So, the U-552 was built in the shipyard of Blohm und Voss, in Hamburg on September 25, 1939. U-552 was part of a sub-series of eight vessels, ranging from 551 to 558.
These units were equipped, apparently, with the volumetric compressor driven by gears (not turbo-charged also known as Buchi Super-charged) and as such, used the G.W engines made by Krupp.
Here is what the Type VIIc Manual tells us:
G.W. diesel engine installation.
Diesel engine installation of some Type VII C boats consists of 2 G.W.-engines with associated auxiliary machinery. The engines are single-acting four-cycle with fuel spray injection and supercharging. Both engines are reversible.
G.W. general and performance figures.
Germaniawerft Marine diesel engine, Model e.v. 40/46 with forced induction by Roots type blower
Full load 1400 SHP
Overload 1500 SHP
Maximum load 1600 SHP
Fuel oil consumption
254 kg/hour (full load)
6 x 57.8 liters
Construction of the GW propulsion plant.
Bed plate and engine block (lower part) are constructed as one welded block, which consists of single, vertically standing cast steel frames, which are held down by welded steel plate. The two cylinder blocks (three cylinders each) are connected with this bed plate by means of tie rods passing through flanges on the lower edge of the cylinders. These tie rods relieve the working cylinders of the combustion gas forces and transfer them to the upper part of the raised bed plate. The cylinder heads are pressed by studs on the working cylinders and thus form the upper abutment for the cylinder cans. The fuel oil is injected into each cylinder by a fuel oil injection pump (Bosch design) and a fuel oil injector (type G.W.) into the combustion chamber. Oil from a fuel oil feed pump (gear pump) is transmitted to the individual fuel oil injection pumps. The delivery rate of the fuel oil pumps is controlled by shifting the regulator linkage (changing volume by means of beveled edge). The control regulator linkage can be moved by governor as well as by fuel oil control lever at the operating station.
The governor is
built as a centrifugal governor and is driven by an idler gear from the camshaft drive. Charging (increase in the amount of air to burn) takes place via a Roots type supercharger (G.W. type). The charger is arranged on the clutch side of the engine and is driven from the crankshaft by means of gear wheels through a hydraulic clutch. This hydraulic clutch and interlocking mechanism allows the charger to be used only while driving forward and engaging and disengaging it only when the engine is running. Cylinder barrels, cylinder heads, exhaust valves and exhaust manifolds are cooled by sea water, which is fed by a coupled cooling water pump (piston pump).
The coupled lubricating oil pumps (gear pump) feed lubricating oil that is used to operate the hydraulic governor as well as for lubrication of the crankshaft bearings, cylinder barrels, camshaft bearings, the fuel oil pump control linkage and drive as well as for supercharger blowers. The engine can be reversed either by means of compressed air or by means of a special hand oil pump. Altering the direction is controlled by shifting the cam shaft in the longitudinal direction, so that the cams for the opposite direction of rotation engage push-rod rolls.
Auxiliary machinery associated with G.W. engines.
In order to have a greater air supply available, a Roots type blower is mounted on the clutch side of the engine. The blower is powered from the crankshaft of the engine by means of gear wheels. The blower is driven by means of a wrap-spring coupling, which prevents torsion vibrations and which takes up the distortion of the rotating motion. The supercharger clutch is a double cone type which can be engaged and disengaged from the operation station only while in "Ahead" drive, and only when the engine is running at full operational speed. In order to preserve the friction lining, the operating speed is to be lowered if possible when engaging to on.
Cooling water pump.
The cooling water for cooling the engine oil cooler, cylinder barrels, cylinder heads and the exhaust is fed by a double piston pump, which is at the front of the engine. The drive takes place via a crank, which is driven by gear wheel transmission from the crankshaft of the engine. A safety valve on the discharge side of the pump should respond when outlet valves are closed.
Lubricating oil pump.
The entire engine as well as all important contact surfaces are lubricated by oil fed from lubricating oil pressure line under appropriate, reduced pressure. The lubricating oil pump is driven by the gear wheel of the cooling water pump drive. It is a gear pump and operates in both directions of rotation. The adjustment of oil pressure takes place via a pressure control valve from the operating station.
