(c) 2001, Johan J. Heiszwolf
Live
Scale Dive Technology
Basically, there are
two ways to submerge a boat: dynamic diving and static diving.
Many model submarines use the dynamic method while static diving is used
by all military submarines. Dynamic diving boats are submarines that inherently
float that is, they always have a positive buoyancy. This type of boat
is made to dive by using the speed of the boat in combination with the
dive planes to force the boat under water. This is very similar to the
way airplanes fly. Static diving submarines dive by changing the buoyancy
of the boat itself by letting water into ballast tanks. The buoyancy is
thereby changed from positive to negative and the boats starts sinking.
These boats do not require speed to dive hence this method is called static
diving.
Modern military submarines dive use a
combination of dynamic and static diving. The boat submerges by filling
the main ballast tanks with water. After that, the buoyancy is accurately
adjusted with the trim tanks. Once underwater, the depth of the boat is
controlled with the hydroplanes.
In the following, the dive methods are
treated in detail. We will start with static diving because this is more
important for real submarines.
Static
Diving
The buoyancy of a submarine
can be changed by letting water into the main ballast tanks (MBT). The
MBT's can be located in three different ways: (a) inside the pressure hull,
(b) outside the pressure hull as additional tanks, and (c) in between the
outer hull and the pressure hull. Figure 1 shows the three possible configurations.
Drawback of having the MBT inside the pressure hull is obvious: it takes
up space that could otherwise be used for equipment, weapons or personnel.
This MBT arrangement was used in the WW-I boats and other early submarines.
The classical example of a boat with MBT's outside the pressure hull is
the German Type VIIC but also American and Dutch submarines in WW-II used
this design. Due to the location of the MTB's, they are called saddle tanks.
Most modern military submarines use the space in-between the inner pressure
hull and the outer hull as MBT.
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| Figure 1: Different locations of the main ballast tank. |
There are two different ways the MBT's
can be emptied and filled. These methods will be referred to as the the
western (USA, UK) method and the Russian method. Please note that the 'Russian'
method is not exclusively Russian because it was also used by for example
the Dutch triple hull Dolfijn class boats. Figure 3 depicts the both methods,
the left hand side of the pictures shows the USA/US method, the right hand
side the Russian method. When surfaced, the MBT are entirely filled with
air and the main vent valves on top of the MBT are closed. In the USA/UK
boats the flood opening at the bottom of the MBT always remains open. Water
is prevented to enter the MBT because the air in the MBT is pressurized,
at about 10 PSI. In the Russian boats, the bottom flood opening is closed
with a valve, a so called Kingston. Because the Kingston prevents water
entering, air in the MBT can be at approximately atmospheric pressure.
To dive the boat, the vent valves on top of the ballast tanks are opened
to let air escape the MBT. Because in the USA/UK boats the air is pressurized,
the air roars out of the vents, resulting in a large spray of water, see
Figure 2.
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| Figure 2: An Ohio class submarine venting the forward ballast tanks. |
In the Russian technology, the Kingstons
at the bottom of the MBT also have additionally to be opened in order to
let water enter the MBT. It is claimed that because the air in the USA/UK
boats is pressurized (more gas in the MBT and larger friction in the vent
valves) the Russian MBT is flooded more quickly.
 |
| Figure 3: Flooding of the main ballast tanks. |
To surface the boat, the water in the MBT's
is forced out by pressurized air. When the boat is deeply submerged, the
water is forced out using high pressure air to overcome the water pressure.
Once the boat is near the surface, the blowing of the MBT's proceeds with
low pressure air. Once at the surface, the Russian boats close the Kingston
valve and then opens the main vent valve briefly to equalize the air pressure
in the MBT with that of the atmosphere. In the USA/UK boats, the main vent
valve remains shut to keep the air in the MBT under pressure. The pressure
inside the tanks remains equal to that of the low pressure air system.
 |
| Figure 4: Blowing of the main ballast tanks. |
Figure
5 shows the location of the MBT's in a modern diesel electric submarine.
The bulk of the MBT's are located at the bow and aft sections of the boat
and a small MBT surrounds the pressure hull in the center of the boat.
