The UMB Mine. The UMB mine was first started in 1941, and it appeared in four forms. It was a moored, contact, chemical- and switch-horn mine, laid by surface craft.

It was an offensive or defensive mine, for use in maximum water depth of 500 feet, against surface craft and submarines. The maximum depth of case when moored is 110 feet.

The Four Forms

UMB with tombac tubing; 1941

UMB with 5-foot chain and mechanical cutter; 1943

UMB with 5-foot chain, mechanical cutter, and cork-floated snag line; 1943/44

UMB with 5-foot chain and two improved mechanical cutters; 1944

Description of Case

Share

Material

Diameter

Charge

90 lb. block-fitted hexanite

Description of External Fittings

Horns

Base plate

Hydrostatic switch covers

Explosive flooder cover

Securing lugs

Snag line (Optional)

79 feet long , secured to center of three-foot length of wire connecting two switch horns. When the mine is so rigged, the chemical horn directly above is blanked off.

The hydrostatic scuttling switch on the upper hemisphere is an anti-shallow-plant hy-drostatic which controls a double-pole switch nromally made to one of its contacts. The hydrostat may be set to any one of four depths: 0, 5, 10, or 15 meters. If, upon laying, the mine moors at a depth shallower than that set on the hydrostat, the explosive floo-der will fire upon closure of the mooring safety switch. If the mine moors correctly, (i.e., at a depth greater than that set on the hydrostat) the switch changes over to the other contact, permanently breaking the flooder circuit.

The hydrostatic arming switch, Figure 40, on the lower hemisphere is designed to open or close the firing circuit when the mine rises above or descends below a depth of six feet. A glycerine-filled dash-pot delays the action of the switch for a period of 20 se-conds. A screw plug, fitted to the center of the switch cover, is painted white when the switch is rigged to operate as described above. If the plug is painted red, however, it in-dicates that the switch has been closed during assembly, being held in that position by a special extension arm added to the screw plug. In this case, the switch will not open under any circumstances.

Operation. The mine takes depth by plummet. The hydrostatic switch closes in six feet of water (if red screw plug is fitted, switch is permanently closed) and, if the mine moors at a depth greater than that set on the anti-shallow-plant hydrostatic switch, the flooder circuit is broken. Dissolution of a soluble plug allows mooring tension to pull out the mooring spindle, closing the mooring safety switch and tripping the booster-release lever, and the mine is armed. A spring-loaded detent is usually fitted to lock the mooring spindle out.

The mine has standard chemical or switch-horn firing. An additional firing method may be incorporated by fitting a "tombac" anti-sweep tubing to the mooring cable. Upward movement of this tubing along the mooring cable, such as might be caused by sweep wire contacting it, will close a switch on the tubing and fire the main charge. Mines fitted with snag lines will not normally be fitted with the "tombac" anti-sweep device nor the locking detent on the mooring spindle.

The mooring safety switch is designed to disarm the mine by opening the firing circuit upon release of mooring tension except when the detent its fitted. The hydrostatic arm-ing switch is also designed to break the firing circuit if fitted with a white screw plug.

Figure 37 – UMB Mine - Cross Section

Figure 38 – UMB Mine

Figure 39 – UMB Mine Afloat

Figure 40 – Hydrostatic Arming Switch for UMB Mine