The Report presents the effect of various subdivision arrangements of ro-pax vessels on the subdivision index. It covers single and double hull sides above car deck with and without buoyancy pontoon of fixed height below the car deck. The width of double sides above car deck read b = 0.2B. Double bottom when not flooded adversely affects damage stability, therefore, calculations were performed solely for damage above the double bottom. The car deck should be of openwork type allowing free flow of air and water. Otherwise the ship is apt to capsize at the initial phase of flooding the inside of the ship. Double sides and double car deck are to improve the subdivision index.
Article  discusses the effect of various subdivision arrangements of ro-pax vessels on damage stability. Old ferries feature subdivision arrangements presented in Figure 1. with the space below the car deck densely subdivided by transverse bulkheads. Most of these compartments were void, not used for the carriage of any cargo or supplies. There were no double side above the car deck. This type of subdivision arrangement was common to the end of the nineties.
Nowadays, space below the car deck is frequently utilised for carrying ro-ro cargo in the long lower hold (Long Lower Hold - LLH), stretching for about half of the ship length. Below the car deck it has double sides, subdivided by transverse bulkheads, usually terminated at the car deck, and no transverse bulkheads in cargo space. For better safety, the double sides should extend above the car deck; see [2, 3].
The reported study chose a ferry built in 1990 (still in service) as a generic ship with calculations made for a number of modified arrangement configurations.
The ship fulfils the requirements of the IMO resolution A.265 , the predecessor of the current harmonised SOLAS . The said resolution does not require reserved buoyancy above the car deck. In the case of a LLH below the car deck, the width of the double sides should be equal to b = 0.2B, while the height of the double bottom should be at least hb = 0.1B. It should be noted that a higher double bottom adversely affects damage stability . In view of damage safety, the height of the double bottom should be as low as possible. The minimum height of the bottom for the ship investigated according to PRS Rules is HB = 1.025 m.