(a) State why cargo ships must have a collision bulkhead. (6)

(b) A double bottom tank is 1.2 m deep and has a sounding pipe extending 11 m above the tank top. The tank is filled with oil (rd 0.89) to the top of the sounding pipe. The double bottom floors are spaced 750 mm apart and are connected to the tank top by riveted angles, the rivets having a pitch of 7 diameters. If the maximum allowable stress in the rivets is 30 MN/m, calculate the pressure in kN/m2 on the outer bottom and the diameter of the rivets. (10)