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    Influence of the piston tank on the fore and aft trim of model submarines. (English Translation)

    david f
    david f
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    Influence of the piston tank on the fore and aft trim of model submarines. (English Translation) Empty Influence of the piston tank on the fore and aft trim of model submarines. (English Translation)

    Post  david f Mon Apr 03, 2023 2:37 pm

    (To see the photos and figures, please refer to the original articles in German in the Files section of our  Facebook site.)

    Two articles by Heinrich Kistenich & Helmut Huhn)

    These are 2 very significant articles from the past about the control and placement of piston tanks.

    2006 Article. (SchiffsModell 4/2006) Piston Tanks.

    2023 Introduction by Heinrich Kistenich.
    I am pleased that our original article of 2006 has now been translated into English. The original  theoretical and practical study of Piston Tanks has been shown to be significant and groundbreaking for our hobby.

    The reason behind our study was that in the years 2000 to 2004, complaints from submarine enthusiasts in the modelling magazines increased because they could not achieve good trim with their boats. If the boat was exactly on the DWL (Design Water Line) on the surface, it was not level when submerged, or vice versa.

    Efforts were made to compensate for this error, but many construction reports ended with the
    resigned statement: “The error had become smaller, but one would probably have to live with
    it in the future.”

    It is fair to say that the  most common piston tank installation is for one piston tank. Double Piston Tanks take up a lot of space, cost twice as much , and are twice the work.

    BUT Double piston tanks have the advantage that with they allow changes to be made
    about the transverse axis. (Trim)

    A model submarine should be able to stay at a constant depth, preferably without moving in altitude. But this is only possible with active depth regulation!

    One piston tank is sufficient for this. But even with one manually controlled piston tank it is possible to put the model on a thermocline (a dense, cold water layer.)

    No one should go to the trouble of building an ARCHI 1-like vehicle in order to use it to determine the installation position of the Piston Tank. This has already been done and this article describes the theoretical basis.

    Our division of labour was that Helmut did the electrics and Hein did the mechanical construction. It took us about 9 months to do this work.

    2006 Article. (SchiffsModell 4/2006)

    In an earlier article in SchiffsModell 9/05,  page 20, heading “Volume and position of the Piston Tank” we described how to determine the best position of the piston tank in a model submarine. Today we will focus on the tools that were needed to confirm the hypotheses of issue 9/05.

    We have used the term “Bathyscaphes” to describe our model in the test tank because we are only looking at the buoyancy forces not dynamic ones produced by motion through the water.

    The weighing apparatus in the aforementioned issue, which is used to determine the symmetry point of a Piston Tank, will not be discussed again here. However, the “bathyscaphes”, which were developed in a short period of time, should be explained in more detail here. During our joint search for solutions to the above-mentioned essay, Helmut Huhn and myself were plagued by doubts. As the great German poet Goethe says so well: "All theory is grey, my friend" which is more prosaically translated into English as “The proof of the pudding is the eating”.

    ARCHI 1
    Therefore, a test vehicle was needed to either confirm or reject our theories. It was named ARCHI 1, in reference to Archimedes. To make the processes inside this test vehicle visible, Acrylic tubing was used for the construction. ARCHI 1 has a construction of Acrylic and lead as its upper deck, which weighs 500 g, but also displaces 0.5 1 . The sliding Piston Tank  also has a filling volume of 0.5 1 and weighs 1,350 g when filled. A "tower" with a periscope was installed halfway along the upper deck. The tower is open at the bottom and carries a thin tube at the tip of the periscope, which is closed with a clamp. The Tower and periscope are transparent.

    The contents of the tower are determined by means of a calibrated scale. One line on the scale corresponds to 1 g. This device is used to adjust ARCHI 1 to the respective water temperature. Both halves of the upper deck are fitted with centimetre measurements, and amidships under ARCHI 1 is an open half-shell that serves as a container for additional weights. Inside, three threaded rods encased in protective tubes hold the six Acrylic frames. At the lowest point, a double T-profile made of aluminium runs from front to back, which serves as a rail for the movable items - the Piston Tank and counterweight. The threaded spindles of the displacement drives form the supporting upper points of the two moving parts. The distances covered by the two movable parts are marked with scales and pointers.

