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Efficiencies of a Marine Electric Drive
A lot of interest has been stirred up with powering a boat with an electric
motor in the last couple of years. Why would a boat owner choose to have an
electric boat? The number one reason is the decrease in pollution for the
environment, both globally and locally. Exhaust fumes, the smell of fuel, noise
and vibration pollution are all decreased or eliminated by this technology. The
consolidation of house and propulsion power on diesel electric boats results in
lower fuel consumption, less maintenance, an ability to choose when to run the
diesel, and overall a more pleasant experience for the boat owner.
The efficiencies of each component of an electric drive system are multiplied
together to get the overall efficiency of the entire system.
Component Low Efficiency High Efficiency
Battery
.8 - .95
Cables
.95 - .98
Controller
.75 - .95
Motor
.80 - .93
Gear Reducer .8 -
.97
Thrust Bearing .9 -
.98
Packing Gland .9 -
.99
Cutlass Bearing .98 - .99
Propeller
.5 - .8
Multiply all the low efficiencies together and you get 14.5%
Multiply all the high efficiencies together and you get 61%
The highest efficiency drive will therefore have a range of over 4 time that of
the lowest efficiency drive. Efficiency matters!
Since efficiency directly affects range and power, it is very important to
optimize every individual component of the system. In addition to the system
efficiencies it is also important to maintain a watchful eye on hull efficiency.
Large, protruding through-hulls create substantial drag. Pitted bottoms should
be faired smooth. Anything that makes the hull have less drag is of particular
interest to the electric sailor looking for increased range and speed. We just
need to talk to the racers at the club to understand the importance of hull
resistance!
BATTERIES
All batteries require care in mounting and wiring. The boat should be able to
withstand a complete knockdown or rollover without the battery moving. The worst
thing that could happen to a boat is an 8D battery coming loose and flying
across the cabin! Batteries should be fused at the battery and switched near the
battery. All batteries should be charged individually with a shorepower charger.
This allows for balancing of the string. You can still charge the entire string
with solar or a genset, but a regular balancing charge will add to the life of
the battery. Make sure the connection to the battery is tight and covered with a
light grease, Vaseline works well. Many battery clamp connectors do not hold up
over time. Use only the best tinned bronze clamp connectors. Pass the clamping
bolt through your battery lug for the best connection. Don’t use the stud on the
top of the connector. Never use a nylock connector on a battery connection, heat
can build up and they can catch fire.
Flooded Lead Acid Batteries are the least efficient and become less and less
efficient with age. I do not like flooded lead acid batteries and refuse to sell
them. As a marine electrician I sometimes refuse to work on a boat with flooded
lead acid batteries for liability reasons. As the batteries age the plates start
to sulfate, and the electrolyte begins to bubble emitting both hydrogen and
oxygen gases, a very explosive mixture. The gases settle on nearby metal and
start corroding. Flooded lead acid battery terminals are notorious for buildup
of acidic deposits that we used to use baking soda to dissolve. The problem with
flooded batteries on boats is the high currents for long periods of time. A
loose connection can cause heat to build up and the battery can explode. I know
because it happened to me, blowing the hole top of the battery off. The bottom
line – stay away from flooded batteries on your boat!
AGM batteries are a good choice for an electric boat. If completely discharged
they are capable of being revived. Taken care of with a good charger they will
typically last 5 to 7 years. In most cases a flooded battery charger will work
well with an AGM battery but it is good practice to observe charging voltages. A
charger with temperature compensation is necessary for accurate charging.
Typically an AGM battery is good for 400 cycles at 50% rate of discharge.
Lifeline batteries are renowned for being the best AGM battery with many
batteries lasting in excess of 8 years.
Due to their weight take extra care as to their placement on the boat. Check the
waterline to make sure the boat is not listing or the bow is too low for the
decks to drain.
2nd Generation AGM batteries are a good choice for an electric boat. Make sure
the charger is powerful enough for the battery. Trickle charging an AGM-2
battery will decrease the life of the battery. Odyssey batteries are known for
their ability to take a very fast charge and discharge, but be careful doing a
complete discharge, you will probably ruin the battery. Odyssey batteries are
rated at 400 cycles down to 80% depth of discharge, effectively doubling the
range of a standard AGM battery. I prefer Odyssey batteries for any diesel
electric hybrid sailboat.
