Stability was of great concern to the Germans. However, according to Schmalenbach the Frahm tanks on PG were almost useless and even counteractive, and making them into passive fuel tanks was the most logical thing to do, and fitting bilge keels aleviated the roll more effectively.
From: Heinrich Evers, Kriegsschiffbau, Springer Verlag, Berlin 1943.
440. Roll Damping Tanks 1.
Roll damping tanks were utilized for the first time in the old English citadel ship “Inflexible”, because her large meta-centric height, produced a very short roll periodicity, made damping mandatory. This consisted of rectangular water tanks arranged athwartships. In the German Navy such tanks were installed on the battle-cruiser “Derfflinger” (Fig. 264a), one small cruiser, torpedo boats, and submarines. In merchant ships, for example, they are used in the steamers of “Deutschland” class. The express steamer “Bremen” also has roll damping tanks (filled with fuel oil) installed, and the bilge keels are entirely omitted.
Fig. 264a shows the older design installed in some ships, Fig. 264b shows the newer form for merchant ships – installed here with the so-called design-stabilizing bulges – and Fig. 264c shows a similar type for U boats.
Fig. 264. Roll damping tanks of various constructions 2.
Both of these designs are similar in principle. The roll damping tank represents a U tube [“communicating tube”] in which a certain mass of water can swing [shift]. The connection between the two vertical legs, in the older version, is a horizontal conduit; in the newer version this is formed by the external water level. The outboard slits in the newer design represent the transfer between connecting conduit and the vertical leg of the older version.
The mode of action of the rolling tank is in principle as follows: The water column in the legs provides a harmonic oscillation whose period (assuming no friction) resolves into
where l represents half the length of the fluid column. The oscillation periods can be adjusted so that they are equal to the roll period of the ship.
If one considers the wave oscillation, the oscillation of the ship, and that of the -water in the tank as an alternating, coupled oscillating system with friction and other forces, then this results in a phase shift of 90 degrees between the oscillating periods of the waves and the ship and a further phase shift of 90 degrees between the oscillating periods of the ship and the fluid column in the tank, overall totaling 180 degrees, i.e., the impulses of the roll damping tank water counteract the impulses of the waves.
In the newer roll damping tanks the flooding of the water is regulated by a gyro-controlled compressor or pumps (active roll damping).
1: Hänert: Angewandte Mechanik (Applied Mechanics), p. 143.
2: From Johow-Förster: Hilfsbuch für den Schiffbau (Reference for ship construction). 5. Auflage (5th edition)
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The incomperable Paul Schmalenbach of “Prinz Eugen” fame verifies in detail the existence of such active “Frahm” tanks aboard. Mr. Frahm himself was in charge of the stability test at Blohm+Voss for BISMARCK! And there is no mention of adjusting or securing any such tanks in the BISMARCK inclining test instruction. They could have even been used in the test itself, had there been any. Here is a quick translation of what Schmalenbach says:
“PG possesses an active roll damping installation which operates according to the principle of communicating tubes [U tube]. The installation consists of two lateral large cells [tanks] positioned in the aft third of section VII. The cells are connected at the bottom by a pipe for boiler feeder-water and on top with a pipe for air. A compressor pushes air into the port and then again into the starboard cell in such a manner that each time the water is pushed into the cell with the rising motion from the cell on the ship’s opposite side. With this the motion is braked or damped. The control of the compressor is initiated by a gyro that is independent of other gyro equipment (compass, artillery, torpedoes). - If the author seems to recall correctly, the equipment was operating only in the spirit for which it was designed, but it was not sufficiently powerful. Therefore, in the winter of 1940/41 a bilge keel was mounted on both sides of the hull and the rolling was immediately lessened, and, then the damping effects of the equipment became adequate. - In order to increase the cruising range – by one fifth! – the equipment was converted from boiler feed-water to fuel oil.
Intentionally switched to the wrong setting, the equipment acts like a roll device, and it is used to exacerbate the conditions during gunnery practice. (Drawing 2. It shows the layout of the generator compartment, turbine 2/3, boiler 1, radio compartment B and anti-roll compartment at 1:200)” From Paul Schmalenbach: Schwerer Kreuzer Prinz Eugen: Tatsachen Bericht, ISBN 3-453-01494-4, pp. 76-77, Wilhelm Heyne Verlag, 1982.