A person with marginal knowledge of submarines wants to know the difference between an ISWAS and a Banjo. Those of you using a Mark 106 Fire Control System with modern computerized instant data, may be interested in the computers that led up to the solid state electronic wonders that we all take for granted.

Submarine captains of the First World War simply carried in their heads several useful formulae, such as the "Three minute rule." Or, 3 minutes is the time it takes a surface ship traveling at 5 knots to cover 500 yards. This is a slight adjustment to the basic equation, Distance = Time x Speed. The approach officer estimated range by estimating ship's length, considering angle on the bow, then matching the observed length to periscope reticules. Target speed was most often an estimate from bow wake, since sonar turn count data was far in the future. His submarine, both Allied and German, made surface attacks; reserving the boat's ability to dive as an emergency evolution. His weapons were elementary steam powered torpedoes and he was constrained to aim his submarine at his target, thus allowing what would later be known as a zero gyro set-up. Of most importance, was the necessity to get in close. Attacks were typically at night to allow the boat to maneuver within shooting range (around 700 yards) without being detected.

The approach officer's task was to mentally calculate the impact point using his basic knowledge of arithmetic. It is surprising that hits were obtained on a regular basis without the aid of any plot or any instrument to back up the captain's experience and judgment.

During the first part of the Second World War fire control had evolved into a science of angles, speeds and distances. Weapons had gyros, (that usually worked) and depth control planes (that usually didn't). Attacks were normally made by submarines remaining on the surface. The master gyro slave Target Bearing Indicator on the bridge combined a compass card and binocular sight to generate a target's true bearing from the submarine. This meant that plotters in the control room could maintain a geographic plot to represent, own ship, target, target's track and projected point of impact. Additionally, two mechanical instruments were used to aid in obtaining a firing solution. The ISWAS came from its function: There he was and there he is. It was a circular calculator similar to a slide rule that was normally operated by the assistant approach officer. Often it was attached to a cord which hung around his neck. The instrument did the D=RxT equations in a few seconds.

The other instrument used during the early part of the war was the Banjo; so named because of its similarity in size and shape to the musical instrument. Its base plate had templates showing graduations representing the desired torpedo. The most often used templates were for the Mk 14 steam torpedo and the Mk 18 electric torpedo. Attached to the base plate was an own ship's disk (showing an image of own ship) through which an arm extended both downwards (showing gyro angle which was matched to the torpedo characteristics on the template base plate) and upwards (to represent the pseudo torpedo run) to a pivot disk that represented the impact point. This disk was the pivoting anchor for the pseudo torpedo run arm and the target speed arm. This latter arm intersected a third line of sight arm running from the own ship's disk and could slide in relation to the line of sight arm. Thus, the operator manipulated a triangular shaped instrument created from the three arms which could be adjusted to represent the relation between own ship, target's current position (line of sight), speed of target (length of target speed arm) and impact point. The instrument could portray the periscope angle, the torpedo track angle, target course and target speed. The track angle was the angle between own ship's course and pseudo torpedo arm.

The major limitations to the Banjo was its static display and its unwieldy manipulations. Still, during the pre-TDC years it was the submarine's best asset in making rapid fire control decisions.

When the Torpedo Data Computer was installed in US submarines the operator could read a continuously generated solution and could enter target angle on the bow, speed and range. Indicator compass cards rotated to show target course, speed, range, torpedo gyro angle and other information. Attached to the analyzer section was the angle solver which generated the proper torpedo gyro information and spread information.

All of this was a far cry from the tools of the modern fire control system.