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Underwater Log Systems
USS Bowfin (SS-287)
Information about the sub
A submarine must operate below the surface a great part of the time while in enemy-controlled waters. She is therefore denied the opportunity to establish her position by means of sights of the sun; and weather conditions often make it impossible to obtain sights of the stars when surfaced at night. Accurate knowledge of her position is vital to the submarine, not only to enable her to avoid the ordinary hazards of navigation, but to assist in her primary mission of locating and destroying the enemy. In order to know her position with accuracy, the submarine must have a means of determining her own speed.

The submarine underwater log system is a device for indicating, in knots, the speed of the ship as it travels through the water, and for recording in nautical miles the distance traveled. The principal components of the system are located in the forward torpedo room, below the light draft water line of the ship.

The rodmeter, commonly called the sword, is located in the forward torpedo room below the light draft water line. It projects through the hull of the ship, into the water, and is the unit in which static and dynamic pressures are produced and transmitted to the other units of the system. When in use, the rodmeter extends into the water for a distance of about 3 feet. Being located in the forward part of the ship, the rodmeter contacts water that is least affected by the movement of the ship or by the turbulence of the water created by the action of the propellers.

The sea valve forms a support for the rodmeter and provides a means of closing the opening through which the rodmeter passes when the rodmeter is withdrawn, or fully housed. It is located in a well below the deck in the forward torpedo room, and is bolted to the inner hull below the light draft water line. A tube extends from the underside of the inner hull to the outer hull where it is welded to a flange and guide bushing. The guide bushing forms the lower support for the rodmeter. When the rodmeter is withdrawn, closing of the sea valve prevents sea water from flooding the forward torpedo room.

The control unit is mounted in the forward torpedo room and is suspended on a gimbal bracket which tends to keep the unit in an upright position regardless of the pitching or rolling of the ship. This unit provides a means of automatically controlling the operation of the rotary distance transmitter. It consists of a sensitive bellows enclosed in a watertight housing, and a set of electrical contacts. The inside of the bellows is hydraulically connected to the pump of the rotary distance transmitter, while the outside of the bellows is connected to the static orifice of the rodmeter. The electrical contacts control the supply of current to the rotary distance transmitter pump drive motor.

The rotary distance transmitter is located in the forward torpedo room, below the light draft water line. It is the unit that develops the force applied to equalize the dynamic pressure produced within the rodmeter. It consists of an electrically driven transtat assembly, an electric motor which drives a centrifugal-type pump, and a distance transmitting unit. By means of these components, rotary motion is transmitted to the master speed indicator, and to the speed and distance indicator.

The master speed indicator is mounted on a panel near the control room steering station. Revolutions, the number of which are proportional to the distance traveled, are received by this unit from one of the self-synchronous transmitters in the rotary distance transmitter. These revolutions are registered on a counter and, by means of a time element, are converted into a speed indication in knots. This indication is transmitted to the speed and distance indicator.

The speed and distance indicator, commonly called the repeater, is mounted in the conning tower. It repeats the speed and distance readings of the master speed indicator.

Some installations of the Pitometer underwater log system include another unit known as the constant frequency supply unit. This unit is designed to supply a constant 60-cycle current at 115 volts to the system.


The Pitometer underwater log, rotary balance type, was made by the Pitometer Log Corporation, New York, New York. This system consisted of five major components, as shown above. Each instrument was watertight, and was designed for either panel or bulkhead mounting.


The underwater log system operates on the principle of hydraulic pressure actuating electrical and mechanical units. These units are so calibrated that hydraulic pressure is translated into terms of speed and distance. The hydraulic pressure acts through the rodmeter. This part of the underwater log system extends through the hull of the ship into the water. There are two passages in the rodmeter. When the ship is at rest, the hydraulic pressure is equal in both passages, and is due only to the weight of the water above the system. This pressure is known as static pressure. As the ship moves forward, the movement creates additional pressure in the forward passage of the rodmeter. This added pressure is known as dynamic pressure, and is the actuating force which operates the system.

The dynamic pressure developed in the forward passage in the rodmeter can be determined by creating a force of known value and using it to equalize the dynamic pressure. The amount of force required to equalize the dynamic pressure is converted into units of speed. These units are registered on indicators calibrated to read in knots, and in distance traveled.

This is the pit sword installed aboard USS Bowfin in the forward torpedo room.














































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