Magnetic - Harbor Defense


♦ Mk 18


Mk 18 is a surface-laid bottom magnetic mine, cylindrical in shape (with the cylinder axis vertical), with a prominent vertical post protruding from it. The mine is provided with wheels beneath the cylinder so that it can be run down a track on board the laying ship. Total weight is 2140 lb, including 1440 lb of HBX-1. Dimensions: diameter 42 in x 72 in maximum height. Mk 18 was exported to U.S. allies in the 1950s, to be used to mine shallow strategic straits. There is no other U.S. ship-laid bottom magnetic mine of the same vintage.

♦ Mk 51

This controlled mine for harbor defense is typically laid in groups of 13. Mk 51 is a vertical cylinder (62 x 37 in), carrying 3275 lb of TNT (total weight is 6200 lb). Mk 51 resembles Mk 18, but Mk 51 is substantially larger. This weapon was probably exported after World War II. Mk 51 Mod 0 was designed to counter conventional submarines, and it was modified postwar (to Mod 1) to counter midgets down to a 30-ton displacement. Mod 1 uses Acoustic System Mk 6, a line of hydrophones. The mine itself has a magnetic induction sensor. The operator ashore, who can monitor 10 hydrophone channels (13 mines/ group), has access to both the acoustic and magnetic sensor outputs. The operator times mine firing based in part on a judgment of the size of target. Target size is indicated in part by the number of mines responding to the target.


Mk 51 Controlled Mines at NSMW-Yorktown-ca 1952
[photo courtesy of John Asmussen]


These controlled mines were stocked at Harbor Defense Units along the East and West coasts of the United States.
The most prominent of these was HDU-Norfolk, operating base for the Hampton Roads area. The entrance to Chesapeake Bay
 was the responsibility of Harbor Entrance Control Post-Ft. During World War II, artillery defenses consisted of 4-16" and 6-6"
 guns, as well as more than a dozen guns of lesser calibers. Story. See this page for more details.



Derick S. Hartshorn - 2009-present
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