    The R/C system is kept simple: Three two-channel switches control the three drive motors. As long as a function is keyed on the transmitter, the corresponding motor runs. When the desired positions are reached, ARCHI 1 can be switched off from the outside with a magnet. Both additional weights and buoyancy bodies can be attached to the outside of the front pressure bulkhead. Different buoyancy bodies are effective both above (only submerged) and below the DWL. In this way, the normally buoyancy-symmetrical ARCHI 1 can be made buoyancy-asymmetrical, and now the sliding Piston Tank  can show that the desired zero load can be achieved both above and below water, even without lead and/or polystyrene.

    ARCHI 2
    Because the project looked promising, another vehicle, ARCHI 2 with 2 Piston Tanks, was built immediately. This bathyscaphe has the same external dimensions as its predecessor, an upper deck has been dispensed with, but a conning tower (in the form of a measuring chamber) could also be fitted here.

    ARCHI 2 has two Piston Tanks with 250 ml each, which are controlled by a Huhn control system. The regulation corresponds in large parts to the technology installed in my submarine model STINT. Compared to the STINT regulation, this regulation has a few new features. Most importantly ARCHI 1 uses both a Piston Tank and counterweight to the stern, both of which can be moved independently  to the bow or the stern.

    . This way the boat would be level under water, but stern-heavy above water. And this is how it looks in the aquarium: If the two movable installation parts are in the wrong position, the boat is stern-heavy above water.

    ARCHI 2 is connected to a triple servo tester. This allows the same manoeuvres to be simulated as with the R/C system in the water

    It is worth mentioning that the new control system moves to the centre of the control window by itself when it is switched on. With STINT, a two-colour LED is used to recognise the control window. Its three switching states provide information about the current electrical position of the control.
    RED = boat too heavy,
    OFF = centre of the control window
    GREEN = boat too light
    Although ARCHI 2 has no rudders, it does have a rudder position indicator for the two functions "position control" and "depth". It is linear up to ± 45°.

    For the "dry operation" of ARCHI 2, a transparent measuring vessel with two chambers of different sizes was built, with which the reaction can be simulated. Both chambers together have a content of 500 ml, i.e. the total content of both Piston Tanks. The smaller of the two chambers has a capacity of 100 ml and thus corresponds to more than the maximum control volume of both Piston Tanks. The max. control quantity is currently ± 12 g. The control volume can be read on a calibrated scale in 1 ml increments. A hose simulates the depth-actual value recording of the respective diving depth with a pressure membrane. Several U-tube manometers, which can also be connected to this hose, are used for exact depth adjustment of the control electronics and for checking and calibrating existing measuring instruments. The smallest readable unit is 1 cm water gauge (w.g.) The largest measuring depth is 210 cm w.g. Another small unit works down to a depth of 150 mm.

    For 'dry operation' on the table, a separate three-axis servo tester serves as a transmitter simulator, with which all functions can be operated by cable as with the transmitter. Both vehicles are kept in a container that is also used for presentation on special occasions. In the base, there is space for all the things needed for operation and maintenance. Tools and equipment are stored in a small box.

    Neither vehicle has a drive or a rudder, because they are "only" running in the aquarium or on the table in test mode - under laboratory conditions, as it were.

    What the vehicles can show in detail:
    Determination of the positions of the Piston Tank  and counterweight.
    Visualisation of the load with partially filled Piston Tank .
    Effectiveness of additional weights and/or buoyancy bodies.
    Change in buoyancy with changing water temperature.
    Buoyancy change with bubbles forming on the outer skin.
    Visualisation of the adhesion of  the water film during descent and ascent.

    An interesting and useful device that also gives answers to all questions of buoyancy and balance in the model submarine.

    SchiffsModell 9/05, “Volume and position of the Piston Tank”


    This paper is a theoretical and practical study of the influence of the piston tank on the fore and aft trim of model submarines.

    The nature of the piston tank.  
    A well-built Piston Tank  is one of the most precise buoyancy modifiers that can be used in a model submarine. The trim problem arises only when the tank is mounted horizontally. If the  threaded spindle is mounted vertically, Curve 1 in the paper would be a straight line. Because the Piston Tank  is installed horizontally, it not only provides the desired change in buoyancy, but unfortunately also a change in trim .

    Basic Principles.
    Let's assume that the model starts off floating level under water.
    If the model , when surfaced, is bow-heavy after emptying the Tank then the Piston Tank  must be moved forward. Every centimetre is significant. If the boat is stern-heavy after surfacing, the Piston Tank  must be moved backwards. Every time the Piston Tank  is adjusted, another weight in the boat has to be moved in the opposite direction.
    In reality, a submarine hardly dives on an even keel because bow and stern, which rise out of the water, are not equal in displacement. The conning tower also plays a role. Accordingly, mounting a piston tank in the centre of the model rarely makes sense.

