Overview of USN and IJN Warship Ballistic Computer Design

by Bradley Fischer
Updated 09 September 2003


The United States Navy and Imperial Japanese Navy both utilized standard analog mechanical ballistic computers during World War II for most of their cruiser and battleship applications. The USN utilizing the Mark 8 rangekeeper for all World War II construction as well as numerous pre-war and refit construction while the IJN using the Type 921 Shagekiban 2 for the following calibers: 41 cm, 36 cm, 20 cm, and 15.5 cm; the calibers utilized in most IJN heavy cruiser and battleships.

Fundamentally, both achieve the same task: calculating future target position and generating gun orders to hit the target ship utilizing available input from such sources as the director, rangefinding equipment, gyros etc. there, however, the similarity ends as both navies’ pursued different philosophies. The USN with greater resources and a long firmly established independent industrial base was able to develop a very advanced fire control system that was highly automated.  The IJN by contrast, had a much younger industrial base and fewer resources to draw upon and thus their fire control system had to rely much more on large group of operators to accomplish the same task.

Systems Overview

The US Navy had several fire control systems in use during World War II, however most share the same general characteristics as the most modern system used by the US Navy’s battleships, the Mark 38 Gun Fire Control System (GFCS). This system is centered on the Mark 8 rangekeeper that is the heart of its operation and is used to compute various data such as target, gun orders, director train, time of flight etc.; all the necessary data to hit a moving target.  The components that fed information into the Mark 8 rangekeeper were:

  1. Mark 38 Director 3
  2. Rangefinding equipment (Stereoscopic rangefinders and radars Mark 3, 8 and 13)
  3. Stable Vertical (Gyro for determining true horizontal plane)
  4. Gyro Compass, and Pitot Log
  5. Miscellaneous Data (wind data, projectile data wear assessment, spot corrections etc.)

The Mark 8 utilized this information to generate the following orders to the guns via electrical output directly:

  1. Elevation Orders
  2. Train Orders
  3. Sight Angle

The Imperial Japanese Navy utilized a different system which was based upon information and techniques practiced by the British Royal Navy.  This data was passed on to the Japanese during the interwar period.  The components that fed information into the Type 92 Shagekiban computer were:

  1. Rangefinders
  2. Type 94 Hoiban 4 director
  3. Type 92 Sokutekiban 5
  4. Ship’s master Compass
  5. Miscellaneous Data (Ballistic corrections, wind correction, spot correction)
Note that the computer in this system computes future target position and basic gun orders, ONLY.  The Type 92 Shagekiban computer then produces the following outputs that are sent back the Type 94 Hoiban director:
  1. Lateral Deflection
  2. Super Elevation 6

The output values from the Type 92 Shagekiban computer are added differentially to director setting and training.  Afterwards, parallax, roll and cross roll corrections are added and the orders are sent to the guns via a follow the pointer system. 7

US Navy Mark 8 Rangekeeper

The Ford Mark 8 rangekeeper is a descendent of the original Mark 1 rangekeeper of the First World War and constituted the state of the art analog mechanical computers for the USN during the Second World War. Operated by a single individual, the panel of the instrument has three distinct sections:

Graphic Plotter

The automatic plotter is used to graph range vs. time (delta R, range rate) as well as spot corrections and salvo times and is well suited for post engagement analysis.

Main Panel

This panel contains the main displays and dials:

  1. Own ship and target course dials
  2. Own ship and target speed
  3. Wind speed and direction
  4. Range rate
  5. Deflection rate
  6. Course change indicator
  7. Time of flight
  8. Advance range
  9. Target and service velocities
  10. Range and deflection corrections (spot corrections).

Side Panel

This section contains the auxiliary dials and displays:

  1. Generated director train
  2. Plot ready signal
  3. Sight deflection
  4. Sight angle
  5. Selected train
  6. Observed director train
  7. Range scale shift
  8. Range receiver
  9. Manual power crank.


