U.S. Mk. 230 Hydrostatic Depth Bomb Fuze

Mk.230 Fuze
The Mk.230 Hydrostatic Fuze was an improved fuze for depth bombs used in anti-submarine warfare and was developed circa 1943. It has a wind vane safety which required air travel before arming. This prevented detonation if submerged accidentally, which evidently was a problem with earlier designs.
The Mk. 230 was used in the 325lb and 350lb depth bombs, and also used in 500lb, 1000lb and 2000lb general purpose bombs.
It was made in different modifications, this one is marked "MOD 4" and dated 1944.

The fuze is made of steel, brass and aluminum, measures about 14 inches long and weighs over 12lbs. Dud fuzes had a reputation of being very dangerous to disarm.

MK53 325lb Depth Bomb Mk.53 325lb Depth Bomb
Overall Length - 54.6" (Including nose fuze)
Diameter - 13.5"
Material - 1/16" Sheet Steel w/ TNT Filler
Fuzes - Mk. 103 Impact Nose Fuze / Mk. 230 Hydrostatic Tail Fuze.

Mk.53 & Mk.54 Depth Bomb
325lb Mk53 and 350lb M54 Depth Bomb w/ Mk.103 Nose and Mk.230 Tail Fuzes

Sectioned Fuze Mk.230 - Mod 4 Function
This is a hydrostatic fuze developed for detonation under the water surface at selectable depths of 25, 50, 75, 100 and 125 feet.
The depth desired is set using a rotating knob on the outside of the fuze body. A pin locks that setting in place.

When dropped, the fuze is armed by a rotating wind vane as it travels through the air. It takes only about 10 turns to complete the arming sequence.

The wind vane cranks a gear system which slowly rotates a brass cup which retains a spring loaded locking pin. After about 10-turns, a cut-out in the cup exposes that pin arming the fuze.

The components in the center set the depth by moving the bellows, adjusting the distance it can  travel.
Safe & Arm

Fuze Bellows

The inertia counterweight (brass) is a safety device. By impact on the water surface it locks to prevent forward movement of the depth spring stem (and piston) and thereby prevent premature functioning of the fuze.
When the fuze has reached the predetermined depth, the hydrostatic pressure is great enough on the bellows to force the piston (moving left) to a point, where a groove in the piston passes over the locking balls, permitting them to shift and release the plunger & compressed firing spring. After igniting the detonator and the 2 lead ins, the main booster charge will explode and then the main charge.