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While
most boats on the water today are powered by diesel engines, and
sail power and gasoline engines are also popular, it is
perfectly feasible to power boats by electricity too.
Electric
boats were very popular from the 1880s until the 1920s, when
the internal combustion engine took dominance. Since the energy
crises of the 1970s, interest in this silent, non-polluting and
potentially energy source has been increasing steadily
again. With the present state of technology so developed, many
believe that the time is right for electric boats to become
popular again.
Components
The
main components of the drive system of any electrically powered
boat are similar in all cases, and similar to the options
available for any electric vehicle.
Charger
Electric
energy will have to be obtained for the battery bank from some
source.
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Mains
charger allows the boat to be charged from a shore-side
power point when one is available. The use of this may call
into question claims that the electric boat is
'non-polluting' and uses 'renewable energy', but only among
the more pedantic purists. The boat is not directly
polluting the water in which it is sitting, as are most
diesel- and gasoline-powered boats. The power stations that
generate the electricity are subject to strict environmental
controls, and an increasing number of these in the near
future will be using renewable and non-polluting energy
sources themselves, see Kyoto
Protocol.
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Solar
panels can be built into the boat in reasonable
areas in the deck, cabin roof or as awnings. Modern solar
panels, or photovoltaic arrays, can be flexible enough to
fit to slightly curved surfaces and can be ordered in
unusual shapes and sizes. It is still true that the heavier,
rigid mono-crystaline types are more efficient in terms of
energy output per square meter. The efficiency of solar
panels rapidly decreases when they are not pointed directly
at the sun, so some way of tilting the arrays while under
way is very advantageous.
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Towed
generators are common on long-distance cruising yachts
and can generate a lot of power when travelling under sail.
If an electric boat is to have sails as well, and will be
used in deep water (deeper than about 15 m or 50 ft), then a
towed generator will help build up battery charge while
sailing. There is no point in trailing such a generator
while under electric propulsion - we are not trying to
create a perpetual motion machine! The extra drag from the
generator will waste more electricity than it generates.
Some electric power systems use the free-wheeling drive
propeller to generate charge through the drive motor when
sailing, but this system, including the design of the
propeller and any gearing, cannot be optimised for both
functions. It may be better locked off or feathered while
the towed generator's more efficient turbine gathers energy.
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Wind
turbines are also common on cruising yachts and can
be very well suited to electric boats. There are safety
considerations regarding the spinning blades, especially in
a strong wind. It is important that the boat is big enough
that the turbine can be mounted out of the way of all
passengers and crew under all circumstances, including when
alongside and when coming alongside a dock, a bank or a
pier. It is also important that the boat is big enough and
stable enough that the top hamper created by the
turbine on its pole or mast does not compromise its
stability in a strong wind or gale.
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If
the boat is to have an internal combustion engine
anyway, then its alternator will of course provide
significant charge when it is running. This does rather
defeat the original purpose, however. The weight saving that
we would expect by not having this engine and all its
associated tanks, pipework and other fittings would help to
add to the efficiency that electric propulsion needs.
In
all cases, a charge regulator will be needed. This is to
ensure that the batteries are charged at the maximum rate that
they safely can stand when the power is available. It must also
ensure that they are not overcharged when nearing full charge
and not overheated when they are discharged and a great deal of
charge current becomes available.
Battery
bank
There
have been significant technical advantages in battery technology
in recent years, and more is to be expected in the future.
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Lead-acid
batteries are the most viable option at ther moment.
Deep-cycle, 'traction' batteries are the obvious choice.
There is no denying that they are heavy and bulky, but not
much more so than the diesel engine, tanks and fittings that
they may be replacing. They need to be securely mounted, low
down and centrally situated in the boat. It is essential
that they cannot move around under any
circumstances. Care must be taken that there is no risk of
spilled, strong acid in the event of a capsize as this could
be dangerous or even fatal. At the same time, venting of
explosive hydrogen and oxygen gases is also necessary.
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Nickel
metal hydride and other hi-tech, solid-state
batteries are becoming available, but are still expensive.
These are the kind of batteries currently common in
rechargeable hand tools like drills and screwdrivers, but
they are relatively new to this environment. They require
specialised charge controllers.