The Mark 8’s two primary controls for tracking a target are the Target Course and Target Speed knobs. With these two the operator can more easily adjust the “solution”.  Before the operator can begin tracking a target, he must input an estimated target course and speed which he gets from CIC or one of the aloft directors.  In addition there are some other variables to be inputted manually:

  1. Powder Charge type and Projectile Type
  2. Wind Course
  3. Wind Speed
  4. Range and Deflection Spot Correction 8

After these are entered, the operator can then begin to receive direct input from the Directors, stable verticals, pitot log, and gyros compass. Below is a listing of data received electrically or mechanically in automatic mode: 9

  1. Observed Director Train (deflection rate)
  2. Range
  3. Range Rate
  4. Level (Stable Vertical)
  5. Cross Level (Stable Vertical)
  6. Own Ship Speed  (Pitot Log via the Stable Vertical Control Cabinet)
  7. Own Ship Course (Gyro Compass via the Stable Vertical Control Cabinet)

Once the operator has set up the rangekeeper, and the director and radar have acquired the target, the operator can now track and build a solution on the target. The primary method of tracking is called rate control. This is a method where the operator compares generated range and bearing rates with observed range and bearing rates from the director and radar/rangefinders.

As stated above, on the main panel there are numerous displays representing target and own ship course and speed as well as bearing and range rate dials. 10 The latter are more important in determining the accuracy of the solution while the former are important for adjusting the target course and speed.   While it takes practice and a good knowledge of vector mathematics, the basic principal behind this operation is to compare what the rangekeeper predicts the range and bearing rates are versus what the director and radar are observing; from experience the operator can then adjust the target course and speed to quickly null out the discrepancies between the observed and generated values. The operator can see the error as both generated as well as observed values have their own separate colored needle or pointer on the respective displays, thus the operator merely has to match the needles much like a follow the pointer system for gun laying.

A interesting feature of the Mark 38 GFCS and the Mark 8 rangekeeper is the ability for the system to operate in a closed loop, thus providing automatic feed back to quickly correct the variances in generated vs. observed values. One method is for the director itself to operated in generated mode where by the director automatically follows the target from training orders produced by the rangekeeper and the trainer only has to make corrections using his hand wheel as needed to track the target precisely. His corrections then modify observed deflection rate (director train) showing as an error on the deflection rate dial.

In addition, with the advent of the Mark 8 Mod 2 radar in late 1943,11 the radar range unit receives the generated range rate from the rangekeeper that controls the range line 12 on the radar display automatically. The rangekeeper operator has an auxiliary 3 inch (7.62 cm) scope that he monitors and, as he observes the range line separating from the base of the target pip, he can then modify the target speed and course to regain a proper track and thus maintain an accurate solution. As an additional duty to tracking the target, the operator applies spot corrections in both range and bearing that he receives orally from the primary spotter or gunnery officer.

While the tracking process is going on, the Mark 8 rangekeeper is constantly producing gun orders directly to the turrets via synchroes for automatic gun laying. It is important to understand that this is a continuous process, even if few ranges were added manually, the instrument is constantly developing gun orders. It is because of synchronous transmission and the stable vertical that USN warships effectively had fully stabilized guns for firing in most weather conditions or sea states.

Type 92 Shagekiban Analog Computer

The Aichi Clock Company first produced the Type 92 Shagekiban Low Angle analog computer in 1932. The most noticeable difference between the USN Mark 8 and the Type 92 Shagekiban is that the latter cannot perform all of the tasks of the USN Mark 8 by itself.  Where the director and stable vertical in the USN Mark 38 GFCS are devices to gather data for the Mark 8, the Type 92 Shagekiban must rely on the Hoiban Director, and Sokutekiban to assist in making the necessary calculations. The Type 92 Shagekiban’s panel is arranged for the 7 operators particular needs, and can be broken down into three basic sections:

Graphic Plotter

The plotter has a similar use and function to the American system however it differs in that it also depicts bearing change vs. time (bearing rate).

Range Averaging Panel

This section is used by the operator to select best ranges from the various rangefinders.

