Question

(a) Why is it important in a tender ship to keep the double bottom tanks pressed up? (6)

(b) A double bottom tank is 23m long. The half breadths of the top of the tank are 5.5, 4.6, 4.3, 3.7 and 3.0m respectively. When the ship displaces 5350 tonnes, the loss in metacentric height due to free surface is 0.2m. Calculate the density of the liquid in the tank. (10)

Answer 1

Part (a)

In a tender ship, which is characterized by a small metacentric height (GM), it is extremely important to keep the double bottom tanks pressed up. Tender ships have a high rolling period and tend to incline easily, taking longer to return to their upright position. If the double bottom tanks are only partially filled, the liquid inside them can move from side to side with the ship’s motion. This movement causes a shift in the center of gravity from G to a virtual position G1 due to the free surface effect, resulting in a virtual loss of GM.

This reduction in GM diminishes the ship’s righting lever, changing it from (GZ sinθ) to (G1Z1 sinθ), which is smaller. As a result, the moment of statical stability is significantly reduced. In a tender ship, this is particularly dangerous, as it can lead to the ship reaching an angle of loll or even capsizing in severe conditions. Therefore, to minimize the free surface effect and maintain sufficient stability, it is crucial to keep the double bottom tanks either fully pressed up or completely empty, with pressed up being the preferred condition for safety in tender ships.

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