A large portion of the space between the pressure hull and the outer hull
is occupied by the fuel tanks. It is important to note that the MBT is
only used to change the buoyancy of the boat from
very positive
(the boat is surfaced) to slightly positive (the boat is
just
still on the surface, decks awash this is called). The optimal rig
for a submerged boat is neutral buoyancy: the boat neither floats nor
sinks. This situation is accomplished by the use of the main trim tanks
(MTT) located in the center of the boat. Once the MBT is full of water,
the MTT is carefully filled with water until a neutral buoyancy is obtained.
For a submarine with a given weight, the amount of water required
inside the MTT depends on for example the salt content and the temperature
of the surrounding water. Maintaining neutral buoyancy in a submarine is
a continuous procedure. For example the diesel engines consume fuel and
the personnel eats food so that the total weight of the boat steadily decreases
during a mission. This means that while progressing with the mission, the
amount of water in the MTT has to be increased to maintain neutral buoyancy.
Also the density of the surrounding water
plays an important role. A well known example is the downstream area of
a river where fresh and salt water mix leading to a different density than
in the open sea. If a submarine enters such a region, the trim has to be
adjusted. For military submarines an obvious action that changes the buoyancy
of the boat is the launch of a torpedo. For this purpose, military submarines
have a special ballast tank located in the vicinity of the torpedo room
to compensate for the weight loss of the torpedo. Usually the water level
in the MTT is adjusted using high pressure pumps rather than high pressure
air because the latter makes much more noise. Some of the MTT tanks can
however be emptied using pressurized air to get a quick blow in case of
an emergency. Once a neutral buoyancy is obtained with the MTT's, the depth
of the boat can be changed using the speed of the boat and the angle of
the dive planes. This is thus dynamic diving, see below.
 |
| Figure 5: Location of the different tanks in a modern diesel electric boat. Picture adapted from Gabler (1987). |
At neutral buoyancy conditions, it is also
important that the submarine maintains a horizontal angle. For this purpose
the submarine is equipped with two sets of trim tanks located in the bow
and aft section of the boat. Both fore and aft trim tanks are connected
with a line so that water can be pumped back and forth to obtain the required
horizontal angle of the boat. Further note that in the military submarine
of Figure 5 a large section of the boat is free flooded. With the use of
the free flooding sections, the overall size of the ballast tanks can be
kept to a minimum.
Dynamic
Diving
Once the boat is trimmed
to more or less neutral buoyancy, the depth of the boat is controlled with
the hydroplanes. To use the hydroplanes the boat requires speed to create
a force on the tilted planes. At slow speeds, the fore hydroplanes are
exclusively used to keep the boat at the required depth. The fore planes
can be located on the hull near the bow or on the sail of the boat. Because
bow mounted hydroplanes are located further from the center of gravity,
the depth control is more accurate with these types. Arguments for locating
the fore planes on the finn of the boat are (a) improved performance of
the spherical sonar array in the bow because the fore hydroplanes generate
noise and (b) bow mounted hydroplanes can be damaged during docking of
the submarine. Penalties for placing the fore planes on the fin are (a)
the operating gear takes up space in the fin where room badly is needed
for the masts, (b) the ice breaking performance is decreased, (c) at periscope
depth the planes are close to the surface so their performance is adversely
affected by the surface turbulence and finally (d) the hydroplanes are
closer to the center of gravity and are thus less effective. Note that
while improving the Los Angeles class submarine (688I) the US Navy relocated
the fore planes from the sail to the bow. At sufficiently high submerged
speed (more than 12 knots), the fore planes are no longer needed to control
the depth of the submarine. At these speeds, they are rotated in a neutral
or slightly dive position. Because the fore planes generate noise, many
submarines are capable of retracting the forward bow planes at high speeds.
All this considering, we may conclude that (retractable) bow planes are
more favorable. It may be added that the author is not aware of boats having
both
dive planes on the bow and on the sail.
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| Figure 6: Location of the dive planes and their angles during the dive of Figure 7. |
Figure 7 shows how the fore and aft dive
planes are used during a dive. At the start of the dive the aft plane is
rotated upwards so that the stern of the boat is forced upwards. The fore
hydroplanes are rotated downwards thus forcing the bow of the boat down.
