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SUBDIVISION INDEX of ropax vessels

Polski Rejestr Statków S.A.
Research & development Division

Gdańsk 80-416
Al. gen. Józefa Hallera 126

email: dr@prs.pl
tel:  +48 58 75 11 248



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 [1] 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 [4], the predecessor of the current harmonised SOLAS [5]. 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 [1]. 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.

full report….




  1. Pawłowski M., Laskowski A. Effect of watertight subdivision on damage stability of ro-ro ferries, Trans RINA, Vol. 156, Part A2, Intl J Maritime Eng, Apr–Jun 2014, pp. A-131–136, DOI No: 10.3940/rina.ijme.a2.2014.283.
  2. Pawłowski M. Subdivision of RO/RO ships for enhanced safety in the damaged condition, Marine Technology, Vol. 36, No. 4, Fall 1999, pp. 194–202.
  3. RINA, Best Ships of 2001, pp. 46-47.
  4. International Maritime Organisation: Regulations on subdivision and stability of passenger ships (as an equivalent to part B of chapter II of the 1960 SOLAS Convention), London, IMO, 1974. This publication contains IMO resolution A.265 (VIII), A.266 (VIII) and explanatory notes.
  5. International Maritime Organisation: SOLAS Convention, Consolidated Edition 2009, IMO, London 2009.
  6. ROROPROB (2000–2003) Probabilistic rules-based optimal design of ro-ro passenger ships, EU Research Project, FP5, DG XII-BRITE.
  7. Ravn Erik Sonne, Probabilistic Damage Stability of Ro-Ro Ships, Phd Thesis, Technical University of Denmark, Department of Mechanical Engineering, Maritime Engineering, Denmark, Kongens Lyngby 2003.