STEAM AND ELECTRIC PROPULSION
BOATS are propelled by two different systems. Some inland-water boats
still employ side paddle-wheels, while ocean-going vessels use the more
modern propeller or screw.
The paddle-wheel really acts as a continuous oar. Such a wheel is shown
in Fig. 33. As the wheel goes around the paddle dips into the water and
pushes the boat forward. If the direction of the boat is to be reversed,
the rotation of the paddle-wheels is reversed.
[Illustration: FIG. 33]
[Illustration: FIG. 34]
[Illustration: FIG. 35]
[Illustration: FIG. 36]
[Illustration: FIG. 37]
[Illustration: FIG. 38]
Before passing onto the screw, it may be well to explain just how a
paddle-wheel causes a boat to move. When a man gets into a rowboat, he
generally pushes himself off by placing his oar against the dock or
shore and pushing on it. That is just what the paddle does in the water.
It dips into the water and pushes against it. It must be remembered,
however, that water is unlike a solid substance and it "gives." When a
man places his oar against the bank and pushes it, the bank does not
move, and all of the man's energy is used in starting the boat. Water,
however, does not remain stationary when the paddles push against it,
and therefore all of the power it not utilized in moving the boat--part
is used in moving the water.
The paddle-wheel is not so efficient in moving a boat as the more modern
propeller--or screw, as it is more often called. The screw receives its
name from the ordinary metal screw, because its theory of operation is
exactly the same. A wood screw, when turned, forces itself into wood. A
propeller, when turned, forces itself (and thereby the boat) through the
water. A small propeller is illustrated in Fig. 34. This is an ordinary
three-blade propeller. (The writer prefers the word propeller instead of
screw.)
From the drawing, it will be seen that the propeller-blades are mounted
at an angle. This angle of the blades causes them to force water back as
they cut through it when the propeller is revolving. This forcing of the
water back tends to produce a forward motion of the propeller, and in
this way the boat on which the propeller is mounted moves through the
water. The propeller is caused to revolve by a steam-engine,
steam-turbine, or gasolene-engine, as shown in Fig. 35. Longer boats
have more than one propeller. A boat that has two propellers is called a
twin-screw boat. A boat driven with four propellers is called a
quadruple-screw boat.
When a machine screw is turned around just once, it moves forward a
certain distance, as a glance at Fig. 36 will show. The distance the
screw moves forward will depend entirely upon the distance between the
threads. The distance between the threads is called the pitch of the
thread. If the threads are 1/32 inch apart, then the screw will move
1/32 inch every time it revolves.
If a propeller acts in the same way as a screw, then it too must have a
pitch. The pitch, or the distance that a propeller will advance in one
revolution, is measured in inches. A propeller with a pitch of ten
inches should move ten inches through the water at each revolution.
However, there is a certain amount of "slip," and a propeller does not
actually advance the distance that it should theoretically. The pitch of
a propeller is really the distance it would advance in one revolution
if it were revolving in an unyielding or solid substance.
To make a simple propeller, first cut out of thin sheet brass three
blades as shown at _A_, Fig. 37. Sheet brass with a thickness of 1/32
inch is very suitable for this purpose. Next, a block, as shown at _B_,
is carefully carved out so that the propeller can be hammered down into
the depression. The same block is used for the three blades, so that
each will have the same curvature. The block should be cut from oak,
since this wood will not split or lose its shape when the forming is
done.
The hub is made next. This is shown at _C_, Fig. 37. The hub, of brass,
is made according to the stream-line method. It is filed to shape from a
piece of round brass stock. A hole runs lengthwise in the brass, as
shown, and a set-screw is used to hold the hub of the propeller-shaft.
The method of cutting the slots in the hub is shown at _D_, Fig. 37. The
hub is clamped between two boards placed in the vise, and a hacksaw is
used to cut a slot in the hub. The hub is then turned around one third
of a revolution, and another slot cut, using the same saw-marks in the
boards, so that the angle of the second slot will be the same as the
first one. The third slot is cut in the same manner. The three blades
that were cut out are now fastened in these slots and held there by
solder. This completes the propeller and it is now ready to be fastened
upon the propeller-shaft.
Let us consider the general method of putting the propeller-shaft in
place. The young boat-builder will readily understand that it would be
very impractical merely to bore a hole in the hull of the boat to put
the propeller-shaft through. In this way water would surely leak into
the hull and the boat would sink in a short time. Some method must be
evolved to keep the water out of the hull, and yet allow the
propeller-shaft to revolve freely.