During the dive the aft hydro planes are moved to the neutral position
and the dive angle is controlled with the fore hydroplanes only. Close
to the the required depth, the aft planes are rotated down and the fore
planes up to level off the boat. At slow speeds the depth of the boat is
maintained by the fore planes only. During the first dive, the water
level in the main trim tanks is adjusted to obtain a neutral buoyancy so
that the required depth can be maintained with a nearly horizontal position
of the hydroplanes.
 |
| Figure 7: Angle of the dive planes during a drive. |
Aft Hydroplanes
Figure 8 shows the
positioning of the aft hydroplanes as used by military submarines. Type
A is the configuration applied by many modern military submarines. The
hydroplanes are located in front of the screw. Note that the rudder blades
are of different size. The bottom plane is smaller than the top one so
that the boat can be put on the bottom of the sea (bottoming). Types
B and C have the hydroplanes behind the screw. This is a configuration
used by older submarines. The hydroplanes behind the screw are still used
by the double screw Russian Tango and India class boats. The arrangement
of D has the rudder behind the screw but has the dive planes in front of
it. This type of arrangement was used for the German 205 and 206 class
boats. Type E has the hydroplanes tilted 45 degrees, the so called X-tail
configuration. No distinction between the rudder and the dive planes can
be made. To steer and dive the boat, all of the four hydroplanes are used.
In old submarines each set of hydroplanes, fore dive, aft dive and rudder,
were operated by a separate person that manually turned a control wheel
to the desired angle. It is obvious that an X-tail can only be operated
by electronics or computer control. Because all four planes are used for
both horizontal and vertical movement, the control of the boat is more
subtle. Due to the 45 degrees tilting of the hydroplanes, bottoming is
made possible without having to decrease the size of the lower dive planes.
The X-tail configuration is used by the Dutch Walrus (Figure 9), the Swedish
Vatergotland and the Australian Type 471.
 |
| Figure 8: Positioning of the aft hydroplanes for single screw boats. (side, aft and top view). |
 |
| Figure 9: X-tail configuration of the Dutch Walrus Class, picture from Miller (1990). |
Model
Submarine Dive Technology
In the previous sections,
the dive technology of real submarines was explained. It was shown that
the bulk buoyancy of the boat is changed with the MBT followed by fine
tuning with the MTT and finally the correct depth is maintained using the
hydroplanes. Of course the ultimate model submarine should operate in exactly
manner. Due to the small scale however, application of the real submarine
technology is not always possible. In the following, some of the available
model diving technologies will be treated.
Dynamic
Diving Technology
The fully dynamic diving
boats are the most simple model submarines available. These boats have
an inherent positive buoyancy which means that they will float back to
the surface if control is lost. This is a major advantage for model submarines.
Two German model manufacturers sell dynamic diving submarines: Robbe
The Seawolf (not the real one) and the U-47 (a Type VII-C boat) and Graupner
sells the Shark. On the internet Charles
Darley has an excellent web site showing the building of a fully
dynamic submarine. To get a positive buoyant boat under water, the force
on the hydroplanes has to overcome the upward force of the floating boat.
This requires a combination of sufficient speed or sufficiently large hydroplanes.
Of course the closer the boat is rigged towards neutral buoyancy the smaller
the required downward force of the hydroplanes. Figure 10 shows the angle
of the dive planes to keep a positive buoyant sub under water. At low speed
both planes have a downward angle. The aft hydroplanes are needed to prevent
the stern of the bow rising above the surface. Just like for the real boats,
at sufficiently high speeds the aft hydro plane can be moved to a neutral
position and depth control can be maintained with the fore plane only.
 |
| Figure 10: Angle of the dive planes, left low speed, right high speed.. |
In general, the force on a hydroplane can
approximately be calculated from the following equation:
|
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| Equation 1: Force acting on a hydroplane. |
For example, if we would have a model submarine
with square hydroplanes of dimension 5 x 5 cm, this leads to an area of
the individual hydroplane of A = 2.5 10-3 m2.
With a boat traveling at a speed of 10 km/h (v = 2.78 m/s) and a
hydroplane down angle of a
= 30 degrees, the down force on the hydroplane is about half a kilogram.
