Measuring Ship’s Speed Using Pitometer Log


All the navigational equipments that are used for measuring ship’s speed are known as logs. Pitometer log is one such navigational equipment that is used to calculate a ship’s speed relative to water.

Used as an underwater electromagnetic log equipment, pitometer is found on both ships and submarines. Let’s find out the construction, principle and working of pitometer log.


Pitometer log is based on the principle of early navigational method, dead reckoning. In the ancient days, a ship’s speed was measured using navigational tools such as chip log. Chip log was the first navigational tool that introduced the usage of knots in measuring ship’s speed.

Pitometer logs were used extensively during the World War Time, especially in the war ships. The logs were directly integrated in to the ship’s navigation system which also helped the ship’s weaponry system to automatically track the targets.

Construction of Pitometer log

The construction and design of a pitometer log is similar to that of a pilot tube of an aircraft. The there main components of a pitometer log are:

Rodmeter – Also known as pit sword, rod meter is the part pitometer that protrudes from the hull of the ship.

Sea Valve – A valve that is used to prevent the ingress of water into the ship from the rodmeter, generally when it is changed or taken out.

Indicator Transmitter – Coverts the measured parameters to voltage for displaying the ship’s speed.



The long tube of pitometer log is fixed in the hull of the ship. The part of this tube that protrudes out of the water is known as rodmeter. Rodmeter has two faces, one facing the direction of the sea water flow and the other perpendicular to the flow. The face of the opening in the direction of sea water flow measures the dynamic seawater pressure where as the perpendicular face measures the static seawater pressure. A difference of both these pressures is used to measure the ship’s speed.

For measuring the speed of the ship, both the static and dynamic pressures are added and using Bernoulli’s principle the velocity of water is found out. However, it is to note that as the velocity of the ship is derived by using the difference in pressure, the result is never accurate when the ship is moving with slow speed, as the difference in pressure is negligible.

The main advantage of pitometer is that it can provide accurate speed without any fluctuations even in the roughest sea conditions. Moreover, it is robust in construction and has a rapid response time.

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