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]

Deflection Ring Markings
Position	Torpedo Speeds in Knots
(blank)	29, 35, 44.5
Medium	23, 24, 25, 26
Extreme	18, 19, 20, 21

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

1921's new scale fitting for "running abaft"

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.

In 1919, the "alter-course" fitting shown in Plate 108 was criticised because a telescope could not be fitted and the line of sight along the V of the open sight was restricted by the alter course arc. New sights being manufactured remedied this by the sector raised ⁹⁄₁₆ inch and the pointer reduced.

In 1920, suggestions were fielded that it was desirable to be able to set the sights by director angle in addition to torpedo deflection. As there were no more places on the deflection ring for this, it was decided to replace the position of the E.R. setting, as it had recently been abolished. Requests to permit use of gyro bearings on the sight were deferred as being too difficult to accomplish.

In 1921, adjustments to raise the Clear Range Indicator so that it would not foul the telescope and the "V" of the open sight were specified, as was a scale to be added to permit easy adjustments for torpedoes which would run abaft their intended path.

An Extant Article

The editor owns an example that is both missing the sighting scope attachments hinted at here as well as the clear range indicator. It also has a cross-piece attachment on the sighting arm to show angular offsets from the line of aim. The graduations for the deflection ring's three positions are simpler than those indicated above, containing markings for

1. deflections in knots for torpedo speeds of 45 and 50 knots, marked every 1 and labelled every 2
2. deflections in knots for torpedo speeds of 35 and 40 knots, marked every 1 and labelled every 2
3. director angles, marked 0 to 50 degrees each side by ones (with 5s and 10s labelled) and then a final tick to show 55 degrees.

The rotating cross-piece is square in cross-section, can be rolled to any of 3 positions. It has tapped holes on three of its sides to receive vertical posts (four still supplied on mine), arraying them in pairs to left and right of the sighting angle. The holes are marked with a number of degrees to reflect the angle difference between a post on the left and the right. The posts, like the fore sight, have an inlay which I presume has a radium element within. The holes are marked as follows:

1. six holes at 6, 14, and 18 degrees, left and right
2. four holes at 10 and 21 degrees, left and right
3. four holes at 9 and 27 degrees, left and right

The function of these posts is not clear to me, but I would guess they were to be used to assess danger to own ships when firing or to calculate the best means of firing to endanger an enemy fleet as a whole.

The handles to facilitate spinning the deflection dial are, in this example, stiff wire loops drilled into the edge of the dial. A spring-loaded button releases and engages the dial in its three positions.

The device is incredibly massive at 32.5 pounds, and requires careful handling. A cold stress fracture in a clamping piece has caused the sighting arm to wobble a bit. It seems that this weak part of the design may be evidence of its alteration from a torpedo director of unknown pattern, but a slot which may have been for the sliding torpedo speed bar is curiously not aligned along the axis of one of the cruciform arms spanning the ring as is shown in all other drawings.

Its base is labelled "Patt 4608 B", and "T.D.S. Mk III/Muirhead & Co. Ltd./No. 201". The sighting bar is marked "P.A.M. Ltd. Patt 12001." I am supposing Muirhead Ltd. was the company presently called Muirhead Aerospace owned by Alexander Muirhead, and that perhaps P.A.M. Ltd. is the same company by another name or an affiliate.