Weights, trim and free surface
Every weight loaded, moved or discharged changes stability and trim: weight low improves stability, weight high erodes it, and weight off the centreline heels the ship. Trim — the difference between forward and after draughts — is set by the longitudinal position of the weights and affects the ship's handling and propeller immersion. The free-surface effect of slack tanks acts as a virtual rise of G and can be large, so slack tanks are minimised and pressed up or run empty. The loading computer is used to verify every significant change against the criteria.
- Weight low improves stability; weight high erodes it; off-centre weight lists the ship.
- Trim is set by the longitudinal position of weights and affects handling and propeller immersion.
- Free surface in slack tanks reduces effective GM — minimise, press up or empty.
- Verify significant loading and ballast changes on the loading computer against the criteria.
WARNING — Free surface is a hidden loss of stability Several slack tanks together can erode stability dangerously without any change in the weights aboard. Keep tanks pressed up or empty and minimise the number left slack, especially in a seaway.
TIP — Trim affects more than looks Trim changes the ship's handling, her propeller immersion and her fore-and-aft stress distribution. It is managed deliberately with the loading computer, not left to chance.