Managing weights and free surface
Every weight loaded, moved or removed changes the ship's stability and trim: weight added low improves stability, weight added high erodes it, and a weight moved off the centreline heels her. The free-surface effect of slack tanks acts as a virtual rise of the centre of gravity and can be large, so tanks are pressed up or run empty rather than left slack, and the number of slack tanks is minimised. Ballast is managed to keep the ship within her approved conditions, correct trim and list, and provide adequate stability and propeller immersion.
- Weight added low improves stability; weight added high reduces it; off-centre weight heels the ship.
- Free surface in slack tanks reduces effective GM — press tanks up or empty, and minimise slack tanks.
- Manage ballast for stability, trim, list and propeller immersion within the approved conditions.
- Use the loading computer to check each condition before and during the operation.
WARNING — Free surface is a hidden loss of stability A slack tank behaves as if G has risen, and 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.
TIP — Check the condition on the computer Before shifting significant weight or ballast, check the resulting condition on the loading computer against the criteria. It is quicker than recovering from a mistake and it is what the examiner expects.