Trim is the difference between the draughts forward and aft. When the forward and aft draughts are the same the ship is said to be on an even keel. Trim is an important consideration when loading and/or ballasting the ship. A small trim by the stern is desirable as most ships are expected to handle better in a seaway in this condition.
A trim by the head should be avoided for the following reasons:
- The rudder will be immersed less making the ship difficult to steer.
- More water is likely to be shipped forward.
- Reduced propeller immersion will lessen propulsion efficiency.
- If the ship is pitching (especially in the light condition) the propeller will tend to ‘race’. This accompanied with increased vibration may cause propeller shaft damage.
- Rudder efficiency will be intermittent as the ship pitches.
- Ballast suctions are sited at the aft end of tanks, a head trim will make these impossible to empty completely.
Excessive trim by the stern should also be avoided because:
- The large wind area forward and too deep immersion of the stern will make the ship difficult to steer.
- Pitching may be excessive in heavy weather causing excessive panting and pounding (this will be evident regardless of trim if the forward draught is too small).
- A large blind area will exist forward, especially with an aft bridge, hindering pilotage and reducing lookout effectiveness.
Consider the ship shown in Figure with draughts Fwd. 2.20 m and Aft 2.68 m.
The trim of the ship is: 2.68 – 2.20= 0.48 m by the stern or; 48 cms by the stern
The same ship is now floating with draughts Fwd 2.70 m and Aft 2.32 m
The trim of the ship is: 2.70 – 2.32= 0.38 m by the head, or; 38 cms by the head.