Torpedo Deflection Sight Mark III

Torpedo Director Pattern 2391a converted to a T.D.S. Mark III.
Note that the deflection ring B is not locked in any of its three positions in this image.

Modified Rings for T.D.S. Mark III
Handles added to alter setting shown in blue.

Modification for T.D.S. Mark III
Modification to show amount of course alteration required to bring sights on.

The Torpedo Deflection Sight Mark III was a British torpedo deflection sight. There is some mystery as to its nature, but I believe this designation was given to all converted torpedo directors other than those for above water tubes (which became Mark I) and the line of sight directors (which became Mark II).

Design

This was to be the universal sight for light cruisers (submerged or above-water firing) and destroyers for fitting in control positions or directly on the tubes.

The directors adapted to this form lost of lot of metal: enemy bar, existing slotted sight bar, and the torpedo bar. In return, it acquired a sighting telescope (probably the Pattern 3341 of 2.5 power, ~25 degree field as adopted in 1915.) and a Clear Range Indicator Mark III.

The slot where the torpedo bar would slide was filled with a piece of metal A. A deflection ring B held in by three clips C and lockable by pin D could be rotated so that any of three deflection scales for various speed settings of the torpedo was in position. The telescope carrier F rode on the sight bar and a pointer G allowed the desired deflection to be read off against ring B.

The entire director would be oriented to match the direction of the torpedo tube plus any gyro angle in effect. In the first image, that would be at the top of the drawing.

This advantage the circular directors and the new 3 sector deflection ring had over the sector-shaped directors that became the T.D.S. Mark I is probably what earned the T.D.S. Mark III the label of "universal".^[Inference]

Allocation

By (T.O. 232/17), they were approved to be issued as follows:

• four to each light cruiser with 21-inch (H.B. or S.L.) for use on fore rangefinder or compass platform and in after control positions.
• Two to Flotilla Leaders and to TBDs "V" class and later for use on bridge
• for use on A.W. training tubes when the supply of earlier types is exhausted.

By July 1919, supply was in this state:

• Four to each light cruiser armed with 21-in H.B. or S.L. torpedoes, to be fitted to fore bridge and after control positions (issuance complete)
• Eight to each of light cruisers Danae class and later (being completed with these supplied)
• Four to each Raleigh class light cruiser
• Four to some "V" class destroyers for use on the fore bridge. (Supplied on completion)
• Two to existing flotilla leaders for use on the fore bridge. (Supply in process)
• Two to nine of the "V" class destroyers for use on the fore bridge. (Supplied on completion)
• Four to flotilla leaders of Scott class and later. (Supply in process)
• Four to "W" class destroyers and later ("W" and "S" were supplied on completion)
• One for each Torpedo Attack Table in Destroyer Depots (Supply in process)

Alterations

By mid 1919, issues with the deflection rings becoming stiff from salt water prompted a modification to add 3 lugs to help move the ring as needed. New sights would be completed with these in place, and the staff of light cruisers were to adapt their existing sights with destroyers having the work performed by their depot.

Similar issues were seen in the need to permit torpedo officers to tell their captain how much more of a turn would be required before the torpedo could be fired, and the work was executed in the same manner.