What is Trim and Why Trim by head is avoided onboard Ships?
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.
Trim is the difference in draft between the forward and aft parts of a ship. It can be expressed in terms of centimeters, inches or feet.
Trim by head means that the draft at the bow of the ship is greater than the draft at the stern. This can happen due to several factors such as loading, ballasting, or wave conditions.
Trim by head is generally avoided onboard ships because it can cause several issues, including:
- Increased resistance and drag: When a ship is trimmed by head, the bow is deeper in the water and the stern is higher out of the water. This increases the wetted surface area of the hull, which results in increased resistance and drag, reducing the ship’s speed and fuel efficiency.
- Reduced stability: Trim by head reduces the metacentric height (GM) of the ship, making it less stable. This can increase the risk of capsizing in rough seas or in the event of an accident.
- Steering problems: When a ship is trimmed by head, it can cause steering problems due to the difference in pressure between the bow and stern. This can make it difficult to control the ship, especially in adverse weather conditions.
Therefore, maintaining proper trim is important for the safe and efficient operation of a ship. Most modern ships are equipped with ballast systems that can be used to adjust the trim by transferring water between the fore and aft tanks.
Why Trim by the head should be avoided onboard Ships?
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.
Why Trim by the Stern should be avoided onboard Ships
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.