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Fuel
cells are going to provide significant advantages in
the years to come, and one day heavy lead-acid batteries
will seem 'pre-historic' by comparison. Today (2005) however
they are very expensive and require specialist equipment and
knowledge, making them all but impractical for any but their
dedicated enthusiasts.
The
size of the battery bank will determine the range of the
boat under electric power alone. The speed that the boat is
motored at will also affect this - a lower speed can make a big
difference to the energy required to move a hull. Other factors
that affect range include sea-state, windage and any charge that
can be reclaimed while under way, for example by solar panels in
full sun. A wind tubine in a good following wind will help, and
motor-sailing in any wind could do so even more.
Speed
contoller
To
make the boat usable and manoeverable, a simple-to-operate
forward/stop/backwards speed controller is needed. This must be
efficient - i.e. it must not get hot and waste energy at any
speed - and it must be able to stand the full current that could
conceivably flow under any full-load condition.
Electric
motor technology is also complex and changing. Permanent magnet,
brushless motors are considered very suitable by some
specialists.
Drive
chain
Depending
on the size of the boat and the choice of electric motor, a
standard propeller shaft, bearings and propellor may be
available. In some cases some reduction gearing may be required,
but from the point of view of efficiency, wear-and-tear and
routine maintenance this should be avoided if at all possible,
perhaps by choosing a different propeller.
Types
There
are as many types of electric boat as there are boats with any
other method of propulsion, but some types are significant for
various reasons.
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River
and lake boats. Electric boats, with their historically
limited range and poor performance against a strong headwind
or current, have tended to be used on inland waterways. In
this environment, their complete lack of local pollution is
also a significant advantage.
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Electric
outboards and trolling motors have been available for
some years at prices from about $100 (US) up to several
thousand. These still require external batteries in the
bottom of the boat, but by being a one-piece item apart from
that, the manufacturers have been able to optimise the
combination of the motor, the speed controller and the drive
chain including the all-important propeller. Electric
outboards have for some years provided an ideal drive
systrem for inland waterway fishermen, being quiet and
pollution-free so as not to scare away or harm the fish and
other wildlife. As technologies improve, they should bring
these benefits to many other dinghy users too, such as for
yacht tenders and other inshore pleasure boats.
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Cruising
yachts usually have an auxillary enine, and there are
two main uses for this engine. One is to power ahead or
motor-sail at sea when the wind is light or from the wrong
direction. The other is to provide the last 10 minutes or so
of propulsion when the boat is in port and needs to be
manoevered into a tight berth in a crowded and confined
marina or harbour. Electric propulsion is ideally suited to
the second case. The first case provides many - especially
beginning yachtsmen and women - with a cause for anxiety, as
a powerful diesel auxillary has often been known to get a
boat into harbour when sail power alone, or sail with
limited help from an electric system, would miss the tide,
the hours of daylight, or a rising gale. In fact with good
seamanship, good passage-planning, sailing skills, the
ability and willingness to keep to the deep sea or to anchor
off in unfavourable conditions, none of this need be a
problem, except perhaps for those in a hurry to get home,
maybe for work in the morning or to feed the cats.
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Diesel-electric.
There is a third potential use for the trusty diesel
auxillary and that is to charge the batteries, when they
suddenly start to wane far from shore in the middle of the
night, or at anchor after some days of living aboard. In
this case, where this kind of use is to be expected, perhaps
on a larger cruising yacht, then a combined diesel-electric
solution may be designed from the start. The diesel engine
is installed with the prime purpose of charging the battery
banks, and the electric motor with that of propulsion. There
is some reduction in efficiency if motoring for long
distances as the diesel's power is converted first to
electricity and then to motion, but there is a balancing
saving every time the wind-, sail- and solar-charged
batteries are used for manoevering and for short journeys
without starting the diesel. There is the flexibility of
being able to start the diesel as a pure generator whenever
required. The main losses are in weight and installation
cost, but on the bigger cruising boats that sit at anchor
running large diesels for hours every day, these may not be
too big an issue, compared to the savings that can be made
at other times.
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