The Rotary Bowcap

© J. D. Perkins 1999

Many of the submarines designed by the Electric Boat Company featured an interesting device known as the "rotary bowcap". This was used for submarines having four torpedo tubes in the bows. These include Design 19E (Canadian CC1), USN H1-H3, USN K, L and N classes, as well as those built to Design 602 including the British (20), Russian (11), USN H4-H9 and the 8 Italian boats. There were undoubtedly others; Electric Boat was a prolific designer and builder of submarines.

Photo 1 (click on it to get an enlarged view)
[Photo1] A very interesting photo of one of the British H-boats under construction. This shows the arrangement of the torpedo tubes and the structure of the bows particularly well. The circular opening in the side of the forward plating is the starboard Fessenden transducer. Only the ten Montreal-built British boats had this equipment. (RN Submarine Museum)

In boats fitted with the rotary bowcap, the torpedo tubes were arranged with a torpedo tube in each corner of a square with the muzzles slightly proud of the foremost bulkhead. (See Photo 1) In the H-boats this was No.1 main ballast tank. To seal the tubes so that the breech doors could be opened, a circular plate having the same diameter as the hull was pivoted on a shaft on the center line immediately ahead of the muzzles. The shaft itself extended aft into the torpedo compartment and was free to move a few inches fore and aft. To seal the tubes, the plate was drawn tightly against rubber-like flanges on the muzzles by exerting a mechanical pull on the shaft to haul the plate tightly against the muzzles.


F-class arrangements, the H-class were similar (click to get an enlarged view)

To permit torpedoes to be launched, the plate was pierced by two 18-inch circular openings 180-degrees apart. When the bowcap was in the shut position, these openings were aligned top and bottom between the muzzles of the upper and lower pairs of tubes. In this position the openings were hidden, and protected, inside the external casing (See Photo 2).

Photo 2 (click on it to get an enlarged view)
[Photo2] Italian H8. As well as showing an incredible wealth of detail, this is a particularly good view of the bowcap in the shut condition. Also visible is the arrangement of the forward hydroplanes showing how they stowed against the casing and giving a glimpse of the gearing. The finish looks a little rough in spots!

Photo 3 (click on it to get an enlarged view)
[Photo3] An Italian boat under construction. The quality and detail provided by these photos is mouth watering. Here can be see how the bowcap has been rotated to the "Starboad upper, port lower" firing position. In the original the lower opening is clearly visible. Note the wooden packing around the sides of the cast steel stempost. Note to the extended fore planes and their distinctive "whale fin" shape.

The tubes were fired in diagonally opposite pairs by rotating the bowcap 45-degrees to left and right of centre (See Photo 3) using manually operated mechanical gearing fitted between the tubes inside the boat (See Photo 4). As the bowcap had to be slacked off to allow it to turn, all four tubes had to be flooded and equalized before firing could commence. According to one British report, four torpedoes could be fired inside of 55 seconds!

Photo 4 (click on it to get an enlarged view)
[Photo4] British H9 post-war. The upper handwheel between the torpedo tubes is for drawing the bowcap tight against the tube muzzles and for releasing it for rotation. The crank below is used for rotating the bowcap. The disc behind the crank is mounted on the end of the pivot shaft. It served both to show the position of the bowcap and as an interlock for the rear doors. The four "lugs" extending from the rear door locking rings to the ribs on the disc are the actual interlocks. Only when the cap was properly aligned and shut could the rear door locking rings be rotated to the open position.

The highly polished state of the rear doors, gauges and other brightwork is indicative of how clean these boats were kept. The lack of water stains in the drip-trays under the main drains points to the boat being in good mechanical condition. The ladder has been rigged in the forward escape hatchway.

To provide for hydrodynamic fairing in the area of the bows, the front of the bowcap plate was fitted with a bowl-shaped cover that filled the cavity between the forward bulkhead and the stem post. To give adequate support, the bowcap operating shaft was extended forward into a bearing in the rear face of the stem post (See Photo 2).

When the British H8 set off a mine while dived off Holland, the explosion blew the bowcap right off, flattened the starboard tubes, damaged the port ones and ruptured No.1 main ballast tank. The captain of the boat stated that the loss of the heavy bowcap offset the flooding and damage the boat suffered forward and was instrumental in their getting back to the surface. His only concern then was that the four warheads and their sensitive arming pistols were exposed to the effects of the seas.

Last Updated: 1 August, 1999.

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