The propeller-shaft is arranged within a brass tube, as shown at Fig.
38. The brass tube should be about 1/8 inch larger in diameter than the
propeller-shaft. A little brass bushing must also be arranged at each
end, as shown. When the propeller-shaft is mounted in place in the
tube, there will be a space between it and the tube. Before the
propeller-shaft is put in place it is well smeared with vaseline, and
when it is placed in the tube the space between the shaft and the tube
will be completely filled with it. This will prevent water from
entering. Owing to the fact that vaseline is a soft, greasy substance,
it will not prevent the rotation of the propeller-shaft. The brass tube
is placed through a hole bored in the hull of the boat. The hole should
be a trifle smaller than the diameter of the brass tube, so that the
tube can be forced into the hole.
[Illustration: FIG. 39]
[Illustration: FIG. 40]
[Illustration: FIG. 42]
One of the simplest methods of propelling a boat is by means of rubber
bands. Such a boat is shown in Fig. 39. This is a small wooden hull
fitted with a two-blade propeller. The propeller is shown at Fig. 40. It
is cut in a single piece and held to the propeller-shaft merely by a
drop of solder since there will not be much strain upon it owing to the
low power of the rubber-band motor. The opposite end of the
propeller-shaft is bent into a hook, and the rubber bands run from this
to another hook placed at the bow of the boat. The rubber bands may be
similar to those employed by model airplane builders. The motor, of
course, must be wound up by turning the propeller around until the bands
become twisted into little knots, as shown at Fig. 39. Boats driven by
rubber bands cannot be very large unless a great number of rubber bands
are used. Even then the power is short-lived. However, building a few
small boats driven by rubber-band motors will do much to teach the
young boat-builder some valuable lessons in boat construction.
Probably the best method of propelling model boats is the electric
method. By building a boat large enough to accommodate two dry batteries
or a small storage battery and a little power motor, a very reliable
method of propulsion is made possible. The boat must have sufficient
displacement to accommodate the weight of the dry-cells and storage
battery. A boat two feet long, with a beam of 4-1/2 inches, is large
enough to accommodate one dry-cell and a small motor, providing the
fittings of the boat are not too heavy.
A suitable power motor for small boats, which will run with either one
or two dry-cells, is shown in Fig. 41. The connections for the motor are
given clearly in Fig. 42, and a suitable switch to control the motor is
shown at Fig. 43.
Owing to its greater power, the storage battery is to be preferred.
Dry-cells are extremely heavy and occupy considerable space. They are
also costly, since they do not last long and cannot be worked too hard
unless they polarize.
[Illustration: FIG. 41]
[Illustration: FIG. 44]
[Illustration: FIG. 43]
[Illustration: FIG. 45]
A very suitable method of mounting an electric motor is illustrated in
Figs. 44 and 45. It will be noticed that the motor is inverted. A small
pinion or gear is mounted upon the armature-shaft of the motor. A larger
gear (about three times the diameter of the small one) is placed upon
the propeller-shaft. This gives a speed reduction of three to one. It
will be seen that the propeller-tube is strapped within a strip of brass
to a small cross-piece nailed to the bottom board of the hull. The hull
is of the built-up type, and the other three boards that go to make it
up are not shown. When the three boards are glued in place, a brass
strip is run across the top board and the base of the motor is screwed
to this. This holds the motor rigidly in place so that it will not move
when the power is turned on. The brass strip used should have sufficient
thickness to hold the motor rigid. It will also be seen that the motor
is tipped slightly so that it will come in line with the
propeller-shaft.
[Illustration: FIG. 46]
[Illustration: FIG. 47]
[Illustration: FIG. 48]
It is not always possible to obtain small gears. For this reason the
model boat builder may find it necessary to use a different method of
fastening the propeller-shaft to the motor. A very good method of doing
this is shown in Fig. 46. Here a coiled wire spring is used. This is
wound to shape on a rod, and a drop of solder holds it to the propeller
and motor shafts. In the method of propulsion shown in Fig. 44 the
armature-shaft of the motor must be perfectly in line with the
propeller-shaft, or the gears will bind and unsatisfactory operation of
the motor will result. With the little spring the motor will not have to
be mounted exactly in line with the shaft, and it will also be possible
to mount the motor standing up. Of course, if the motor is mounted in
this way it will be necessary to make the propeller-shaft longer, as is
shown in Fig. 47.
