- Duchesne, Tim
- Ship design and development
- RAN Ships
- None noted.
- December 2003 edition of the Naval Historical Review (all rights reserved)
The rapid post-war development and spread of high definition centimetric radar, surface and airborne, denied submarines the safe use of the surface and impelled the USN and RN to develop their own systems designed to permit submarines to run their diesels at periscope depth. The system fitted in the Type XXI U-boat was too advanced to be fitted in the British ‘T’s, ‘S’s and ‘A’s but was incorporated in the designs of the ‘T’ Conversion and the totally new Porpoise Class. Existing operational boats nonetheless had to be given a snorting capability. The developing Cold War lent urgency to this requirement. From the end of the War the Admiralty devoted increasing design effort into meeting this requirement, and ‘snort’ systems began to be fitted to ‘T’s, ‘S’s and ‘A’s from about 1948. As engineering background, these boats were described as ‘direct drive’ submarines; in other words, each propeller shaft consisted of a ‘main engine’ (diesel), occupying much of the submarine’s Engine Room with, further aft in the Motor Room, a ‘main motor’ (electrical) with an ‘engine clutch’ between engine and motor. Next aft came a ‘tail clutch’, followed by a ‘thrust block’, a stern gland (through the pressure hull) and, finally, the propeller.
The normal propulsion configuration on the surface would be to have both clutches ‘in’ on each shaft and the diesels thundering away at whatever revs (rpm) were ordered. A ‘running charge’ for the submarine’s main battery was obtained by adjusting the revs and using the motors as generators – the amount of charge depending upon the proportion of engine load devoted to charging as opposed to propelling. With a fairly full battery, it was sufficient merely to ‘float the load’ (the ‘load’ being the domestic electrical load). To charge the battery in harbour, both tail clutches were taken out permitting the main engines to turn the main motors as generators. At sea, surfaced or snorting, one engine and motor (on the same shaft obviously) could provide a standing charge by taking out the tail clutch on that side. Depending on circumstances, this could be combined with main motor propulsion on the other side, or main engine propulsion to provide a supplementary ‘running charge’.
In contrast, in ‘P’ and ‘O’ Classes, the diesels were pure diesel-generators and mechanically isolated from the propeller shafts. The main motors were fed directly from the battery, which in turn was supplied by the diesel-generators. This configuration also applies to the Collins Class and is known as ‘diesel-electric drive’ (as opposed to the earlier ‘direct drive’). The ‘T’ Conversions – lengthened and streamlined ‘T’s with an additional battery section – could be configured in either ‘direct drive’ or ‘diesel-electric’ modes.
The early RN snort system consisted of a hinged ‘snort mast’ with its base in the casing at the after end of the bridge structure. When lowered, it rested in a cradle let into the casing with the head valve aft. When raised (hydraulically) the snort head valve (a balanced double-seated valve, working on the same principle as a lavatory cistern), the top of the snort mast would be, ideally, just clear of the surface. The ‘snort exhaust’ was a separate but conjoined steel tube with its horizontally bent, open end about four feet lower than the head valve.
To start snorting, the snort mast would be raised and locked in position with a ‘snort mast locking pin’ (manually operated from within the Control Room); the mast and the rest of the induction system between the base of the mast and the ‘snort induction hull valve’ in the Engine Room would then be drained into two ‘snort drain tanks’. While this was going on, the engine on the side selected for the standing charge would be ‘blown round’ and made in all respects ready. The engine clutch on that side would then be put ‘in’ and the tail clutch taken ‘out’. From this point, the propeller on that side was no longer available for propulsion.
When completely drained down, the CO would order, ‘Open the snort induction hull valve’ That having been done, the ERA of the Watch would check that there was no water in the system and open the back-up flap valve. The ER telegraph governing that side would then be put to ‘Standing Charge’. Using a 50lb ‘exhaust mast blow’, the water in the snort exhaust (about 300 gallons) would be blown clear, whilst the Trimming Officer would flood in an equivalent amount from sea into a trimming tank to preserve the boat’s trim.
On the report, ‘Exhaust mast clear’, the ERA would start the diesel. The engine’s exhaust would prevent water from flooding back down the exhaust mast, provided the boat remained at or near its ordered depth. (Any significant depth ‘excursion’ would cause sea pressure to overcome the engine exhaust pressure with disastrous consequences for that engine unless the well-drilled ER team stopped the engine and shut the ‘group exhaust valve’ in time.)
The Motor Room watchkeepers would then, when given the ‘OK’ from the ER, put in the main motor starting switches and load up the diesel so that maximum current started flowing into the grateful battery. The ‘snort induction flap valve’, mentioned above, had a sad and interesting history: When the lost Affray was finally discovered on the bottom of the Hurd Deep in the English Channel, it was noted that the snort mast had fractured and that the submarine was flooded. Water would have poured into the boat at a huge and overwhelming rate through the open ‘snort induction hull valve’ which had a diameter in excess of 12 inches. As one of the many results of the Board of Inquiry, a flap valve was inserted just inboard of the induction hull valve which would be tripped automatically by any inrush of water.