Since the boat has two forward dive planes the nett down force would be
close to a kilogram. This means that the positive buoyancy of the boat
can also be allowed to be a kilogram. However, at lower speeds than 10
km/h or at hydroplane angles less than 30 degrees, we will not able to
submerge such a boat! Equation 1 can be used to design the size of the
hydroplanes or the allowable positive buoyancy for a given hydroplane size.
Please note that the value of the friction coefficient C is dependent
on the design of the hydroplane, C=01 is the maximum value. In updates
of this page more accurate values of the friction coefficient C
will be given. Generally, for fully dynamic diving models one would want
the boat rigged close to neutral buoyancy. That is, the decks awash
situation. In that case, the boat can be submerged at low speeds with a
realistic size of the dive planes. However if the boat is already close
to neutral buoyancy, the surface running is not very realistic. To obtain
realistic surface and submerged operation ballast tanks are needed.
Static
Diving Technology
In real submarines,
MBT's are filled by venting the air inside the tanks and are emptied by
blowing compressed air in to them. For model submarines a number of alternative
methods are available.
Vented
ballast tank
The vented tank (Figure
11) can be used to decrease the buoyancy of the boat from positive to slightly
positive (decks awash). If the flood valve is opened, the air can escape
through the vent and water fills the tank. The tank can be emptied by pumping
water out of the tank while air is sucked back into the tank through the
vent. Note that in order for this system to work, the top of the vent line
must be above the water level. That is why the vented tank cannot be used
to give the boat neutral or negative buoyancy. With a filled tank the boat
can dive using the hydroplanes. Note that if a bi-directional pump is used,
the flood valve is not needed. To prevent water getting in to the ballast
tank when running submerged, the diameter of the vent line should be kept
small. Please note that the vented ballast tank is not very convenient
as a ballast system.
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| Figure 11: Vented ballast tank. |
Flexible
ballast tank
The flexible tank (Figure
12) consists of a rubber balloon placed inside a rigid tank. To flood the
tank, the valve is opened and water is pumped into the tank. The valve
is closed to prevent water getting out once the tank is flooded. The air
originally present in the rigid tank is vented into the pressure hull of
the boat. This will lead to an increase of the pressure inside the hull.
If the volume of the ballast tank is not to large compared to the air volume
inside the pressure hull this is not a problem. Note that the inside of
the submarine is usually packed with equipment so the air volume is certainly
not equal to the hull volume.
 |
| Figure 12: Flexible ballast tank. |
Wilhelm
Sepp, builder of a model of the Nautilus, used an inflatable toy ball
as flexible tank, see Figure 13. The ball is filled and emptied by a Robbe
gear pump. The tilted section of the wooden panel is connected to a micro
switch and rests on the balloon. Once the balloon is completely filled
the micro switch closes so that the power to the pump is cut off and the
balloon cannot be filled beyond its capacity.
|
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| Figure 13: Flexible tank, empty and full, by Wilhelm Sepp. |
Pressure
ballast tank
The pressure ballast
tank (Figure 14) consists of a sealed ballast tank capable of with standing
a significant pressure increase (5 bar or so). To flood the tank water
is pumped into the tank with a high pressure water pump. Because the air
in the ballast tank cannot escape the air is compressed. To empty the tank,
the water pump pumps the water out of the tanks again. Note that because
the pressure build-up inside the ballast tank it can never be completely
filled. Assuming a maximum pressure of 5 bar inside the tank, about 80
percent of the volume of the ballast tank can be used.
 |
| Figure 14: Pressure ballast tank. |
On the internet, the company SubTech
sells a special T-valve that can be fitted on the ballast tank. This T-valve
releases air into the boat when the tanks is filled and lets air into the
tank if the tank is emptied. In that way, the ballast tank does not have
to withstand high pressure. In my opinion a drawback of this system is
that the ballast tank is connected to the interior of the boat. When the
tank overflows, the water ends up inside the pressure hull containing the
electronic RC equipment. A maximum water level detector that cuts the power
to the pump can prevent this.