    Archimedes plays a role here: a body has as much buoyancy as it displaces in water. The weight of the material is meaningless here, because a cubic centimetre of lead generates the same buoyancy under water as a cubic centimetre of foam. The balance of the volumes of the superstructures of a submarine (turret, weapons, railings, etc.) in front of and behind the ship's centre of gravity above the DWL (Design Water Line) cause the load, because during descent the difference in displacement in front of and behind the centre of gravity comes into play. Example: If a submarine is long and pointed at the front and the deck is also very thin-walled, then it has relatively little water displacement volume at the front. In this case, the boat lifted at the underwater centre of displacement would be stern-heavy.

    Here's another example: The boat is first surfaced with an even keel on the DWL, everything is OK. The Piston Tank is filled and the submerged boat is now bow-heavy. Obviously the boat is too heavy at the front because the Piston Tank  sits too far forward. Therefore the Piston Tank  should be  moved further back so that it makes the boat heavier at the back when flooding. That is exactly the solution! It is possible, however, that the boat's interior is designed in such a way that the Piston Tank  cannot be moved. Then the  only solution is to install buoyancy above the DWL in the free-flooding space, so that the displacement is greater under water on the bow side. The installation of buoyancy bodies below the DWL is useless in this case!

    If the boat is stern-heavy when it is submerged, foam must be fitted above the DWL on the bow side and lead must also be fitted here so that the model trims level when it is above water. The submerged boat is now on an even keel when the buoyancy of the foam cancels out the weight of the lead. The amount of foam needs to be determined by experiment.

    The effectiveness of foam and lead used in combination increases the further they are from the centre of the boat. The more buoyancy bodies are used, the deeper the DWL  will sink when the boat is surfaced.

    Installation of  a single Piston Tank in a new-build.
    Since Archimedes' law of buoyancy comes into play here, the boat must be completely finished externally. As curves 1-8 show, a Piston Tank  ultimately behaves like a water tank that only knows two states: "empty" or "filled without bubbles". Its position determines the possible correction of the buoyancy of the superstructure, its filling volume determines the position of the DWL .

    The second solution for the correct positioning of the Piston Tank  but without calculation is as described above under the heading "The nature of the Piston Tank ", but now in more detail. Batteries, remote control, additional weights and the Piston Tank  are placed in the boat and the Piston Tank  - after it has been filled and emptied - is moved until the boat is zero-loaded above and below water. During this action, the batteries and additional weights are adjusted accordingly in the opposite direction. This is the correct  position for the Piston Tank.  

    The other parts can be placed anywhere, taking into account the trim. Disturbances such as air bubbles or a change in the temperature of the water, which require a buoyancy correction via the Piston Tank , also cause a small change in ballast.

    All the statements in this article assume that the threaded spindle of the Piston Tank  points to the stern. If it points to the stern, the boat would be stern-heavy during filling.

    If the installation position is correct, the boat lies on an even keel both above and
    below the water. Only during filling or emptying will the boat be tilted!

    If two Piston Tanks are installed, which run symmetrically and whose threaded spindles face each other, there is no load during filling or emptying.

    A final thought. Since our models, in contrast to the originals, do not experience any load or consumption, such as fuel or food, during their operation, there are no significant changes in buoyancy and load.

    Published with the kind permission of:
    Wellhausen und Marquardt
    Mundsburger Damm 6
    D 22087 Hamburg
    Publisher of the magazine SchiffsModell
    AMS Forum Owner

    Posts : 3026
    Join date : 2010-11-05
    Age : 64
    Location : Nevada, USA

    Influence of the piston tank on the fore and aft trim of model submarines. (English Translation) Empty Re: Influence of the piston tank on the fore and aft trim of model submarines. (English Translation)

    Post  Tom(ADMIN) Sun Apr 30, 2023 10:30 am

    What is the original FaceBook link as mentioned on the first sentence?
    david f
    david f
    AMS Treasurer

    Posts : 2401
    Join date : 2010-11-10
    Age : 73
    Location : Cumbria

    Influence of the piston tank on the fore and aft trim of model submarines. (English Translation) Empty Re: Influence of the piston tank on the fore and aft trim of model submarines. (English Translation)

    Post  david f Mon May 01, 2023 4:46 pm

    Hi Tom,

    Should be here:

    Let me know if that works.


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