Does a ferry float on a deckhouse?
Anders Björkman wonders

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Does a ferry float on a deckhouse?

Picture right of the transverse section of a ro-pax ferry illustrates how it floats and heels <37° with water on a car deck in a superstructure above waterline.

With no water on the car deck the ferry is evidently upright with its centre of gravity G above the hull centre of buoyancy B.

With water on the car deck it heels, G shifts to the heeling side and B shifts as well - there is balance and a stable condition - with heel!

Easy to show with proper stability calculations.

The next picture right shows what happens when more water is loaded on the car deck and when the (green) deckhouse becomes submerged below the waterline.

G shifts outside of the hull centre of buoyancy B and the ferry will capsize (turn upside down), unless it floats on the watertight deckhouse! Heiwa Co described it in a book 1998.

The ferry will only float (and never sink) and will not capsize as long as the red part of the deckhouse is 100% watertight!

Does anybody believe that the Estonia deckhouse was watertight? There were 200 windows in the side between decks 4 and 7!


No ferry has ever floated on a deckhouse! According international standards a deckhouse is immediately flooded when submerged and does not provide any buoyancy.

Estonia had about 50 windows in the side of deck 4 with a total area of abt 18 m² or just 5% of the total wall area. The window panes brake at about 0.1-0.2 bar pressure and the inflow of water is then of the order 4 800 m3/min. When the windows of deck 5 comes under water the inflow rate is the same, while it increases to say 8 400 m3/min for the windows at deck 4 as they are submerged more. Total inflow is then of the order 13 200 m3/min. When the windows of deck 6 comes under water the inflow there is 4 800 m3/min (same as deck 4 in the beginning) while the inflow at deck 5 becomes 8 400 m3/min as they are submerged more and 10 800 m3/min for the windows at deck 4 as they are now submerged most. Total inflow into the deckhouse is then 22 400 m3/min and it increases rapidly when the side is submerged more. As the total volume of the deckhouse is abt 22 000 m3, it is clear that the deckhouse fills up 100% within less than two minutes ... and the ferry capsizes like Herald of Free Enterprise. It is not possible that Estonia floats on the deckhouse for 20-25 minutes full-scale (or 3-4 minutes model-scale) as in the SSPA tests!

So how happened that the SSPA model didn't capsize with a submerged non-watertight deckhouse? Was the deckhouse arranged with hidden floating aids inside (like some rubber balloons that were deflated during the tests or some other arrangement to prevent water inflow and escape of air to fake slow flooding of the deckhouse and associated slow heeling and sinking)?

The deckhouse (picture left) has no windows or openings in the port side at all (!) that would allow air to escape when the starboard side becomes submerged and the deckhouse is flooded with water.

SSPA was supposed to provide full description of the deckhouse and associated stability calculations by 5 May 2008 to be further discussed at an international ship safety symposium on 23 May 2008 at Stockholm.

SSPA was supposed to also explain how the compressed air trapped in the hull after capsize managed to escape allowing the model floating upside down to actually sink.

However, it was never done! SSPA cheated and allowed air to escape from the capsized hull floating upside down through hidden valves!

Furthermore, all documentation promised to be available on the SSPA web site disappeared. The owners of SSPA, Chalmers University of Technology, was simply asked by the Swedish government and Vinnova to fake all the research!

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