Lithium Iron Phosphate batteries are the lightest and most powerful battery
commercially available today but hold onto your wallet when you get the bill.
Most of us do not need the 2700 charge / discharge cycles down to 80% depth of
discharge, but if that is what you are looking for there is no substitute.
Lithium batteries require a battery management system which balances each cell.
Some BMS’s can get quite complicated and costly. The technology is rapidly
improving and cost is coming down. If you have time to wait, or can afford them
now, this is definitely the battery to go with. Monohulls can handle the extra
weight of a lead acid battery, but if you are equipping a multihull, Lithium is
the way to go.
CABLES
Choose the biggest cables you can afford for the least voltage drop and highest
efficiency. We typically use a 2/0 cable for DC loads of 100 amps. Kapowai has
two 48 volt strings so each string is providing 50 amps and we still use a 2/0
cable. Smaller cable has higher resistance and heats up wasting energy. You
won’t notice the heat by touching it but line losses are a very real phenomenon.
CONTROLLERS
The simplest controller was a bypass resistor in series or parallel with the
motor. The resistor heated up and diverted the power away from the motor. These
old controllers were ridiculously inefficient! Luckily advancements in
electronics have trickled down to the controllers and the new controllers are
remarkably efficient. But take advantage of the new technology. Many of the
controllers on the market today are still using 10 year old technology. A lot
has changed in the last 10 years, and even the last 2 years.
Find out what the controller consumes in the idle state. This will give a good
indication of the efficiency of the controller in the lower power bands. The
magic of an electric boat is in providing small amounts of power to the
propeller. Firstly the propeller drag is eliminated which adds a knot to boat
speed and effectively gives you less resistance than a feathering propeller. The
next 100 to 200 watts of energy can make apparent wind and have you sailing when
your friends boat is luffing. An electric motor is like having another sail and
light air is where it shines, providing the controller is efficient at all power
levels, not just full throttle!
The latest controllers like Sevcon’s Gen 4, is can bus compliant. This means
that you can tap into the controller with a computer and change settings on the
fly. This controller also has a remote screen that monitors all aspects of the
electric motor and controller including voltage, ac and dc current, battery
state of charge, torque, rpm, motor and controller temperature, and some
programmable functions. The screen eliminates the need for a separate shunt for
a multimeter making the installation simpler.
The Gen 4 controller is a variable frequency, variable voltage, variable
current, DC to 3 phase AC inverter.
MOTORS
There are many different types of electric motors that have been used for boat
propulsion. The most common are brushed DC motors, AC induction motors or
permanent magnet motors.
Permanent magnet motor have the highest efficiency. A permanent magnet motor
relies on the magnetic field of rare earth magnets on the rotor and does not
require energy to create an electromagnetic field which results in higher
efficiency.
Brushes require maintenance and create electromagnetic interference, both of
which we are trying to avoid on a boat. Brushes are used on DC motors as a
mechanical means of converting Direct Current to Alternating Current. Today’s
high efficiency controllers do this electronically.
The highest efficiency, lowest maintenance, lowest EMI motor available today is
the brushless permanent magnet AC motor.
GEAR REDUCERS
There are 3 different types of gear reductions available for boat propulsion:
gearbox, belt or chain drive.
Both belts and chains require maintenance and an enclosure for safety. You do
not want to get your clothing caught in a belt or chain drive while the motor is
running. If you did choose a belt of chain drive, the enclosure should be
transparent so that you can visually inspect the belt or chain. Belts have the
advantage of being very quiet. Typically a belt or chain drive may be 90 %
efficient though some of the newest belt drives boast better efficiencies. You
will still require a thrust bearing with the belt setup.