Piston
ballast tank
The piston ballast
tank (Figure 15) consists cylinder and a movable piston, just like a giant
syringe. The piston can be moved with a thread, a cogwheel and a small
motor. The outer end of the cylinder is directly connected to the surrounding
water. In the piston ballast tank no air is present. Just like the flexible
tank the pressure inside the boat increased if the piston tank is filled
with water. If the position of the cylinder is measured, for example with
a linear potentiometer connected to the thread, the buoyancy of the boat
can very accurately be adjusted. Due to the large stroke of the piston,
these types of ballast tanks are mostly fitted horizontally, like in Figure
12. This means that during filling of the tank with water the axial center
of gravity of the boat is affected. For example if the boat is balanced
to run horizontally with a full ballast tank, the angle of the boat is
no longer zero with an empty tank. This drawback can be overcome by using
two piston tanks in the aft and bow section of the boat.
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| Figure 15: Piston ballast tank. |
The piston tank can be purchased from Norbert
Bruggen, see Figure 16. In this system, a small electric motor drives
the piston in the plastic cylinder. Two micro switches interrupt the current
to the motor if the piston is in either fully retracted or fully deployed.
 |
| Figure 16: Piston ballast tank by Norbert Bruggen. |
Membrane
ballast tank
The membrane ballast
tank (Figure 17) is a simplified version of the piston tank. It consists
of a rigid disk that can be moved up and down with a thread connected to
a motor, just like the piston tank. The disk is connected to the cylinder
via a flexible rubber membrane. When the disk is retraced, water is allowed
into the boat. A nice aspect of the membrane ballast tank is that the water
tight sealing is very easy. As long as the rubber membrane is properly
attached to both disk and tank, leaking is not possible. In a piston tanks
the sealing between the piston and the cylinder is quite critical. Drawback
of the membrane tank is that the stroke of the piston is not very large
so the change in buoyancy of the submarine is not very large. To make optimal
use of the membrane tank, the diameter of the cylinder should be rather
large compared to its height. The system is however ideal for small, or
micro, model submarines. Thorson Feuchter
has a nice collection of boats based on this principle.
 |
| Figure 17: Membrane ballast tank. |
Bellow
ballast tank
The bellow ballast
tank (Figure 18) is a variation on the membrane ballast tank. Instead of
a flat membrane a rubber bellow is used. This has the advantage that the
stroke of the disk is increased so that more water can be taken into the
boat. Rubber bellows of sufficient diameter, 5 to 10 cm or so, can be found
in car parts shops. In cars they are for example used to seal off moving
parts of the steering equipment. Under pressure, the zig-zag wall of the
membrane may pop out, resulting in a sudden increase of the ballast volume
(and sinking of the sub). To prevent this, it is recommended to fit the
bellow inside a cylinder.
 |
| Figure 18: Bellow ballast tank. |
Gas
operated ballast tank
The liquid gas system
(Figure 19) consists of a storage cylinder with pressurized gas, a ballast
tank and two valves. This system resembles the ballast system of a real
submarine very closely. To flood the tank, the valve in the vent line is
opened and water is allowed into the tank via the opening in the bottom.
If the required volume of water is taken in, the vent valve is closed.
The tank can be emptied by forcing pressurized gas into the tank by opening
the blow valve. If we want the model boat to be able to blow the ballast
tank a number of times, the stored amount of gas should be sufficient.
Carbon dioxide (CO2) is an option because cylinders with this gas are relatively
cheap and readily available from Paint ball shops. In paint ball, cylinders
of 50 to 500 gram are commonly used. If CO2 cylinders are used a reduction
valve to bring back the pressure to about 2-3 bar is necessary. CO2 cylinders
are also used in model Warships, an excellent web site giving information
on CO2 cylinders is R/C
Warship.
 |
| Figure 19: Gas operated ballast tank. |
 |
| Figure 20: A CO2 cylinder (14 oz). |
An interesting alternative to CO2 is the use liquefied gas, for example canisters used for air brushing (propane), canisters used to clean photo equipment or electronics called 'dust-off' (dimethylether/tetrafluorethane) or propell (tetrafluorethane). Because these gases are stored as a liquid, the amount of gas that can be stored is quite large. It is also very easy to refill the gas tank in the submarine form a larger stock cylinder. With CO2 do it your self refilling is not that easy so that spare cylinders have to be taken to the lake. An additional advantage of liquid gas is that the pressure inside the storage vessel is about 3 to 4 bar so there is no need for a pressure reduction vale. A very serious draw back of these gases is its flammability. If the storage vessel leaks an explosive mixture may form inside the pressure hull of the boat. The sparks of the electric motor are sufficient to detonate it! The only real safe liquid gas is tetrafluorethane. The dust-off product contains about 20 percent of the flammable dimethylether and is potentially hazardous.
