Long before maritime aircraft had made the surface so
unsafe for submarines, the German Navy had been experimenting with closed-cycle engines
with the object of producing a true submarine: one that could operate continuously
submerged with a speed and radius comparable to that of the diesel-engined boats. War had
naturally given impetus to these experiments, and the critical year of 1943 now made
imperative the successful conclusion of this prolonged research work if German submarines
were to continue to operate in the radically changed conditions that now existed.
In fact, the very survival of submarines was at stake, and the introduction of
Schnorchel could only be regarded as a stop gap measure at the best. The most promising
development lay in the Walter turbine, which had been fitted in the experimental submarine
V.80 as long back as 1940, but this installation had encountered many difficulties which
were never completely overcome although sufficient success had been obtained to merit its
adoption.
The
essence of the Walter system was a closed-circuit turbine activated by the thermal energy
produced by the decomposition of a high concentration of hydrogen peroxide (perhydrol).
This resulted in the formation of hot gas under considerable natural pressure quite
sufficient in itself to drive a turbine. The system was a complex one by which the
perhydrol was forced up, by water pressure from its containers stowed at the bottom of the
hull, to a porcelain-] ined chamber where it encountered the catalyst necessary to bring
about its decomposition.
This resulted in steam and
oxygen at a high temperature (1,765°F) which passed to a combustion chamber where they
met to ignite the oil fuel, while water was sprayed on the gas to increase its volume and
descrease its temperature (to 986°F). This combination of gas and steam was then led to
the turbine, and from there to the condenser, where the water was extracted and the
residual carbon dioxide generated in the combustion chamber drawn off.
The
greatest difficulties, however, arose with the manufacture and storage of the perhydrol
which re-acted powerfully to any impurity present. This latter could serve as a catalytic
agent, causing decomposition and a rise of temperature until spontaneous combustion
resulted. Absolute clinical cleanliness was essential in preparing and storing the
concentrate (which was expensive to produce costing approximately eight times as much as
fuel oil), and after a variety of tests flexible synthetic rubber was found to be the most
suitable storage container. For submarine propulsion a further advantage of the Walter
system, beside it being quite independent of external air, was its power/weight ratio of
about 11.25 Ib./S.H.P. which resulted in a compact unit, but this was rather offset by its
high rate of consumption at full speed.
The
first naval boat to receive the Walter turbine, the U.791 had a two-shaft, installation
developing S.H.P. 4,360 for a speed of 19 knots. The boat was not, in the final instance,
commissioned, but proved a suitable test bed from which to plan a production series.
Conventional diesel/electric drive was also provided to expand the radius which, at full
speed, was limited to a little over 200 miles with the Walter turbine.
The type
XVII, therefore, heralded a new generation of German submarines, and were coastal boats
with only a single shaft to which was coupled two Walter geared turbines and the normal
diesel/electric motors. The designed speed of 26 knots with the turbines was not,
evidently, quite attained, but this in no way detracted from what was otherwise a major
advance in submarine propulsion. The two succeeding types, types XVIIB and XVIIG, were
slightly larger but only had one geared turbine coupled to the single shaft, although two
had been originally intended, but even then the submerged speed was in excess of 20 knots.
The type XVIIK. used a closed-cycle diesel engine drawing on stored oxygen but were far
less successful than the turbine boats, and none were finally completed. Limited success
with the Walter system then inspired the ocean-going type XVIII with turbines coupled to
three shafts and diesel/ electric motors on the centre shaft only. This too rapid advance,
however, outstripped progress and the project had to be abandoned.
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