How airbag quantity and working pressure are worked out from a vessel's weight and contact length, and why the bursting-pressure margin matters.
Quantity follows the design load
Sizing starts from the vessel's launching weight distributed over the length of hull actually resting on airbags at any instant. Applying safety and load-distribution factors gives a design load per metre; dividing by the rated bearing capacity of the chosen airbag diameter gives the number of airbags — always rounded up, with a spare in the working set.
Pressure and the safety margin
Working pressure follows from the airbag diameter and the required bearing capacity, and is deliberately kept well below the rated maximum so the bursting-pressure ratio (typically at least 3.0×) is preserved throughout the launch. The whole calculation is carried out per vessel to ISO 14409 — never guessed from a previous job.