Main Panel

This panel contains the heart of the panel and contains the following displays: 13

  1. Range Rate
  2. Present Range
  3. Range Correction (Spot correction)
  4. Present Range
  5. Own Speed
  6. Bearing
  7. Wind (deflection only)
  8. Deflection
  9. Bearing Correction (Spot Correction)
  10. Compass Card
  11. Bearing Rate


The Hosen Shiki Sochi (LA fire control system) is much more dependent on the operation of the Director than is the US system and essentially the functionality of the USN Mark 8 rangekeeper is split up between the Type 92 Shagekiban and the Type 94 Hoiban director along with its associated Type 92 Sokutekiban.  The operation of the Shagekiban is therefore interrelated with its supporting instruments to a much larger degree than the USN rangekeeper.

The Shagekiban receives range information from the rangefinders and target course and speed information from the Sokutekiban. After receiving this information, the Shagekiban now has enough information to generate gun orders. These are then transmitted back to the director where calculations are made for roll in the line of sight, cross roll for elevation and train, and parallax.  Once that is done, the director transmits the orders to the guns via a follow the pointer.

Type 92 Sokutekiban

The Sokutekiban is a device that is unique to the Imperial Fleet and has no equivalent to any other navy.  Mounted aloft on the control deck, two decks down from the director, 14 this instrument is operated in a similar fashion to a director and is used to determine the target’s course and speed for the Shagekiban.  The instrument requires an eight-man crew to operate, whose duties are as follows:

  1. Compass Follow-up
  2. Enemy change of bearing 15
  3. Trainer
  4. Inclination
  5. Target length setting and range difference setting
  6. Inclinometer angle follow-up
  7. Present range follow-up 16
  8. Target speed and target angle follow up transmission

The instrument has two methods by which to calculate course and speed. The first is when the inclinometer is used to measure target course17 relative to own course.  With the course established the speed can be solved; this is calculated with the formula (R + delta R) × sin delta B, 18 where:

R is the new range to the target
delta R is change in range
delta B is change in bearing
If the target’s length is not known with enough confidence, then the calculations for course must be accomplished together with the afore mentioned velocity calculation. This is done using rate integrals along with a constant time mechanism to properly space the time of readings. 19

Now that the target’s course and speed have been calculated, they are sent down to the Shagekiban for calculation of future target position.

Type 92 Shagekiban

The operation of the Type 92 Shagekiban computer is very similar to the operation of the Sokutekiban and requires a 7-man team to operate it, organized as follows:

  1. Supervises and operates the range and bearing plots
  2. Turns the range and range rate hand wheel at his right and left hand respectively, so as to match the range indicated by the control officer and range rate from the slope of the range plot by matching pointers
  3. Follows up future range
  4. Sets initial bearing angle (B), total deflection correction to bearing (indirect fire), setting of own ship’s speed, target speed and target inclination
  5. Follow up compass course as received from the gyrocompass
  6. Follows up bearing rate from the bearing plot
  7. Range averaging man. This operator observes the ranges received from the different rangefinders, cuts out those, which are either inoperative or inconsistent, and provides the selected range for the range plot.

The operation of the Shagekiban computer is broadly similar to the operation of the Mark 8 rangekeeper, however, the Shagekiban requires much more manual input and includes no less than 7 operators vs. the one operator for the Mark 8.  One important feature present in this Japanese computer that is not in the American instrument is the range averaging section.  This section contains a dual pointer dial and indicating lamp, which is illuminated when there is a malfunction with the rangefinder it serves.

Inside the dial, there is a range receiver, a mean range receiver and a range transmitter.  The range receiver receives the signal directly from the rangefinder to which is serves.  The transmitter transmits that same signal to the range averaging unit, while the mean range receiver receives the signal from the range averaging unit.  On the face of the dial there are two pointers, one for the range signal from the rangefinder, and one of the mean range.

The operator uses these to observe patterns in the rangefinders that are feeding data to the Shagekiban.  With these, the operator can eliminate as many as he sees fit if he feels they are unreliable or are not producing “honest ranges”.  The range averaging unit now sends its only output signal directly to the mean range receiver where it is one operator number 2’s job to match his pointer with the average range pointer that he received from the range averaging unit.

