About unguided torpedoes

This blog post is going to be reminiscent of my much earlier blog post about late propeller era combat aircraft, as it attempts to sum up concisely an obsolete topic. From time to time I gather the necessary motivation for such blog posts out of sheer frustration that I did not find such a summary anywhere.

The introduction of the steam engine in warships by the mid-19th century returned an element of independence from wind that battle fleets outside the Mediterranean and Baltic Sea had not enjoyed for centuries. Soon thereafter, said battle fleets rediscovered both the potency and the difficulties of ramming tactics. More warships have been sunk by accidental ramming than by intentional ramming during the past 150 or so years, but the initial interest in the rediscovered tactic was great and battleships showed off their impressive ramming bows well into the First World War.

A much safer approach to sinking ships close-up was to use the self-propelled torpedo (torpedo initially being a word for naval mines), after its invention by Whitehead in the early 1860's. The earliest torpedoes had indeed such poor characteristics that they were at best a weak replacement for ramming. Later on, more capable torpedoes eventually replaced ramming entirely when improved gunnery made gun firepower unbearable at short ranges.

Torpedoes replaced the hazard of more disastrous accidental collisions with the hazard of possible secondary explosions (if stored above waterline).

The Whitehead torpedo, late 19th century

The torpedo created new warship classes, forced navies to alter their tactical doctrines and provoked quite extreme ideas about its relative merits in comparison to the very expensive gun-armed battleships.

A most important tactical characteristic is that its speed was (and is) not much higher than the target's speed. The best ratio to be expected against a warship fleet in not extraordinarily rough seas was 52 kts versus 18 kts, pitting Japanese high-performance torpedoes of WW2 against a battleship line including the slowest British or American battleships of the time. The ratio between artillery shells and ships is incomparable: More than 1,300 kts to 18 kts.

As a result, the relative position and vectors of movement were very important for the utility of a torpedo. The shorter the running time till impact, the lesser the problems of dispersion, poor fire control and evasion.

Case #1

Case #2

First, an illustration about the use of a torpedo against a ship while on pursuit. The torpedo speed shall be four, both ships' speeds shall be three. The illustration shows the difficulty of catching up. The torpedo takes a usually unacceptable long time to reach the target.

Second an illustration of the opposite case: The pursued ship fires the torpedo on a pursuer. The speed of torpedo and target add up and the torpedo quickly reaches (or misses) its target. Meanwhile, the torpedo user can get away and avoid closing with the pursuer.

The third case shows two ships at equal position, in a broadside duel. The torpedo would need to be fired with deflection and would thus need to run a longer distance than the distance between the combatants. The duration required was usually unsatisfactory under such circumstances.

The fourth illustration shows two ships in a broadside duel with one enjoying the advantage. The speed of both torpedo and target ship add up at least partially and the odds of a hit under such circumstances were often satisfactory unless the combat ranges were long.

Case #4

Case #3

Please note that case three can easily be turned into case four by altering one's course some degrees away from the enemy. The transformation to case four is complete once the aggressive enemy pursues.

The torpedo was favouring the defence in a surface tactical engagement. It was very well-suited as an anti-pursuit weapon.

A 1930's baseline technology torpedo (wet heater type)

The use of torpedoes as envisaged by pre-First World War navies was never very successful: The navies expected seagoing fast and small torpedo boats and torpedo boat destroyers to rush through the friendly battle line towards the enemy battle line well into range for a torpedo attack. Two reactions were advisable for the battle line attacked like this: A simultaneous turn of direction either away from the threat or towards the threat. Either move would expose a much shorter effective target length to the torpedoes. The move towards the threat could protect the vulnerable aft section with its shafts, screws and rudder while the more careful turn away from the threat would exploit case one's dynamic and offer a better chance to avoid a hit altogether.

These overt attacks were historically not very successful, and this is more a problem of the aforementioned reaction (the British Grand Fleet turned away from the torpedo threat in 1916) than of the torpedo's performance.

These overt attacks lacked the one ingredient that proved to be a huge success multiplier in the employment of unguided torpedoes: Surprise.

There were several ways of achieving surprise:

  

(1) Unexpected long effective range (Japanese torpedoes in 1942)

  

(2) Employment by hardly visible vehicles under limited visibility conditions (submarines, torpedo boats, British nightly torpedo bomber attacks in 1941/42)

  

(3) Avoidance of the tell-tale sign of a visible stream of bubbles (German G7e torpedo, Japanese Type 92, 93 and 95 torpedoes).

   

(4) Unexpected high speed, pre-empting evasion manoeuvres (Japanese type 93; up to 52 kts)

The most powerful surface combatant of the Second World War for short and medium range combat was probably the Japanese light cruiser Ōi, which was one of two cruisers re-built into torpedo cruisers. It carried up to forty torpedo tubes for 610 mm Type 93 torpedoes and could have wreaked havoc on a hostile battleship line if positioned as in case four. The range, speed and explosive power came as a surprise to the Allies in 1942, and some cruiser and destroyer battles in the Salomon island campaign were decided by a weaker torpedo salvo than Ōi's twice 20 torpedoes.