Fuel oil feed pump.
The fuel oil is supplied by a special feed pump from the gravity fuel oil tank to the fuel oil injection pumps. It is a gear pump and is suitable for both directions of rotation. It is arranged over the camshaft.
Auxiliary cooling water pump.
For the cooling of the engines in case of failure the coupled cooling water pump an electrically driven centrifugal pump is provided in the diesel engine room as an auxiliary cooling water pump. The pump performance is 48 m³/hour of water against 30 meters H2O. The pump housing is directly connected with the associated driving motor.
Hand cooling water pump.
The hand operated water pump is designed as a double-acting piston pump. It draws sea water and discharges it to the cooling water control manifold.
Auxiliary lubricating oil pump.
The auxiliary lubricating oil pump serves for the lubricating of the engines in case of failure of the coupled lubricating oil pumps and before start-up. The pump is an electrically driven vertically arranged screw pump and supplies 38 m³/hour of oil with discharge head of 50 meters H2O and suction head of 5 meters H2O. The auxiliary lubricating oil pump can be also be used for the distribution of fuel oil.
Hand lubricating oil pump.
The hand lubricating oil pump is used to transfer lubricating oil from supply tanks to the collective tanks, and in case of failure of the auxiliary lubricating oil pump to pump oil before the start-up of the engines. It is designed as double acting piston pump and its performance is 4.2 m³/hour.
Governor, filter, lubricating oil cooler, starting air tanks, lubricating oil purifying system.
A centrifugal regulator is used as a governor which is mounted on the clutch side of the engine and is driven by an idler gear from the cam shaft drive. The governor prevents exceeding the maximum permissible engine speed by more than 10 percent. If the maximum permissible engine speed is exceeded, the governor pushes control linkage towards the zero position.
As soon as the number of revolutions decreases, the governor returns to its previous position and the control linkage brings the fuel oil lever back to the operational position by means of a spring.
Fuel oil and lubricating oil filter.
The purifying of the fuel oil is done by means of a system of EC-edge disk filters. To protect these filters against coarse impurities in the fuel oil
a coarse sieve is inserted in the line before the fuel oil gravity tank.
For purifying lubricating oil an EC-edge disk filter is used for each engine. The bearing lubricating oil as well as oil for the clutches and the hydraulic governor is filtered by these filters after branching from the oil cooler.
Lubricating oil cooler.
The entire lubricating oil cycling installation is cooled by a lubricating oil cooler attached to each engine. The cooling effect is achieved by forcing oil through coils, around which sea water flows. The cooling water installation is so designed that oil cooler can be bypassed as a whole or partially.
Starting air tanks.
Starting and reversing the engines is done by means of compressed air at 30 at. The compressed air is stored in dedicated tanks of capacity 200 liters each, connected together to the high pressure distributor (high pressure air bank 1 – high pressure manifold) through the reduction valve. The pressure reducing valve works automatically and holds the pressure in the starting air cylinders at 30 at. The tanks have safety valves set at 30 atmospheres at as well as necessary connecting valves, drain valves and control valve with pressure gauge.
Lubricating oil purifying system.
An electrically driven purifier with a capacity of 250 liters/hour is provided in the diesel engine room for purifying dirty lubricating oil. The purifier is equipped with an electrical oil pre-heater and a hot water pre-heater. Directly coupled with the drive shaft are the pure oil and dirty oil feed pumps. These two geared pumps arranged one after the other. The attached dirty oil pump sucks the contaminated oil from the lubricating oil collecting tanks and passes it over the oil pre-heater into the lubricating oil purifier. The cleaned oil is then pumped by the pure oil pump.
The waste water from the oil purifying process drains into the bilge. A hot water pre-heater is provided for better cleaning of engine lubricating oil.
1-1 Comparison between engines M.A.N. - G.W.
Number of cylinders
Length (whole engine)
Width (whole engine)
Weight to power ratio
Fuel oil consumption (+)
Buchi Exhaust gas turbine B.B.C.
Roots type charger G.W.
Fuel oil regulation
Over flow adjustment
Chamber volume by means of beveled edge adjustment