Still another method of driving the propeller is illustrated in Fig. 48.
This method is so simple that the author feels explanation to be
unnecessary.
Clockwork can often be employed for propulsion purposes, but this method
is not very satisfactory. It is also very difficult to obtain suitable
clockworks to install in a boat. Oftentimes it will be possible to
salvage the works of an old alarm-clock, providing the main-spring is
intact. It is a very easy matter to mount the clock-spring and connect
it to the propeller. Any one of the aforementioned methods can be
employed.
Steam propulsion has its advantages; but, on the other hand, the writer
is not inclined to recommend it as strongly as the electric method for
reliability. Of course, steam is a more powerful agency in the
propulsion of small boats and thereby greater speed is attainable by its
use.
[Illustration: FIG. 49]
[Illustration: FIG. 50]
[Illustration: FIG. 51]
Here is a very simple small power plant suitable for driving boats up to
3-1/2 feet in length. The boiler is shown in Figs. 49 and 50. The method
of assembling the boiler is pictured clearly in Fig. 49. A brass or
copper tube about 2-1/2 inches in diameter is used. Two end pieces are
cut to shape and forced into the boiler ends. A hole is drilled in the
center of these pieces before they are put in place. After the end
pieces are forced in place solder is carefully flowed around their
edges. The brass rod is then threaded at each end and placed
concentrically within the boiler, as shown in Fig. 49. A nut is placed
on each end of this rod and tightened. The nut is then soldered in
place. This brass rod, called a stay-rod, prevents the end of the boiler
from blowing out when the steam pressure has reached its maximum value.
Three holes are drilled in the brass tube, as shown. One is to
accommodate the steam feed-pipe that goes to the engine; another is for
the safety-valve, and still another for the filling plug. The
safety-valve and filling plug are both shown in Fig. 51. The little
spring on the safety-valve is adjustable, so that the valve can be
regulated in order to prevent it from blowing off at pressures lower
than that at which the engine operates.
[Illustration: FIG. 52]
A suitable firebox for the boiler is shown clearly in Fig. 52. This is
cut to shape from stovepipe iron and held together with small rivets.
Holes should be punched or drilled in the side of the firebox to give
the burner a sufficient supply of air. The burner is illustrated
clearly in Fig. 52. The fuel-tank can be made from an ordinary tin can
with the cover soldered on, and a hole made for a cork by means of which
it is filled with denatured alcohol. A little pipe runs from the
fuel-tank to the burner. It is advisable, if possible, to place a small
valve in this pipe to cut off the fuel supply when necessary. The only
other method of putting the burner out would be to stand it on its end.
The burner consists of a rectangular tin box with a top cut out as
illustrated. A piece of brass or copper gauze is placed in the top.
Asbestos wool is used to fill the can, and the alcohol is drawn into the
wool by capillary attraction, where it burns with a steady hot flame at
the surface of the copper gauze. In the corner of the can near the
feed-pipe another small piece of copper gauze is soldered as shown. This
covers up the feed-pipe entrance so that the asbestos will not plug up
the pipe.
[Illustration: FIG. 53]
[Illustration: FIG. 54]
The engine to be used in connection with the boiler just described is
shown in Fig. 53. This is a very simple engine of the oscillation type,
and there should be little trouble in making it. A more mechanical
drawing of the engine is shown in Fig. 54. The details of the engine are
shown in Fig. 55.
[Illustration: FIG. 55]
The cylinder of the engine should be made first. This is made from a
piece of brass tubing with an internal diameter of 3/4 inch. Two end
pieces, or a cylinder-end cover and cylinder head, must be cut to fit
inside the cylinder. These should be cut to shape from 1/16 inch brass,
and a hole drilled in the cylinder head 1/8 inch in diameter to
accommodate the piston-rod. The cylinder head is then soldered in place.
The cylinder-end cover should be left until the piston-rod and piston
are made.
The piston head is cut to shape from a piece of 3/16-inch sheet brass,
or it can be cut from a piece of 3/4-inch round brass with a hacksaw.