The IJN's cruiser Ōi before it was rebuilt into a torpedo cruiser

No submarines were equipped with a comparable torpedo salvo capability, albeit not the least the minelayer submarine designs showed that such a design was likely feasible. The submarine's assumed stealthiness and ability to close with the enemy for aimed torpedo employment and the costs of torpedoes were likely the reasons for this. A typical torpedo costed about as much as an early 1930's fighter plane without engine. The slower, shorter-ranged yet bubble-free electric engine torpedoes with energy storage in batteries were much cheaper. The Japanese torpedoes with highly concentrated oxygen as oxidizer were more expensive, as the engineering problems caused by the aggressive oxygen required a more sophisticated production.

Late-WW2 autopilots for unguided torpedoes such as the German FAT autopilots which made torpedoes run in circles or other patterns in order to increase the chance of a lucky hit after an initial miss were worth their money. Still, this added to the price of an already very expensive piece of ammunition. Torpedoes were in short supply until about 1943 because of the difficulties and expenses of production.

There were two substitutes for surprise in torpedo employment: Numbers and coordinated multi-axis attacks.

Multi-axis attacks were feasible almost exclusively for aircraft, which had enough of a speed advantage over warships to manoeuvre into position quite freely.  A simultaneous torpedo bomber attack from two directions (90° or more angle) left no promising evasive manoeuvre options to the attacked ship's captain. Even a non-simultaneous attack from two torpedo bomber teams offered an advantage, as a turning ship quickly lost speed and could not recover it quickly. The maximum deflection required for a hit during a follow-up attack would thus be smaller than during the first attack.

A note about aerial torpedoes; the first ships were sunk by aerial torpedoes during the First World War, but the device used back then were about as small as today's lightweight anti-submarine torpedoes and it took till the 1930's for really good aerial torpedoes to appear.

First World War: Sopwith Cuckoo releases a torpedo

The mature unguided aerial torpedo was robust enough for a drop from impressive altitudes and at impressive speeds. Some poor designs from the 1930's were still capable of withstanding drops only from ludicrously low altitudes (such as 25 m) and at slow speed. Later, reinforced designs could be dropped from hundreds of metres and at speeds above 400 kph.

These relatively high drops didn't only make the attack much easier for the pilot and possible at night in the first place (altimeters of the 1930's were unreliable at low altitude because ground level air pressure was unknown - radio altimeters solved this problem during WW2). The torpedo travelled at approx. aircraft speed for the seconds till impact on the water, and this was much faster than its own submerged speed. This capability allowed drops from much longer distances (important in face of effective light anti-air weapons) or alternatively made the torpedo speed much less relevant for drops at short ranges.

Aerial torpedo attacks benefited as much from surprise as other torpedo attacks. The Royal Navy's Swordfish torpedo bombers were so very much obsolete in their performance that they had to prefer night attacks, and their successes against both the Bismarck (1941) and the Italian fleet at Taranto (1940) were impressive. German torpedo bombers also preferred low visibility conditions and often timed their attack into a time window of a few minutes at dawn when their targets' silhouettes were easily visible while the two-engined torpedo bombers were hardly visible.

Torpedoes were usually fused by an impact fuse in the nose, but other fuses appeared later and were quite successful. The most notable one was a magnetic fuse. Fuses caused great problems to the British (malfunction of aerial torpedoes, Germans (torpedo crisis early in WW2) and Americans (torpedo crisis till autumn 1942). The old impact fuse had provoked torpedo defence systems for battleships, which often involved an anti-torpedo bulge that added displacement and depth and was meant to absorb the explosion. The ships with such protection were few, though. The great strength of the impact fuse remained; it provoked the damaged ship to list, counter-flood (almost doubling the water intake) or risk capsizing.
The magnetic fuse was meant to detonate under influence of the ship's magnetic field distortions. The explosion was meant to happen below the ship, avoiding the bulges. The proper countermeasure was a triple bottom with empty compartments at the bottom followed by heating oil-filled compartments above. The detonation's power (apparently aided by a bubble effect) would largely be absorbed by the empty compartments. A magnetic fused torpedo's great strength was its ability to cut small ships in half or to cut off end portions of a large ship (even battleships had no substantial armour at their ends, concentrating their armour protection on firepower- and mobility-related compartments only).
The introduction of the tiny and relatively cheap magnetic fuse did thus cause major headaches to battleship and at times also cruiser designers and influenced battleship designs a lot.
 

I hope this will save others the effort and time to accumulate all this from a multitude of sources over time. I release this blog post into the public domain.

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