TRMA Tech Feature of the Month
June 2005

Titanic's Double Bottom
 

Titanic was constructed with a double bottom, giving the lowest part of her hull a “second skin”.  It’s a common belief that the purpose was to make the hull more watertight and contribute to the ship’s being “unsinkable”.   That’s true, but only up to a point.   The double bottom also provided additional strength and was integral to the workings of the ship.

First, let’s look at how Titanic’s double bottom was constructed.  It’s easily visible in the early construction photos, as seen here:

 

The double bottom was essentially a cellular construction. Inside, running fore-and-aft along the ship’s centerline, is the keelson.  This acted as the spine of the ship.    On either side of the keelson, running cross-wise at regular intervals, were vertical steel plates called floors.  Unlike the floor of a building, which is horizontal, the floors in a double bottom were not what you’d stand on.  Instead, they added strength in a perpendicular (cross-wise) direction. The diagram below shows the general construction of such a double bottom:

Additional longitudinal (lengthwise) strength was provided by a number of other steel girders running parallel to the keel, one of which is shown in the middle of the diagram above.  The diagram isn't as clear as it could be: the side girder actually runs along the top of the floors, and is represented by the single line running along the top of the intercostal plates.  The intercostal plates stood vertically between the floors to add rigidity.  (The above diagram, being rather simplistic, also doesn't show how massive the keelson really was, nor does it represent the side girders as thick as they really were.)

The overall result was a cellular bottom construction of great strength and rigidity.   Even more strength and rigidity was provided by the shell plating that formed the outer skin of the hull, and the steel plating on top.  This top plating that covered the double bottom was, in fact, Titanic’s lowest deck surface.  

A ship like Titanic needed massive strength and rigidity in her bottom for several reasons.  First, the hull itself was an important contributor to the strength and rigidity of the whole ship.  The bottom also had to support the weight of the reciprocating engines - each of which weighed 990 tons each - plus the turbine, the shafts, the coal in the bunkers, and much more.  (To provide additional strength under the engines and other areas, the floor plates were thicker steel, and spaced closer together).   A double bottom was also considered essential to protect the ship against grounding. Grounding - the hull coming in contact with the bottom in harbor approaches and coastal waters - was a very real concern at that time.  Charts were often incomplete or in error, and echo-sounding to determine depth had yet to be invented.  Almost every ship could expect to ground at least once during her life, as Olympic did on at least one occasion.  A double bottom would better bear the ship’s weight if partially grounded, and it would prevent flooding in the event the outer hull plating was breached.   In 1912, a ship was more at risk from damage by grounding than any other cause, and so this was a key factor in hull design.  As the drawing below shows, a great deal of damage could be absorbed in such a manner:


Now we come to the usefulness of a the double bottom in another way.  S
ince the bottom of the hull plating had to be watertight, as did the plating across the top, the compartments within the double bottom could also be used as water storage tanks if made watertight on the sides,  This could be accomplished simply by using floor plates without holes where required, and waterproofing the joints.  In total, there were 44 separate compartments within the double bottom.  The majority of the tanks were used for water ballast:  as the ship's coal bunkers were depleted, this expenditure of weight was compensated for by taking on seawater to maintain stability.  (Otherwise, she would have been riding too high out of the water.)  Other tanks stored fresh water for the boilers. 

The tanks could be inspected by means of manholes in the top with covers that were normally bolted in place.  This would have been relatively easy for a man to do, as Titanic's double bottom was over five feet high.  The diagram below shows a cross-section of Titanic's bottom. The large holes reduced the overall weight of the floors without compromising strength, allowed inspection (the space inside was over five feet high), and permitted water to flow freely throughout the tank. The smaller holes near the bottom permitted low levels of water to flow from one compartment to another so the tank could be pumped nearly dry. The tanks could be inspected by means of manholes in the top with covers that were normally bolted in place.

  

This dual purpose of this double bottom gives Titanic's lowest deck its name, the Tank Top. 

For those interested in further information on the arrangement of tanks within Titanic's double bottom, the following article on the Encyclopedia Titanic site is excellent reading.

  http://www.encyclopedia-titanica.org/articles/halpern_deck.pdf 

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