The operators now track the target using the following follow the pointer:

  1. Mean Present Range
  2. Range Rate
  3. Bearing Rate
  4. Future Range
  5. Deflection

This method is different than in the USN method of rate control, where the operator can observe errors in the fire control solution by observing variances between generated and observed range and bearing rates.  On the Shagekiban, the operators are merely inputting data or transmitting data via their pointers, thus there is no way of gauging the accuracy of the solution.

The data is now transmitted in the form of elevation and deflection orders directly to the Type 94 Hoiban.  The Hoiban on all Japanese warships serves as the primary control station for the battery and it is from here that the final calculations are added and the guns are fired. The operation of the director is similar for most directors of the era with the exception having a higher manning requirement.

One operator not found in the USN Mark 38 Director has the duty for correcting for the movement of the ship along with parallax error. This operator task uses follow the pointer of the received elevation and deflection orders.  His instrument in the director automatically calculates the corrections for roll, cross roll and parallax. In addition, some ships received gyro installations that could be manually followed up at the director for use at night or obscure horizon 20.


It should come as no surprise that the newer USN Rangekeeper, and for that matter the Mark 38 GFCS, has an edge in operability and flexibility. The US system has the ability to operate in a closed loop fashion allowing the plotting room team to quickly identify target motion changes and apply appropriate corrections.  The newer Japanese systems, particularly the Type 98 Hoiban and Shagekiban on the YAMATO class were more up to date, this system eliminated the Sokutekiban, and however, it was based on the same philosophy and still relied on 7 operators.

 This is not to say that the Japanese systems were inaccurate, certainly the IJN demonstrated their gunnery proficiency during the Guadalcanal campaign, just perhaps not quite as flexible. They did, however, have more points for the introduction of inadvertent errors.  Relying solely on optical range finders, lack of gyro for an artificial horizon, and manual follow-ups on the Sokutekiban, Shagekiban, Hoiban as well as guns themselves.  Those types of errors tended to manifest themselves as battle wore on and crews became fatigued.  This was a problem for both USS MASSACHUSETTS21 and HMS DUKE OF YORK at Casablanca and North Cape, respectively. This could have played a role in Center Force’s battleships dismal performance off Sumar in October 1944.


1 “Type 92” denotes model year, or more appropriately, when the design for the device was started. The year is according to the Imperial Japanese calendar, but subtracting 60 can convert this to the last two digits of the year. Thus the Type 92 Shagekiban was designed in 1932, while the Type 94 Hoiban was designed in 1934.

2 Shagekiban – Low Angle Computer

3 The Mark 38 director, unlike the Mark 34 for cruisers, is unable to generate any gun orders in an auxiliary or back mode.  The turrets, however, do have Mark 3 rangekeepers that function in a similar manner to the Mark 8 and can take in data from operable directors and radars.

4 Hoiban – Low Angle Director

5 Sokutekiban – Quoting from Naval Technical Mission to Japan:  “There is no good English equivalent for the name of this device. The instrument is designed to provide transmissions of target speed and target course to the L. A. table (SHAGEKIBAN). In appearance it is somewhat like a director and has to be laid and trained like a director.”

6 “Super Elevation” is the angle that the gun must be elevated above the line of sight to compensate for the curvature of the trajectory caused by the force of gravity acting on the projectile.

7 Naval Technical Mission to Japan:  Ordnance Targets O-31, pg 13

8 The purpose of the spot correction during the set up phase is to correct for variations of the range tables, in this case cold gun correction as well as an arbitrary ballistic correction. The latter is a table devised by the gunnery department from previous firings to correct for indeterminate errors in the fire control system. After the first salvo, the cold gun correction is typically removed.

9 In an emergency or due to a cut off in electrical or mechanical link, the listed inputs can be added manually.

10 The target and own course dials are grouped together and the pointers are shaped like ships in order to enhance the operator’s situational awareness. In addition, on the own ship dial, there is an additional pointer depicting the observed relative bearing of the target.

