Torpedo Control Plotting Instrument

Torpedo Control Plotting Instrument (sometimes called a Bearing Rate Instrument) is a generic-sounding name for a specific series of British torpedo control instruments. They were manufactured by Elliott Brothers to be positioned in the Conning Towers or Torpedo Control Towers of capital ships where they would plot bearings and then calculate the deflection to be placed on a Torpedo Deflection Sight. Somewhat similar was the Dreyer Torpedo Control Table, an adapted version of the fire control table.

Bearing Rate Instrument

Initial Design and Testing

The first two articles may have been built for trial in Queen Mary and King George V in 1913.

The devices consisted of a bearing-taking instrument and a plotter/calculator. The bearing taker, generally the rangefinder trainer in the TCT or a man at a periscope would train upon the target, his instrument thereby passing the relative bearing to the calculator over a flexible shaft. A gyro compass receiver would rectify these to compass bearings, and a clockwork Bearing Plot very similar to that in a Dreyer table would record the bearings when a plunger on the calculator was worked. A bearing rate grid could be aligned to the trend on the plot, indicating the corresponding rate (up to 15 degrees per minute, increasing or decreasing on a Mark II device).

A Forbes Log Indicator or similar means would supply own ship's speed for entry, and a bearing dial would show the relative bearing of the target. The other details of the calculator appear to advance across the Marks.

Mark I

Periscope

The Mark I instrument was equipped with its own periscope by which the target bearing would be observed. This periscope was regarded as optically inadequate, but there was no consensus on a magnification or field of view that would be ideal, but a majority felt that binoculars with a power of 6 would be an improvement.^{[Citation needed]}

The calculator was a simple double slide rule, presumably taking range, bearing rate, bearing and own speed as inputs somehow to create a deflection output.

By the end of 1916, TCP Mark Is were supplied to ships of the Orion and Colossus classes as well as Lion, Princess Royal, Erin and possible Agincourt to be fitted in their CTs.

By mid 1919, the magnification of the Mark I's periscope was judged to be too weak and it was recommended that a new lens might resolve the issue. One periscope was converted to a power of 5 and field of view of 6.5 degrees. Other ships received Pattern 343 binoculars with a power of 6.

Mark II

T.C.P. Mark II with drum-based calculator

By the end of 1915, Mark II TCPs had been supplied to the four ships of the Iron Duke class, and Centurion, Warspite, Queen Elizabeth and Tiger for trial in their TCTs.

By the end of 1916, these ships were augmented by Ajax, King George V (still with her experimental set from 1913), Canada, Barham, Valiant, Malaya, all of the Revenge class except perhaps Ramillies (who likely was to receive a Dreyer Torpedo Control Table) — all in their TCTs.

The Bearing Plot similar to that on a Dreyer Fire Control Table Mark III with a rate grid that could show rates as high as 15 degrees per second to either side. A gyro compass receiver would rectify relative bearings that were introduced by a flexible shaft where 1 revolution conveyed a half a degree change in bearing. A plunger knob on the device could be pressed when the bearing instrument (a rangefinder or a periscope) was on the target, or a remote clacker hand and wire located near the observing instrument could effect this. A switch permitted the plotting paper drive to start or stop.

The calculator was much improved from the Mark I, taking the form of cyclometric dials. The left side's dials recorded range and bearing rate, generating the Dumaresq Deflection. The right side's inputs required the user to copy in the relative bearing on the large dial and own speed to obtain the speed-across attributable to own ship's motion. This figure would either be added to or subtracted from the dumaresq deflection (as commanded by the add/subtract disc to obtain the torpedo deflection to put on the sight.

By mid 1917, the ships of the Revenge class and remaining 3 ships of the Queen Elizabeth class had been fitted, as well as Ajax and Canada.

They were criticised for having too much backlash in flexible shafting, and for lag in a follow-up between the man working the range-finder or bearing instrument and the man who uses the recording knob which marked the paper. A automatic disc indicated "add/subtract" and an improved calculator.

The "add/subtract" disc, however, was found to leap between its indications under own ship's yaw, and the bearing rate scale was too broad and did not afford enough accuracy at low rates. By the end of 1915, these problems had been overcome for future deliveries.