The piston-rod is soldered into a hole in the piston-head. A small
square piece of brass is placed on the opposite end of the piston-rod to
act as a bearing. This little piece is cut and drilled as shown in the
drawing. Before it is soldered in place on the piston-rod the
cylinder-end cover should be placed on the rod. Both the piston and the
cylinder-end cover can then be placed inside the cylinder, and the
piston-end cover is soldered in place. Before final assembling the
piston should be made to fit nicely into the cylinder. This can be
brought about by applying emery cloth to the piston-head until it slips
nicely into the cylinder with little or no play. Thus a steam-tight fit
is made, and this contributes greatly to the efficiency and power of the
engine.
[Illustration: FIG. 56]
[Illustration: FIG. 57]
The cylinder blocks are shown in Fig. 55. These are cut and brought to
shape with a hacksaw and file. With a half-round file one side of one of
the blocks is filed slightly concave, so that it will fit on the outside
of the cylinder. Two 1/8-inch holes are drilled in this piece as shown
in the drawing. The hole at the top is the steam entrance and exhaust
for the engine; that is, when the cylinder is at one side steam enters
this hole, and when the crank throws the cylinder over to the other side
steam leaves through the same hole after having expanded in the
cylinder. This cylinder block is soldered to the piston as shown in
Fig. 56. The pivot upon which the cylinder swings is then put in place
in the hole at the bottom of the block. Solder is flowed around the
pivot to hold it securely in place.
The second cylinder block is now finished according to the drawing. This
has two holes 1/8 inch in diameter bored in it. One of these holes is
the steam inlet and the other the exhaust. When the cylinder is at one
side of its stroke the hole that was bored in the top of the steam block
which was soldered on the cylinder is in line with the inlet hole in the
block under consideration. Steam then enters the cylinder and forces the
piston down. This turns the crank around, and the crank in turn pulls
the piston over to the opposite side, so that the hole in the first
piston block of the cylinder now comes in line with the exhaust hole on
the second cylinder block. The steam in the cylinder escapes and the
same operation is repeated over again. Of course, it must be understood
that this steam admission and exhaust takes place very rapidly. The hole
in the second cylinder block, which goes over the pivot, must be made a
trifle more than 1/8 inch in diameter, so that it will slide freely over
the pivot.
The engine is mounted on a very simple frame, which is a piece of
1/16-inch brass cut and bent as illustrated. After it is cut and bent to
shape the second cylinder block is soldered in place. The cylinder can
then be mounted. It will be seen that the pivot goes through both the
second cylinder block and the engine standard. A small spring is placed
over the protruding end of the pivot and a nut put in place. By turning
this nut the pressure on the face of the two cylinder blocks can be
adjusted, and the model engineer must always remember that the pressure
on these springs must be greater than the steam pressure in the
feed-pipe. Otherwise the steam pressure will force the cylinder-block
faces apart and steam leakage will result. On the other hand, the
pressure of the spring should not be too great, since that would
interfere with the free movement of the engine cylinder.
Nothing now remains to be made except the crank and the flywheel. The
crank revolves in a small brass bearing which is soldered in place on
the engine standard. It will be seen that the sheet brass that makes up
the engine standard is not thick enough to offer a good bearing for the
crank. The crank is bent to shape from a piece of 1/8-inch brass rod,
and the author advises the builder to heat the brass rod red-hot while
the bending is done. This will prevent it from fracturing, and will also
permit a sharp bend to be made.
The flywheel is a circular piece of brass 1 inch in diameter. Its center
is drilled out and it is soldered to the crank as illustrated in Fig.
54. Two other holes 1/8 inch in diameter are drilled in the flywheel as
illustrated, and two small brass pins are cut out from 1/8-inch brass
rod and forced into these holes and then soldered. These provide a
method of driving the propeller-shaft that is shown very clearly at Fig.
57.
The steam feed-pipe that runs from the boiler to the engine can be of
small copper tubing. It may be necessary to mount the engine on a small
block, as shown in Fig. 53. After the steam in the boiler has reached a
sufficient pressure the engine crank should be given a couple of twists
in order to start it. Before operating the engine a little lubricating
oil should be run into the cylinder through the inlet or exhaust ports.
The cylinder should always be kept well lubricated. The contacting faces
of the cylinder blocks should also be kept lubricated.
_Caution._ Always keep water in the boiler. Never permit it to run dry,
as this would cause a boiler explosion. When the engine is started and
cannot be made to run, take the burner from under the boiler so that
steam will cease to be generated. With the safety-valve the model boat
builder need have little fear of an explosion. Nevertheless the
foregoing directions should be carefully adhered to.