WWI War Bonds Poster

WWII Merchant Ship Torpedo Vulnerability

by Paul F. Watson
December 2011


Introduction: I recently wrote a short research paper on Torpedo Accuracy of U.S. Submarines during WWII. While performing the analysis, I needed to understand the average number of torpedos that actually struck a merchant ship prior to its sinking. This central question & surrounding data motivated the current paper.

The Sinking Problem: There are four event sequences by which a torpedo might sink a ship.

  1. Detonation on side of the hull. Compartments fill until bouyancy is lost.
  2. Detonation on side of the hull. Flooding of side compartment overturns ship.
  3. Detonation below the keel of a ship, breaking it in half. Each half then sinks.
  4. Detonation causing internal explosion. Ship explodes into pieces.

Torpedo Summary: Several different types of torpedos were used during WWII.

  1. Steam powered torpedos (that left a visible trail of bubbles)
  2. Electric powered torpedos
  3. Torpedos that exploded upon impact with the underwater hull.
  4. Torpedos that exploded beneath the hull, by magnetic detection.
In practice, either type of propulsion could be matched up with either type of exploder. The U.S. Mark XIV torpedo had both installed with either or both selectable.

The U.S., Germany and Great Britain all used both types of "exploders" for their torpedos. Contact exploders & magnetic exploders.

Contact exploders caused detonation when a torpedo struck the underwater hull. All major powers used this type of exploder. Both U.S. and German contact exploders were defective when WWII began. Germany quickly corrected their torpedo problems, but the U.S. remained "in denial" until well into the war.

Magnetic exploders caused detonation as the torpedo passed underneath the ship & were designed to break a ship in half by shattering its keel. WWII performance of magnetic exploders was poor. Both Germany & Great Britain discontinued their use after about one year of wartime experience. U.S. submarine command was "in denial" and continued their use through 1943.

Pounds of explosive in the warhead varied by nation. U.S. submarine torpedoes used 500 pounds of TNT, but later changed to torpex which was far more powerful. Torpedos of most other nations were more powerful. German steam torpedos used 1100 pounds of torpex and German electric torpedos used 636 pounds of torpex. A number of sources indicate that of all the navies, the Japanese had the best torpedos.

It is generally true that any torpedo used in WWII was capable of breaking a huge hole in the side of anything less than a battleship. The breached compartment would fill with water. Only a battleship had sufficient protective measures to resist underwater explosions with minimal flooding. (see Anti Torpedo Defense Systems, 1900 to 1940 by Watson at the www.dionysus.biz web site.)

Ship Vulnerability: With that introduction to torpedos, we proceed to the central issue of this paper, i.e. the question of how many torpedos were required to sink a ship.

Methodology: The author compiled a tabular listing of approximately 150 prolonged attacks that resulted in the firing of 775 torpedos achieving 298 reported hits for 65 ships sunk. This list identified the type of target, the number of hits and whether or not sinking occurred. In all cases, data was obtained from war patrol reports written by USN commanders. As such, reported sinkings are inflated by about 30% relative to post war verification studies.

Once the tabular list of attacks was extracted from combat reports, development of statistics was performed by:

  1. Sorting by type of ship
  2. Counting occurrences of each hit count & characterizing by result (sunk or not)
  3. Producing tabular summary of data & percentages.

Results: We may summarize vulnerability by type of ship as follows:

Table 1 below summarizes the probability of sinking based on number of hits.

FREIGHTERInstancesSinkingsProb of Sink
1 hit 32 17 53%
2 hits 15 11 73%
3 hits 7 5 71%
4 hits 2 2 100%
TANKERinstancesSinkingsProbability of Sink
1 hit 8 6 75%
2 hits 3 3 100%
3 hits 2 2 100%
Table 1

Conclusions: Statistics describing the typical number of torpedo hits required to sink ships of different types have been accumulated and presented. For merchant ships, the probability of sinking based on the number of hits received was determined and presented.

A companion paper by the same author was (see www.dionysus.biz) addresses the accuracy of torpedos in terms of probability of hitting as a function of range. The paper also discusses various issues related to the accurate firing of torpedos using early to mid WWII technology.

Sources of Data

Contact the author paul@dionysus.biz by e-mail.
Copyright 2012 by Paul F. Watson
All Rights Reserved

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