Mark III

TCP Mark III w/ rangefinder

The Mark III improved on the Mark II by further automating the calculator so it only required range and own ship's speed to be entered manually. Additionally, the bearing taker could himself record bearings on the plotter by using a pneumatic remote handle when his scope was on target.

This model was to be provided all ships built later than those with the Mark I and Mark II or the Dreyer Torpedo Control Table, but the only ships mentioned by name as supplied by them in sources discovered so far are Renown, Glorious and Courageous.

Tricks of the Trade

The Handbook of Torpedo Control, 1916 advised that when the enemy was in line ahead, it might be best to take bearings on a ship ahead of the target vessel while ranging on the target, as this would allow you advance word of his course in future should the line have already begun a turn. When this was not possible operators were encouraged to consider using a dumaresq to forecast the deflection to use when a presumed mean heading during the time-of-flight was attained.

Performance

Alteration proposed in 1917

Late in 1916, people were questioning the efficiency of the devices, and in January 1917, the ships fitted with TCPs were asked to report on several questions:

• should the device be in the forward TS, with bearings being sent down electrically?
• should the bearings be taken through a periscope, telescope, or binoculars?
• should those bearings be taken from the Fire Control Top or other such position (rather than in the TCT)?
• how can the periscope of the Mark I be improved in terms of power and field of view?

There was no agreement on the specific points of inquiry, and some ships flatly questioned the device's utility:

• it did not seem accurate enough for the E.R. speed setting for torpedos which emerged after the devices were designed
• the receiver should remain where it is — perhaps a second plot recorder could receive transmitted data?
• the means of taking bearings from the rangefinder trainer's periscope on the Mark II and II devices would be better replaced by binoculars transmitting bearings electrically.
• the Mark I periscope would be better replaced by binoculars with power 6

The proposed alterations were to be prototyped and trialled using equipment from Agincourt. A pair of binocular-based observing instruments would be capable of fast and slow training to acquire and track a target and transmit its relative bearing over step-by-step circuits. The slow, tracking mode was to have 2 arc minute granularity — better even than the tardily-improved bearing steps used by the Dreyer tables which went from 15 minute to 4 minute some time after Jutland.^{[Citation needed]} The gyro compass receiver which would rectify the relative bearings to true bearings would also be in a fine 2 minute granularity.

Each observer would have a key he could press when he was on the target. The key could cause marks on a 6.5 inch wide clockwork-driven bearing plot featuring an improved speed regulator. The paper for the plot would be divided into two separate plotting areas for the two transmitters, and a bearing grid could be used to judge the rate on either transmitter's data. Each observer would also have a bearing indicator showing him where the other observer was training, to permit discrepancies to be detected.

When the observation instruments cannot be placed on the centre line, port and starboard pairs for each would be used with a COS to select which to use.

This device was trialled with the dual plotter similar to that in Ramillies's Dreyer Torpedo Control Table located in the TS, plotting bearings taken from the forward control position and the torpedo control tower.

Abolition?

By mid 1919, a meeting of the Fleet Torpedo Committee decreed that "deflection plotters" should be abolished, as their results, if accurate, were stale. The momentum in torpedo attack from the battle line was progressing toward attack against the entire enemy battleline, based upon its direction of advance. It seems plain that the TCPs were to be encompassed in this decision, along with the existing Dreyer table design.

See Also

• Dreyer Torpedo Control Table

Bibliography

• Admiralty, Gunnery Branch (1917). Handbook of Torpedo Control, 1916. C.B. 302. Copy No. 141 at The National Archives. ADM 186/381.
• H.M.S. Vernon. Annual Report of the Torpedo School, 1916. Originally C.B. 1329. Copy 4 at The National Archives. ADM 189/36.
• H.M.S. Vernon. Annual Report of the Torpedo School, 1917. Originally C.B. 1474. Copy 7 at The National Archives. ADM 189/37.
• H.M.S. Vernon. Annual Report of the Torpedo School, 1918. C.B. 1527. Copy 143 at The National Archives. ADM 189/38.