In the gas ballast system the electric valves used in the gas line (the blow valves) can be standard solenoid valves used in laboratory equipment. To prevent draining of the batteries, valves that are normally closed should be used. Using CO2 with a pressure reduction valve or liquid gas, the pressure at which they remain closed should be about 5 bars. Miniature solenoid valves can be obtained from Clippard.
The vent valves that let air out of the
ballast tank to submerged the boat are different. The pressure difference
between the air in the tank and that of the ambient air is only a couple
of cm water. Therefore the opening of the vent valve should be quite large
to let our air at a sufficient flow rate to get a realistic dive. Because
the pressure difference is also quite small when the vent valve is closed,
and thus the boat is submerged, we can make these valves ourselves. Note
that many of the above-mentioned solenoid valves have an opening of less
than 1 mm and do usually not like water getting in to it, these types of
valves are not very suited.
Compressed
air ballast tank
The ballast tank system
that uses compressed air is identical the one used in real submarines.
This system is similar to the gas operated ballast tank but in this case
the gas bottle is replaced by a cylinder that is filled with a compressor.
Small compressors can be found in car accessories shops. They sell small
12 volt compressors that are intended to inflate car tires. These compressors
are capable of compressing air to about 6 to 8 bar. These pressures are
high enough to be careful with the storage cylinder. It is smarter to buy
a commercial cylinder or use a empty CO2 cylinder than to make one by your
self. The pressure is however relatively low pressure if you consider the
amount of gas that can be stored. If we would assume that the compressed
air cylinder is half the size of the MBT, we can only blow the MBT two
to three times. This is not much compared to CO2 or liquid gas systems.
In general, boats with on board air compressors refill the air supply each
time they run on the surface after a dive. Special care should be taken
to prevent water being sucked into the compressor. To prevent this, the
air intake should be fitted with a valve that closes if the boat is submerged.
 |
| Figure 21: Gas operated ballast tank. |
The use of a gas compressor is applied in a boat made by Harry Grapperhaus. Two cylinders of 0.75 liter and 0.1 liter are filled with the compressor to about 6 bars. The main ballast tanks is about 3 liters so that with a full air cylinder only one blow of the MBT is possible. Harry uses the MBT also to control the trim of the boat. For this purpose the MBT is fitted will a total of 6 solenoid valves. Two valves are used for the controlled blowing of the MBT. These valves are connected to the air 0.75 liter cylinder via a pressure reduction valve (2.5 bar). By using two blow valves, the air flow rate can be more or less be regulated. One solenoid valve is directly connected to the 6 bar small cylinder of 0.1 liter. This valve is used for quick blowing of the MBT.
The venting of the MBT is controlled by
three solenoid valves so that the air flow rate can be adjusted in three
steps. To get a realistic dive of the model all three valves are opened
simultaneously. Once close to neutral buoyancy, only one valve is used
to regulate the depth of the model. In the model of Harry Grapperhaus,
the MBT always remains partly filled.
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| Figure 22: Arrangement of the valves in the boat of Harry Grapperhaus. |
Note
on gas ballast tanks
Remember the distinction
made between the Russian and US/UK boats in section Static
Diving? The Russian boats use a valve, the Kingston, to seal the bottom
opening of the ballast tank to prevent water entering. The US/UK boats
keep the ballast tank under pressure to prevent water entering. The designs
of Figure 19 and 21 do not have a Kingston valve. In one only uses a gas
ballast tank to adjust the buoyancy of the boat, one can run in to trouble.
Let us assume that the ballast tank is halfway filled with water to get
the boat at neutral buoyancy and the boat is at a depth of 1 meter.
