Both the British and the Japanese practised torpedo dropping in shallow water, to attack an enemy fleet at anchor. For the Taranto raid, the Fleet Air Arm knew they would have problems because of the shallow harbour, so they arranged a wire drum under the nose of the Swordfish, the wire being attached to the nose of the torpedo, pulling it up as it was launched, so that the torpedo hit the water in a belly flop rather than a nose dive. To stabilise it further they had used breakaway wooden tail fins.
The Japanese Assistant Naval Attaché in Berlin, Lieutenant Commander Takeshi Naito flew to Taranto to assess the raid, and later discussed his findings with Commander Fuchida, who would go on to lead the attack on Pearl Harbor.
THE JAPANESE TYPE 91 TORPEDO
‘Type 91’ refers to the date when the design was started, ‘91’ being the Japanese year 2591, or 1931 AD. When in 1944 (Japanese year 2604) the Type 91 was simplified for ease of production and strengthened, the new model was known as the Type 4.
The Type 91 aircraft torpedo was the model which sank or disabled a large number of US ships at Pearl Harbor. The Japanese claimed that just seven Type 91s had been used to sink HMS Prince of Wales, and seventeen were thought to have struck HMS Repulse, explaining why the latter vessel, dating from the Great War, sank so quickly. Some were of the original type with an explosive charge of 330lbs (150kg), and some the Modification 2, with a charge of 450lbs (204kg).
In 1942 the Mod 3 carried an increased charge of 528lbs (240kg), the Mod 4 (Strong — see below) of 1944, 675lbs (306kg), and the final Modification 7 (Strong) brought the charge to 920lbs (417kg), or almost three times the explosive charge of December 1941. The explosive used was Type 97, being 60 per cent TNT and 40 per cent hexyl. The exploder was an inertia type fitted in the top centre-line of the torpedo, and armed by water travel.
The following US battle damage report of the torpedo hits on USS Houston shows the kind of damage inflicted by these 18in aircraft torpedoes.
On 14 October 1944 off Formosa, at 1641 the 10,000–ton Cleveland class cruiser USS Houston was heeling to port to make a high speed turn to starboard when she was hit amidships by an aircraft torpedo, which detonated in contact with the bottom. Her № 1 propeller shaft was broken, and the drag on the propeller pulled the broken shaft 51/2ft [1.7m] towards the stern, damaging the bulkheads and flooding an engine room. Houston took on a 15-degree list to starboard, and her main deck was awash when she rolled. Some 6400 tons of seawater had entered the hull.
By midday on 16 October her crew had successfully undertaken damage control measures and reduced her list to 8 degrees. They had succeeded in pumping out 1700 tons of water, and the main deck was no longer under water when she rolled. Unfortunately, at 1348 a second Japanese aerial torpedo hit her, this time in the stern. Houston took in another 1100 tons of seawater, but her list to starboard was slightly counteracted and reduced to 6 degrees.
She was towed to Ulithi for temporary repairs, where blankets and lifejackets were stuffed into the gap made by the propeller shaft tube, allowing the flooded engine room to be pumped out. Critical bulkheads were shored up, wooden cofferdams were placed around hatch coamings, and a patch was welded over the hole in the hangar floor. She then proceeded to a dry dock at Manus where she was sufficiently patched up to return to the States.
In 1944, in order to punch through the multi-layer torpedo defence system (TDS) fitted to US capital ships, the Japanese introduced the ‘V’ warhead, which was a shaped charge type. They tested it against a multiple compartment TDS of the type fitted to the US Colorado-class battleships, and the charge punched completely through all layers of the TDS. However, only three of these ‘V’ heads were used in combat before the war ended.
All Type 91 and Type 4 torpedoes were powered by an eight-cylinder radial engine, producing 210bhp and a non-adjustable speed of 41 or 42 knots depending on the model. Cylinders were of phosphor-bronze and the pistons of bronze. The contra-rotating propellers had four blades. To prevent damage to the warhead on dropping, approximately the first 24in (600mm) of the torpedo nose were covered with a rubber sheath 0.4in (10mm) thick, which would shatter into small fragments when it hit the water. They had captured and examined several US Mark 13s with the wooden nose shroud fitted, but Rear Admiral Shoji Naruse, head of the team which designed the Type 91, told American interrogators in 1945 that he did not approve of such a fitting.
The Japanese preferred a length/diameter ratio of 11 or 12:1, and they considered the US Mark 13 to have a relatively poor shape for good underwater stability. They had the experience of testing the very large 27.5in torpedo manufactured for them in around 1900 by Whitehead in Fiume, which had suffered from poor underwater stability due to the same problem.
They had tested Type 91 torpedoes with eight tail fins, which gave steadier water travel characteristics than the four-fin tail, but they discovered that recovery from the initial dive took too long. From 1940 the Japanese had fitted small anti-roll flippers to the sides of their aerial torpedoes, controlled by the second gyro, and reported they were a success. Before this innovation, they had been forced to try to counter rolling by only partially filling the warhead space, to keep the centre of gravity as low as possible. With the anti-rolling stabilisers they could now fill the warhead with explosives, which greatly increased its destructive effect.
The wooden tail appendages were added to stabilise the torpedo after release from the aircraft. They also reduced the risk of the torpedo entering the water rolled slightly to one side, when the horizontal control surfaces would act as rudders and deviate the torpedo from its course. The plywood structure was offered to the tail diagonally, passed over and in front of the fins then pulled back into place, being held by air pressure. For aircraft that carried their torpedo internally, a smaller frame was used.
On water entry, the weak points of the aerial torpedo were the warhead joint, and the torpedo body shell of the engine room/buoyancy chamber. If the entry angle was reduced because of a high-speed/low-altitude drop, the Type 91 risked buckling in the area of the engine/buoyancy chamber. The Modification 3 (Improved) strengthened this area with T-section longitudinal ribs. In Modification 3 (Strong) these reinforcing ribs were changed from T-section to I-section. When the revised Type 4 went into production, it was again reinforced, and maximum launch speed was increased to 400 knots. All the Type 91 and Type 4 torpedoes were controlled by two air-blast, air-sustained gyros. One controlled the steering and the other the anti-roll stabilisers.