11 The Mark 8 Mod 2 radar is really a modification of the Mark 8 Mod 1 where by most of the controls are relocated from the director in to the main battery plot. A nice feature was the Mark 3 Mod 1 range transmitter that automatically sent radar ranges into the Mark 8 Rangekeeper.

An interesting historical side note is that the Mark 8 Mod 2 radar began as a field modification on board the USS NORTH CAROLINA and USS WASHINGTON in November 1943. The results were so encouraging that Bureau of Ordnance developed a modification kit for existing Mark 8 Mod 1 radars that was issued to the fleet in June 1944.  There is evidence that these field modifications to convert to the Mod 2 continued as USS INDIANA describes her Mark 8 in a radar spotting report dated April 1944 as “Modified Mark 8 Mod 1”.  In the report she summarizes the new capabilities that are exactly those of the Mark 8 Mod 2, this predates the Mod 2 kits by almost three months in availability.

12 The range line is the pointing device of the radar display and is where the actual ranges are generated from in the B scope plan view presentation.

13 Each display has an associated crank for use by the operator to input the proper data. Note this is one of the great differences between the Japanese and American units, the American unit is much more automated with most of the data automatically inputted.

14 This author has the bridge diagrams for KONGO and HURUNA and it is clear from these diagrams that the Sokutekiban installation has a restricted arc of train limited to roughly 300 degrees.

15 Apparently the Type 92 Sokutekiban receives its bearing rate data from either the Shagekiban (which in itself comes from the director) or the Hoiban director. It also is not clear as to why this information is needed if the instrument is tracking the target itself.

16 It is not clear where the ranges are received from. This author believes that they are received from the Type 92 Shagekiban after the range-averaging operator has selected the best range.

17 If the target’s length is known as well as the present range, the operator measures apparent length of the ship in the form of a bearing measurement (using the stern as the reference point). The formula is: inclinometer angle = L × Cos Ø / R, where: L is the length of ship, Ø  is target angle and R is present range.

18 The entire formula is (V Cos Ø - V Sin ß) × t = (R + delta R) * Sin delta ß, where Ø  is target angle, ß is the bearing angle and R is present range, delta R is the change in range and delta ß is the change in bearing angle over the period t.  Target angle Ø is assumed to be constant over the period t. The delta R term is often expressed in yards/minute and/or knots: 333 yds/min or 10 knots.

19 The formula used when the inclination is not known is (V Sin Ø - V Cos ß) × t =  R + (R + delta R) × (1 + Cos delta ß).  It is used in conjunction with the formula in Note # 18.

20 Apparently the IJN gyro development was not too advanced and the gyros that were installed in IJN vessels were neither accurate nor reliable.

21 USS MASSACHUSETTS had not yet received her receiver regulators for her turrets, thus they had to operate in “follow the pointer” mode for the entire engagement.  Her receivers were not installed until she returned to the yard after the end of her participation in the invasion of North Africa.


General Works

Campbell, John 1985, Naval Weapons of World War Two, Conway Maritime Press, LTD

Friedman, Norman 1983, US Naval Weapons, Conway Maritime Press, LTD

Sumrall, Robert 1989, Iowa Class Battleships:  Their Design, Weapons and Equipment, United States Naval Institute Press

Official Publications

US Navy Bureau of Ordnance Publications:

   Bulletin of Ordnance Information, No. 3-44

   Bulletin of Ordnance Information, No. 4-44

   Bulletin of Ordnance Information, No. 1-45

   Fire Control Equipment, Fire Control Radar, Mark 8

   Source Book of US Naval Radar, 1947 Chapter 13 and 17

US Navy Technical Mission to Japan, Ordnance Targets O-31, Japanese Surface and General Fire Control

Internet Pages

Eugene Slover Navy Pages @ http://www.eugeneleeslover.com/

These pages are copies of NAVAL ORDNANCE AND GUNNERY VOLUME 2 FIRE CONTROL, Prepared by the Department of Ordnance and Gunnery United States Naval Academy.  Edited and produced by the Bureau of Naval Personnel NavPers 10798-A 1958.


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