Abstract: Described are instruments for measuring troll depths of a line, of a known length, drawn through water and having a proximal line end out of the water and a weighted, distal line end in the water. The instruments include a reference to align with the line near the proximal line end to sense the insertion angle of the line. A nonlinear depth scale marks troll depths as a polynomial function of the length and angle of insertion, the angle of insertion measured relative to e.g. a horizontal or vertical reference.

Abstract: An explosively separable casing including a cylindrical afterbody and a circular cover for one end of the afterbody is disclosed. The afterbody has a cylindrical tongue extending longitudinally from one end which is matingly received in a corresponding groove in the cover. The groove is sized to provide a pocket between the end of the tongue and the remainder of the groove so that an explosive can be located therein. A seal is also provided between the tongue and the groove for sealing the pocket from the atmosphere. A frangible holding device is utilized to hold the cover to the afterbody. When the explosive is ignited, the increase in pressure in the pocket causes the cover to be accelerated away from the afterbody.