Question

(a) Explain the effect on GM during the filling of a double–bottom tank (6)

(b) A ship of 8,000 tonnes displacement has KM 7.5 m, and KG 7.0 m. A double bottom tank is 12 meters long 15 meters wide and 1 meter deep. The tank is divided longitudinally at the centre line and both sides are full of salt water. Calculate the list if one side is pumped on until it is half empty. (10)

Answer 1

Part (a)

During the filling of a double-bottom (DB) tank, which is located at the bottom of the ship, mass is added down in the vessel. As a result, the center of gravity (G) shifts downward, leading to an increase in GM (metacentric height) and, consequently, an improvement in the ship's stability.

However, if the ship is heeled or trimmed during filling, the liquid inside the tank may collect on the lower side, causing both the center of gravity (G) and the center of buoyancy (B) to shift to that side. This results in a virtual loss of GM and a reduction in stability.

Once the DB tanks are fully pressed up, the liquid is evenly distributed, the ship returns to an upright position, and the center of gravity remains at its lowest point, resulting in maximum GM and optimal stability.

✓ Verified Model Answer (Text Solution)

During the filling of a double-bottom (DB) tank, which is located at the bottom of the ship, mass is added down in the vessel. As a result, the center of gravity (G) shifts downward, leading to an increase in GM (metacentric height) and, consequently, an improvement in the ship's stability.