Torpedo Control Disc

A Torpedo Control DIsc was a compact torpedo-control instrument helpful in solving many torpedo-related problems.

The term is probably a generic one, but the British used it refer to a specific series of instruments identified by Mark.

Mark II

These seem to have supported enemy speeds of up to 28 knots.

These discs were supplied by mid 1918.

The table below indicates which were etched for the Extended Range "E.R." or the new "Medium" range speed setting for each torpedo.

Torpedo Control Discs Mark II Supplied by mid-1918
Stamp No.	for Torpedo Mark	Ship	Number Supplied	Medium?	E.R.?
T. 135	21-in II-II**** E.R. 3	Destroyer Depot Ships	1 for every 4 T.B.D.s attached	Yes	No
T. 136	21-in II***** & IV	Destroyer Depot Ships	1 for every 4 T.B.D.s attached	Yes	No
T. 132	21-in II-II****	Destroyers	2 per ship	Yes	No
T. 135	21-in II-II**** E.R. 3	Destroyers	2 per ship	Yes	No
T. 136	21-in II***** & IV	Destroyers	2 per ship	Yes	No
T. 138	21-in I-I*	Destroyers	2 per ship	Yes	No
T. 139	18-in VII-VII****	Cruisers, Light Cruisers, and Destroyers	2 per ship	Yes	No
T. 140	18-in VII**-VIIIIII E.R. 3	Cruisers, Light Cruisers, and Destroyers	2 per ship	No	Yes
T. 151	21-in II-II**** E.R. 3	Light Cruisers	2 per ship	Yes	Yes
T. 152	21-in II***** & IV	Light Cruisers	2 per ship	Yes	Yes
T. 139	18-in VII-VII****	K class submarines	1 per sub	Yes	No

Torpedo Control Discs Mark II Supplied by mid-1919
Stamp No.	for Torpedo Mark	Notes
T. 211	21-in II-II*****	35 knot, 6,000 yard

Mark II*

Torpedo Control Disc Mark II*

By 1918 or so, because so many variations of Mark II disc available, and the Mark IV* torpedoes now had four speed settings, a new, universal design was desired to reduce the number of discs needed in service.

It was to be a model similar to the T.C.D. Mark II supporting "Xylonite" discs 7.2 inches in diameter and 0.1 inches thick that could be slipped into the device which would bear the markings. A small index hole in this slip-in disc was engaged by a pin brought in by a leaf spring on the back of the frame to lock it into position. The Xlyonite disc was double-sided, so the four-speed torpedoes only required 2 slip-in discs, and the unused one kept safe in a leather wallet in the storage box (which stored 2 T.C.D.s).

The speed of enemy was increased to 35 knots as a precaution against faster enemies, which required the deflection lines to be 0.1 inch apart rather than 0.125 inches apart on the Mark II devices.

The markings were coloured on a white background for easy viewing, and the markings were cleaned up and slightly reworked so that enemy inclination was read off his head, and that inclination and deflection to left or right would be on the stated side of the device rather than reversed. The extreme firing range circles were graduated on both sides of the centre line so that one range graduation on each circle would always be visible.

Slip-in Discs for T.C.D. Mark II* in mid-1919
Stamp No.	for Torpedo Mark	Speed, Endurance side 1	Speed, Endurance side 2
T. 196	21-in IV*	25 knots, 15,000 yards	29 knots, 11,000 yards
T. 197		21 knot, 18,000 yards	35 knot, 6,000 yards
T. 209	21-in Mark V	29 knot, 15,000 yards	35 knot, 10,000 yards

Other Parts for T.C.D. Mark II*
Stamp No.	Component
T. 172	wooden handle for the back face
T. 167	pin the for spanners
T. 203	base plate with distance piece and gradauted ring
T. 167	storage box

Mark III

'Torpedo Control Discs Mark III Supplied by mid-1918
Stamp No.	for Torpedo Mark	Ship	Number Supplied	Medium?	E.R.?
T. 141	21-in II-II**** E.R. 3	Battleships and Battlecruisers	2 per ship	Yes	Yes
T. 142	21-in II***** & IV	Battleships and Battlecruisers	2 per ship	Yes	Yes
T. 143	21-in Weymouth	Agincourt and Erin	2 per ship	Yes	No
T. 144	18-in VI**-VII***** E.R. 3	Battleships and Battlecruisers	2 per ship	No	Yes
T. 145	18-in VII** H - VI**** H	Battleships and Battlecruisers	2 per ship	Yes	No

Torpedo Control Discs Mark III Supplied by mid-1919
Stamp No.	for Torpedo Mark	Notes
T. 205	21-in IV*	29/35 knot, double disc
T. 212	21-in II-II*****	35 knot, 6,000 yards

Mark III*

Similar to the Mark II/II*, the Mark III received a make-over in response to proposals from Resolution:

• a separate man with binoculars should just inclination, as the ranges are too great to do this
• the Mark III was deemed to small to read easily. A 10 inch diameter would be better
• the disc had 2 speed settings now, and this is hard to read, especially when inclinations were not near 90 degrees
• deflection should be marked in single knots, not every 4 knots as now
• the "cyphers" showing thousands of yards are not needed.
• the ranges should be written 3 times along the longer curves
• coloured lines on the enemy bar to facilitate use of Bell's Station Keeper should be omitted
• the arrow of the "Bearing of enemy" line should be continued to the outer edge of the disc so the bearing can be set accurately
• the pieces securing the enemy bar should be made of brass
• black curves might be added for alternative settings so that a different speed setting might be decided upon

These suggestions were taken into account in delivering a new design that also increased the enemy speed limit to 35 knots and added the use of the Xylonite slip-in discs as used in the Mark II* sight. This design was submitted to C-in-C, Grand Fleet and elicited this supplemental list of features from the Fleet Torpedo Committee:

• the binoculars for judging enemy inclination should be used by a separate man, and not fitted to the device in any manner
• the largest possible transparent window should be added to the enemy bar to help read the disc
• limiting enemy speed to 30 knots rather than 35 would permit better spacing of lines
• discs should be engraved with curves showing when possible shot range is the same for different torpedo settings
• discs should be as light as possible, usable by hand or when shipped in a socket
• the "upper part of the circle corresponding to the setting in use should be graduated in degrees for reading off the director angle."

The advice was taken together with a few final graphical improvements to create the Mark III* design. The Fleet Torpedo Committee's report had specified that brass should be used, but this would weigh 14 pounds. Duralumin was chosen as having sufficient resistance to corrosion if cared for with varnish and oil, and the device would weigh 4.25 pounds. The devices were to be kept oiled and boxed when not in use in order to ensure they lasted more than 3 years.

it was proposed in mid 1919 that these be allocated:

• one to each battleship, battlecruiser and cruiser for use in primary control position
• four to Eagle
• six to each of the Admiral class
• one to Defiance and "Actaeon and two to Vernon for instruction

Slip-in Discs for T.C.D. Mark III* in mid-1919
for Torpedo Mark	Speed, Endurance side 1	Speed, Endurance side 2
21-in Mark II-II***	18 knots, 18,000 yards	23 knots, 15,000 yards
21-in Mark II****	19 knots, 18,000 yards	24 knots, 15,000 yards
21-in Mark IV-IV*	25 knots, 15,000 yards	29 knots, 11,000 yards
	21 knots, 18,000 yards	35 knots, 6,000 yards
21-in Mark V	29 knots, 15,000 yards	35 knots, 10,000 yards

Bibliography

• Template:BibUKHandbookOfTorpedoControl1916
• Template:BibUKARTS1917
• Template:BibUKARTS1918