Ship motions impose heavy loads on the deck cargo which would soon move unless secured in place. Containers carried on deck may be secured by twistlock alone. The integrity of the stow then relies on the racking strength of the containers. Up to approx. 50t stackweight, the lowest container is able to carry the transverse and longitudinal forces on top by itself. With higher stackloads, the stack must be “ reinforced” by lashing bars: single or double cross lashings depending on the stackweight. The effectiveness of double lashings is 1.5 times that of single lashing, unless a load-equalising device is fitted. The general principle is that securing in the forward part of the ship are designed to be suitable for forces increased by 20%, unless a breakwater or similar substantial protection is fitted.
The 40ft ends have a lashing gap of minimum 700mm, so that lashing can be applied. If two 20ft containers are positioned on one 40ft place, the gap is 76mm wide and it is not possible to use lashing and the stack weight is limited to 50t.
It is a rare practice to link adjacent stacks of containers. Skiiping the job substantially simplifies loading and discharging operations. It is, however, sometimes necessary to link an outboard stack to the adjacent stack to help resist wind loading.
At a wind speed of 90 miles per hour, the wind force is about 2 tonnes on the side of a 20-foot box and about 4 tonnes on a 40-foot box.
Container ships are likely to be driven hard in order maintain very tight operating schedules. As a consequence, thousands of containers are lost overboard every year and continue to be the source of substantial claims for P&I clubs.