A tender ship has a small metacentric height and a long, slow roll period, giving uncomfortable motion, risk of rolling in resonance and reduced stability reserve. Pressing up (filling completely and venting air from) the double-bottom tanks achieves several things:

• It lowers the centre of gravity of the ship. Water ballast occupies the very low double-bottom, so its addition lowers KG, which increases GM and makes the ship stiffer and more quickly self-righting.
• It removes the free surface effect. A completely full tank (no free liquid surface) contributes no loss of GM, whereas a part-full tank produces a free-surface effect that reduces effective GM. In a tender ship this loss of margin can be dangerous, so tanks are pressed up to eliminate it.
• It improves stability in that the weight is concentrated low and centrally, and it reduces the free-surface damage risk when other tanks are used.
• Pressing up the double-bottom tanks also lowers the centre of buoyancy proportion and helps trim/heel correction and, because the tanks are deep (low) and of small individual breadth, even when ballasting the free-surface effect is small.

Hence for a tender ship ensuring double-bottom tanks are pressed up restores an adequate, safe GM and a more comfortable, lower-amplitude roll.

For geometrically similar ships, the linear scale is the cube root of the displacement ratio:

Linear scale = (2000/6000)^(1/3) = (1/3)^(1/3) = 0.6934.

Speed scales as the square root of the linear dimension: V2 = V1 x sqrt(scale) = 15 x sqrt(0.6934) = 15 x 0.8326 = 12.49 knots. Answer: speed of the 2000 tonne ship is about 12.5 knots.

Wetted surface area scales as the square of the linear dimension:

S2 = S1 x scale^2 = 2500 x (0.6934)^2 = 2500 x 0.4808 = 1202 m2. Answer: wetted surface area about 1202 m2.

Skin resistance R = 0.45 S V^1.83 N, taking V in knots and S in m2:

R = 0.45 x 2500 x 15^1.83.

15^1.83 = 141.9, so R = 0.45 x 2500 x 141.9 = 159,700 N.

Answer: resistance of the 6000 tonne ship is about 160 kN.