CZK - Z-32-R (říční mina)

Z-32-R (river mine)
Dear colleagues,
many years ago I wrote down some notes about river mines produced in ČKD. Apart from the prototype below, these were U-28, U-36 and Z-28 mines in modifications for the Czechoslovak army and for export to Yugoslavia and Romania. Allegedly, these mines were also used by the Wehrmacht during the DSV.


Copy of archival document:


Confidential!
Report on internal tests with Rudmann - ČKD mines (type Z-32-R), regulated to low current.


Tests carried out on 25.IV., 5.V., 12.V.1939.


Prototype for small river current (lower Danube)
After the demonstration of the ČKD-Rudmann mines in Galaci, we were invited by the Romanian Ministry of the Navy to demonstrate this year a sample device that would also suit the minimum current occurring near the mouth of the Danube. For this purpose, the Romanian Ministry of Defence gave us the opportunity to investigate the current speeds of the Danube bed at all their stations along the Danube from Stara Ršava to Valçova. According to the tables sent to us, the velocities are in the range of 1-0,5 m/sec.


The existing instruments supplied by CKD, i.e. series I, series II and the Yugoslav series, were built for currents greater than 1 m/sec. During experimental throwing of such devices into currents lower than 1 m/s it was shown that in some cases the mines ceased to function as far as the depth below the surface was concerned. At current values even lower, e.g. 0.8-0.9 m/sec, the mine sank to the bottom. The value of 0.8 is critical because the flow of water in the river is not laminar but fluctuates around a mean value, notwithstanding the fact that the current at the surface is somewhat lower than at a depth of 1 m below the surface, because in the vertical profile from the surface to the bottom of the river the current rises to a certain maximum, then falls and is at its lowest again at the bottom. This unevenness of the current, together with the turbulence of the water in the river bed, has a very unfavourable effect on the navigation function of the Rudmann principle mine.


Thus, if a mine of the present design was thrown into a current of 0.8 m/sec, then due to the fluctuation of the current, the current velocity at a certain moment drops to, for example, 0.7 m/sec and at this critical moment the mine sinks below the water and does not reappear on the surface. It is not impossible that in deeper layers with a higher current speed it will settle so that it does not fall to the bottom, but its navigation function is not fulfilled because it does not maintain the tolerated depth prescribed, i.e. 75 cm below the surface.


Mine series I and series II, supplied by the Czechoslovak Republic. were designed for the Bratislava Danube, which has a considerable gradient and an annual average current of 1.7 m/s, i.e. considerably high /currents less than 1 m are almost non-existent/, the requirement of a function in a small current was not an option at all. In the lower Romanian Danube, however, the current velocity is between 0.5 and 1 m/sec, while at the same time the depths of the river bed, especially nearer the mouth of the Danube (Galac, Braila and Ismail), are on average 25, maximum, up to 40 m. The combination of 0.5 m/sec current velocity and 40 m river depth is extremely unfavourable for the operation of this Rudmann principle instrument. Demonstrations on a tugboat in Galati have shown by successive anchor drops to depths that the mine, with proper head relief, works, with the original wing design, to depths of 0 to 10 m. At greater depths, the mine no longer maintains the prescribed striking depth, due to the heavy load on the head from the rope resistance, which increases directly with depth. On the basis of this experience and theoretical studies, a prototype for low current was designed, differing from the original design by the following changes:


1.
Increasing the lifting force, by increasing the wings 1 1/2 times while keeping the centre of gravity of the dynamic lift.


2.
Lightening the mine weight in the water to 0.39 kg /for Bratislava it was 1.9 according to the former method of balancing/*


3.
Increasing the stability of the float by moving the centre of gravity to the stern.


4.
A more perfect way of static balancing of the body centre of gravity in the water, in the water tank, allowing to make the same sailing characteristics for all pieces, in advance already in the CKD workshops. In this way, the previously long handover of the instruments on the river can be eliminated and the handover can be shortened as much as possible in the eventual series.


Practical testing of the designs ad 1-4 was carried out in the Vltava in Štěchovice.



Tests of navigation function in small current.


1.
Litter with a balanced instrument for the Bratislava Danube. Current velocity measured at 0.6 m/sec, after throwing the instrument the mine sank to the bottom after normal filling of the head with water, without maintaining the impact depth. Thus, the mine with the design for the Bratislava Danube did not work in the given current.


2.
The balance set was tested so that the mine had its weight gradually lightened in the water:
a/ 1.9 kg
b/ 1.2 kg
c/ 0,8 kg
d/ 0,2 kg


In the Bratislava current, the weight of the mine in the water was 0.9 kg (1.9 ??) and the mine worked. Here, at a current of 0.6 m/sec, the mine sank to the bottom in all 3 cases a, b, c. At balance d, i.e. 0.2 kg of weight, the mine remained near the surface, but the mean impact depth was too great (ca. 90 cm). A suitable weight in the rear rudder translated the centre of gravity 160 mm aft, whereupon the mine then maintained the correct mean depth of impact tolerance of 75 cm. The mine, thus tested and compliant, was weighed in the water bath in the factory laboratory by weight on a pulley and found to weigh 0.39 kg in the water.


