CATCHING A TRIM

The process of accurately adjusting the state of a submarine to neutral buoyancy and longitudinal balance is termed ‘catching a trim’. This is an important operation which is carried out when a submarine first dives when going on patrol, having left harbour and started passage on the surface. It provides a datum state for the submarine from which subsequent changes in weight or buoyancy can be easily compensated for, and enabling subsequent checks on trim made regularly during the patrol to be soundly based. The importance of keeping in trim is heightened should a submarine be required to remain quiet for any length of time because the process of catching a trim involves running pumps and so tends to be noisy.

It is desirable that when the submarine first dives at the start of a patrol it should not be grossly out of trim, and this requires that a good approximation to being in trim should be achieved before diving. The first step towards that end should have been taken when the submarine returned to harbour from its previous patrol, because it ought then to have been in good trim. From then on all changes in weight, e.g. due to re-storing, are carefully recorded and adjustments made to the contents of the Trim and Compensation (T & C) Tanks in order to correct for them. A methodical way often used for the purpose is by means of a ‘trim crib’, which has been previously drawn up for the submarine and gives the change in amount of water in the T & C Tanks appropriate to each weight change expected to be made during storing.