This post presents an interesting concept illustrated in diesel locomotive
drive train description.
There multiple methods of converting and transmitting energy but they all
have one thing in common you never get out as much energy as you put in.
The difference between energy in and energy our is energy converted to some
other form that is not useable by the system (such as heat from friction).
System efficiency is a mathematical description of the amount of lost energy
compared to input energy.
The object of any sailboat drive is to move it through the water without the
use of the sails and wind. The typical drive system converts chemical
energy (in the form of fuel) into rotary motion to spin the propeller. The
most efficient drive system is a direct drive (such as directly attaching
the propeller to the rotary motive device's output shaft). Unfortunately,
most motive devices such as diesel engines do not produce an output a form
of energy that is directly useable by a propeller and also they do not
operate well when submerged. Hence the need/existence of some type of
transmission and shafting which introduce drive system losses. A gear box
type transmission has the least amount of losses. a hydraulic transmission
(either in a single case [like found in modern automobiles] or separate
pump/motor drive) have greater losses. The electric drive systems have more
losses that a gear box but depending upon design can be more or less
efficient that a hydraulic drive.
Electric drive losses come from wire line losses expressed as voltage drop,
battery losses in both charging and discharging, electric energy generation
be it through a solar cell or engine, wind, water driven generator, motor
losses converting all that electricity into rotary motion to spin the
propeller, and depending upon the size and style of electric motor used,
shafting and associated bearing, coupling and seal losses.
My bottom line is that there are many ways to move the boat but none are
perfect with no losses. The final design for each individual is a
compromise based upon how much the owner/builder is willing to spend and how
much energy loss "he" is willing to accept. Done right each design has its
good points and its bad points but none give a free ride.
Two pluses in favor of an electric drive are; 1. the electric power can be
derived from both combustion (such as a diesel gen-set) and non-combustion
(such as solar panels or wind mills) sources. 2. The stored electrical
energy can be used for "hotel" related boat uses freeing up space and the
cost of a second generator set.
"Ted" tedwilliams@nospam wrote in message
...
On Fri, 03 Jun 2005 14:33:22 -0300, Terry Spragg
wrote:
Mic wrote:
Perpetuated Motion
Electric propulsion for boats: A century-old technology may just be
the wave of the future
LINK:
http://www.cruisingworld.com/article...ID=396&catID=0
" Advocates for diesel-electric propulsion list among its virtues that
it's clean, quiet, efficient, and requires very little maintenance.
Another advantage frequently noted in connection with the STI system
is the ability to make electricity--to "regenerate"--when the boat is
under sail."
"While internal-combustion engines are typically described by their
horsepower rating, STI's motors are named for the torque they develop.
An ST 37 puts out 37 foot-pounds of torque or 6 horsepower; Tether
recommends using it on monohulls up to 32 feet and 10 tons or to
replace diesel engines of up to 24 horsepower. An ST 74 puts out 74
foot-pounds of torque or 12 horsepower; Tether recommends using it on
monohulls up to 50 feet and 16 tons or to replace diesel engines of up
to 48 horsepower."
The statement above "STI's motors are named for the torque they develop"
is
absolutely meaningless without rotational speed. Torque is force at zero
speed and is measured in foot pounds or inch pounds. Torgue is simply the
force exerted in pounds at a given distance from the center of the motor
shaft. For example: 12 lbs-in of torque means that imagining a lever arm
connected to the shaft, at 1 inch up on the lever shaft you would measure
12
lbs of force. At 12 inches up, you would measure 1 lb of force. No
rotational speed is considered.
To propel a boat, the propellor has to turn. A million lbs-ft with no
propellor speed won't move a toy boat.
I could take a toy motor that produces .001 lbs-ft of torque and connect
it
through a reducer of say 10,000:1. At the output shaft of the reducer
(assuming no losses) I would measure 10 lbs-ft of torque. If my toy motor
top speed was 1,000 rpm, the rpm at the output shaft of the reducer would
be
0.1 rpm. So, bearing that in mind, I could tell people that my electric
motor and gear combination that runs on a common D cell can produce 10
lbs-ft of torque and I wouldn't be lying.
In essence, torque produced by a motor without factoring in speed and time
is meaningless. It cannot be equated to horsepower. The following formuala
is used to convert a known (speed in rpm) and (torque in lbs-ft) into HP.
HP = (rpm x T(torque))/(( 5252(constant))
For the example of my toy motor and gear combination I can easily
calculate
the HP.
HP = (.1x10)/5252
HP = 0.000190403655750
Not much HP is it? But I get 10 lbs-ft of torque!
Thrust is similar. Assume no boat or other losses have an effect on the
motor/prop combination.
.
For an example we'll use a trolling motor rated at 40 lbs of thrust at .25
mph at the motor's maximum speed.
Changing the prop to get 1 mph for the motor maximum speed, the thrust
would
be 10 pounds.
Change the prop again to get 2 mph at the motor maximum speed would result
in 5 pounds of thrust.
Once again, thrust like torque must be related to speed to be meaningfull.
I am well aware of electric motors being used on submarines and such. I
have
worked on a submarine motor that was salvaged and is in use today on a
cold
rolling mill. The motor is rated at 2500 hp at 600 volts at 2650 amps.
That
would take a lot of battery power to operate it in a submarine.
The diesal powered locomotives you see are not driven directly by the
engine
as in an automobile. There is no gear train to the wheels. The diesal
engine
turns a generator. The diesal engine is known as the Prime Mover and runs
at
a constant speed. The field of the generator is electrically excited
producing flux in the generator fields. That in turn produces volts at the
output of the generature. The generator is wired to electric traction
motors
that turn the wheels. To change the speed, the amount of generator field
excitation is varied and thus the generator output volts going to the
traction motors.
There is a lot more to motors, thrust, torque, hp, and batteries than I
can
possibly describe here.
I just wanted to throw this in so nobody gets confused and thinks they are
getting something for nothing.
Oh, and by the way, if you had an electric trolling motor on your
sailboat,
and the you were sailing faster than the trolling motor's theoretical
maximum speed with the given prop, the motor would turn faster and
regenerate putting power back into the battery. Of course you are
converting
the energy produced by the wind in the sails and thus motion into
electrical
energy so unfortunately no perpetual motion. grin
Ted