At 1 meter below the surface the pressure of the surrounding water is 0.1
bar as a result the pressure of the gas inside the ballast tanks is also
at 0.1 bar. If we would move this boat upwards, the water pressure will
decrease
resulting in an expansion of the gas in the ballast tank. The expanding
gas will force water out of the ballast tank so that the boat gets lighter
and will rise even more. On the other hand, if we would move the neutral
buoyancy boat downward, the gas in the tank is compressed and more water
gets in to the ballast tank. This will sink the boat. We may conclude that
boats with a partially filled gas ballast tank are inherently unstable.
For model boats this may not be a problem as long as the depth of the boat
is controlled by the hydroplanes. Stable depth control at zero velocity
is however not possible. Of course if the boat is fitted with Kingston
valves water cannot enter the ballast tank and the problem is solved. The
author is not aware of any model boats equipped with Kingstons. A different
way to get a stable depth control is to use the MBT either completely full
or completely empty. The trim of the boat is obtained with separate trim
tanks. This is the hybrid ballast system, see below.
Hybrid
Ballast Systems
Just like real submarines
use main ballast tanks (MBT) to submerge and main trim tanks (MTT) to rig
the boat for neutral buoyancy, the ideal submarine should operated likewise.
A nice example of such a boat is the one made by Ralf Diederich. This boat
has a compressed air system as MBT. The air cylinder has a volume of 1.2
liters and is filled by two compressors to 6 bar. The tank is filled by
two compressors to reduce the filling time.
 |
| Figure 23: Two compressors in Ralf Diederich's boat. |
This results in a stored gas volume of
7.2 liters. The MBT has a volume of 3.7 liters. This means that with a
full compressed air cylinder only just two full blows of the MBT
are possible. The exact buoyancy of the boat and the horizontal
trim is controlled by two piston tanks located in the bow and the stern
section of the submarine. The model boat is prepared for operation as follows.
First the both piston tanks are emptied. Then the main ballast tank is
flooded completely. The boat will sink to some extent but will still float
on the surface. Then the both piston trim tanks are carefully flooded.
The pistons inside the trim tanks are moved using two proportional
channels of the radio transmitter. The water level in the trim tanks is
adjusted in such a way that the boat is kept horizontal. The trim of the
boat is completed once only the top of the sail is above the water level
and the boat has a horizontal angle. Since in this condition the boat has
a nearly neutral buoyancy, the depth of the boat can be adjusted using
the dive planes even at a very low speed. When RC operating the submarine,
the trim of the piston tanks is left unaffected and the depth of the boat
is only controlled by blowing or emptying the MBT's and by using the dive
planes once the boat is submerged. This is pretty much the same way real
submarines operate!
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| Figure 24: Two compressors and one piston tank in the aft section of Ralf Diederich's boat. |
Do not think hybrid ballast systems are
a recent invention of model submariners. Figure 25 shows a submarine desing
by George Garrett in 1878. The boat has a main ballast tank (A). Water
from the ballast tank can be removed with a piston pump (B) and the tank
can be flooded with the vent valve (C). The trim of the boat is made with
a piston ballast tank (D).
 |
| Figure 25: Design by George Garrett, 1878. [Compton-Hall, 1999]. |
Conclusions
For model submarines,
a number of different ballast tanks systems were identified. These types
can roughly be grouped into three different ways of operation: (a) mechanical
attenuated systems (piston, membrane, bellow), (b) pump systems (flexible
tank, pressure tank) and gas operated (CO2, liquid gas and pressurized
air). The mechanical attenuated tanks are ones that control the buoyancy
in the most accurate way but they are rather slow. Pump systems are relatively
easy to construct because they use few parts. The gas operated tanks are
the most complex systems but are very similar to the live scale submarine
technology. With gas systems the blowing of the MBT can be carried out
very fast, in fact even an emergency blow can be carried out! An accurate
and realistic model boat can be constructed using a mix of these ballast
systems.
Glossary
MBT: Main Ballast Tank
MTT: Main Trim Tanks
Literature
David Miller, 1990, 'Moderne Gevechtswapens:
Onderzeeboten', Uitgeverij Helmond, Helmond, The Netherlands.
Ulrich Gabler, 1987, 'Unterseebootbau, Bernard & Graefe Verlag, Koblenz, Germany, ISBN 3-7637-5286-2.
Compton-Hall, Richard, 1999, 'The submarine Pioneers, Sutton Publishing, ISBN 0-7509-2154-4.