3.
With the mine thus adjusted in the small current, a cast was then made in the higher current, namely 1.8 m/sec, which was found directly below the dam in Štěchovice. The 40 m anchor line remains the same and nothing else has been changed on the mine. The wings in tests ad 2 and ad 3 were fitted 1 1/2 times larger, so that the centre of gravity of the dynamic lift remained at the same point as before. The mine was then observed and the mean impact depth below the surface measured, finding a mean value of 77 cm at a current of 1.8 m/sec. At low current, i.e. 0.6, the mean measured depth was 74 cm. The percent fluctuations in the first case were 0% (small current), in the second case 4% (large current), or all below the permissible limit. The permissible limit for the mean impact depth of the Čs. 75±12,5 cm below the surface (for the lower Danube we suggested to the Romanians 75±25 cm, more advantageous for us, the tolerance can be increased there, as it is a ship of large draft). For the percentage of fluctuations up to a current of 2 m/sec, the prescribed allowable value is 20%.


Result
The new method of balancing and the enlarged wings have proved their worth, the stability of the sailing and the dynamic buoyancy have been solved in values allowing a reliable sailing function already from 0.5 m/sec minimum river current. The maximum speed limit at which this mine still works will be determined after hydrometric tests in the water channel in Podbaba. For the time being, as already mentioned, the mine has been tested in a current of 1.8 m/sec, where it has fully passed, so that it is already evident that the mine is suitable for a range far greater than the Romanians require for the lower Danube.


4.
War litter
Mine with new wings fitted and balanced to 0.39 kg weight in the water inserted into the original anchor and warp cast made from the buoy. Water depth at the casting site measured 4 m. Current velocity measured 0.55 m/sec. After the cast, correct function of both the unwinding and the mean depth of impact after settling was observed. The settling time (i.e. the time from the moment of the cast to the moment when the mine assumed the correct depth) lasted 7 minutes. As observed, this time is sufficient to fully unwind the entire line from the anchor in even the weakest current. For this purpose, however, it was necessary to enlarge the head by 1 dm3 and to adjust the beds of the anchor winding shaft in such a way as to make the unwinding of the line from the anchor as easy as possible.


5.
Anchor test with intermediate float
One cast of the full anchor and intermediate float was carried out at a river depth of 4 m and a current of 0,8 m/sec at a random location in Štěchovice. The unwinding of the mine line was correct, also the function of the anchor was correct. The float stayed on the anchor (unwinding of the float should occur only at depths greater than 8 m). The float test at great depth has not yet been carried out due to lack of time. Due to the great weight and the difficulty of handling this anchor, it was found that the throw to great depths could only be carried out by means of a small tugboat, which provides permanent towing service on Lake Vrana. The aim is to carry out a minimum of two tugboat casts to different depths in order to test the handling of the cast and the function of the apparatus.


Conclusion:
From the practical tests carried out so far, it is evident that the following ranges of speeds and depths will be possible in the new design of the ČKD - Rudmann mine:


a/
with the original anchor in the range of 0,5-3 m/sec of current speed,
with the original anchor in the range of 2-10 m of river bottom depth,


b/
with an anchor with an intermediate float in the range of 0,5-3 m/sec of current velocity,
with an anchor with an intermediate float in the range of 2*4-0 m of river bottom depth, i.e. to practically any depth.


The exact definition of the values of the mentioned ranges will be determined by measuring the hydrometric values of the mine body itself and the resistance of the anchor cable in the hydraulic channel, where it will be possible to carry out continuous measurements successively at different current velocities from 0.5 m/sec up to 3 m/sec in the shortest test time.


On the basis of the values thus measured, the values of the filling diagram will be revised, for which so far CKD has been using the diagram of the Engineer Regiment 6, which is not reliable, especially for a small current. It will also be possible theoretically to work out an accurate guarantee for the navigation function at certain precisely defined limiting current speeds and river depths. After the completion of these tests, the prototype will be ready for demonstration on the lower Danube.


Libeň, 18 May 1939.


Source
Extinct archive of VHJ ČKD Praha, fund AS, carton 361, folder 1735 - Rudmann mine.
URL : https://www.valka.cz/CZK-Z-32-R-ricni-mina-t34843#126135 Version : 0
River mine from ČKD for the small stream of the Danube River.


Source
Minei Z 32, published by ČKD after 1932
CZK - Z-32-R (říční mina) - Tabulka č. 10 miny Z-32-R

Tabulka č. 10 miny Z-32-R
CZK - Z-32-R (říční mina) - Tabulka č. 1 miny Z-32-R

Tabulka č. 1 miny Z-32-R
URL : https://www.valka.cz/CZK-Z-32-R-ricni-mina-t34843#126143 Version : 0
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