We advocate the use of a gearbox. The gears are 97% efficient and the seals are
99% efficient for an overall efficiency of 96%. Gearboxes have an integral
thrust bearing which makes for a very elegant installation. Gearboxes function
as a heat sink for the motor allowing the motor to operate at higher continuous
power outputs. There is some noise associated with a gearbox but we have found
that it is minimal when using a large, slow turning propeller and high torque
motor. Multiple gear ratios allow precise tuning of the motor with the propeller
creating substantial efficiency gains. The efficiency gains associated with
tuning the system more than offset the losses associated with the gears and
seals.
THRUST BEARINGS
The thrust bearing allows the conversion of rotation force into a push or pull
force. We have not yet quantified the efficiency of the thrust bearing. Thrust
bearings located in a gearbox are lubricated with gear oil which is more
efficient than grease typically used on stand alone thrust bearings.
PACKING GLANDS
Dripless packing glands have a carbon disc that rides on a ultra thin layer of
water on a stainless disc and has almost no resistance at all.
Flax packing glands compact flax around the shaft until a drop of water appears
every 60 or 90 seconds. Flax packing glands have considerably more resistance
than a dripless packing gland.
CUTLASS BEARINGS
Although it is hard to quantify the efficiency of the cutlass bearing, it should
be noted as part of the system. Check the cutlass bearing for excessive wear and
replace if necessary. A wobbly shaft will not permit the propeller to operate
efficiently.
PROPELLERS
At 50 to 80 percent efficient, the propeller is one area that has the most to
gain. Todays propeller technology is far better than even 10 years ago. Computer
numerically controlled (CNC) manufacturing processes coupled with advancements
in design contribute to far greater efficiencies. Large slower turning
propellers with bigger pitch are far better suited for the torque of an electric
system, plus contribute to the overall whisper quiet operation of an electric
drive.
TUNING THE SYSTEM
Another area of efficiency gain is in matching the motor to the propeller.
Though direct drive systems are very quiet, there will be only one size of
propeller that will be right for the motor. Choosing the correct gear ratio
allows for the motor to be running at the optimal rpm, voltage and current. Each
completed installation contributes to a database of information that we draw
upon for every new boat that we re-power. However, we consider ourselves lucky
if we get it right the first time so we have implemented a gearbox exchange
program to allow gear optimization.
What we are looking for is a balancing of DC current with AC current. After
installing a Mars motor with 1.8: 1 gear ratio on a 12” diameter x 15 inch pitch
propeller on our Catalina 30 we started motoring around only to find that at 100
amps DC the motor was overheating. Why? We then measured the AC current and
found it was 160 amps. The motor was not turning fast enough and was drawing
excessive amounts of current to make up for not enough voltage. Changing the
gear ratio to 2.2 : 1 we found that the motor was drawing 130 amps AC for 100
amps DC. We are now installing a 2.8: 1 gearbox and eagerly await the results.
The correct gear ratio also has an effect on the total continuous power
developed by an air cooled motor. If the motor does not spin fast enough the fan
does not cool the windings and the motor will overheat. The amount of power that
a motor develops is directly proportional to the speed that the motor turns at.
If you can double the r.p.m., you double the power. Another way to look at this
is a motor can be half the size if it turns twice as fast. Consequently, gear
mounted motors are a fraction of the price of direct drive motors.
The only disadvantage of mounting a motor to a gearbox is the noise of the
gearbox. This can be augmented by using a larger, slower turning propeller. The
bulk of the noise created by the gearbox is made on the output shaft of the
gearbox. This is because the gearbox acts as a speaker for a fast turning
propeller and driveshaft. The noise can further be decreased by using a motor
with more turns on the windings. By using a longer winding the motor creates
more torque at a lower rpm allowing the input side of the gearbox to run at a
decreased speed. However more turns on the motor requires a longer winding of a
smaller diameter of wire which has more resistance and thus creates more heat.
The offset is the motor is running at a higher voltage for the same rpm. The net
effect can be less power made by the motor. There is a sweet spot to choosing
the right number of windings for power, torque, resistance, and efficiency and
that has to be balanced with noise created by the gearbox. We have found that
the noise created by the gearbox is not noticeable under 2,000 motor r.p.m. and
1,000 propeller r.p.m. We tune our systems to run at 1,800 motor r.p.m. and 800
propeller r.p.m which makes them whisper quiet.
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