigation, all of the above except air conditioing can be obtained with less than 1000 watt usage.)

As a guide the maximum power for any device that can be plugged into a domestic socket is 2400 Watts.
Any equipment with a large electric motor requires much more power to start it than it needs to run, hence a 10KWatt generator might be installed to operate a 4 KW air conditioner. This can be overcome by using an inverter capable of assisting a smaller generator to supply brief high loads.

Inverters will normally be between 1600 and 3000 Watts. These will draw between 140 and 250 Amps from a 12V battery at full load! It is possible to connect two or more inverters together to supply more power.

Many modern inverters can act as a two way device, producing 240V when only 12V is available and charging the 12V battery when 240V is available. Their control systems are also able to automatically coordinate all of the power requirements of the boat including:
starting and stopping a generator,
temporarily reducing battery charger current to make more power available for other devices disabling some devices such as air conditioners or electric hot water services when only 12V power is available.

It is not difficult to design a power system around an inverter that will provide continuously available AC power with minimal generator running time and fully automatic control.

When setting up a power system for a live aboard cruising boat it is well worthwhile to get some expert assistance at the design stage, but you will still need to have a grasp of the above information to end up with a boat that functions well from a power perspective.

Next subject: Water

Sail-World Cruising would like to thank : Multihull Solutionsfor the use of Peter Salisbury's article. For all your multihull needs, click herefor the best solutions.

by Peter Salisbury/Sail-World Cruising

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