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Floors and Double Bottoms

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Floors and Double Bottoms

• Bottom Structure– acts as lower flange – along with the keel acts to resist longitudinal

stresses produced by the uneven support of hull by crests and “hollows” of waves at sea

– must be strong enough to withstand concentrated pressures due to dry docking, grounding, and the wgt. of cargo

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Floors

• some of the additional strength and stiffness comes from the “deepening” of transverse frames (ribs) across the bottom of the hull.

• these structural members are called floors

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Floors

• not horizontal

• not like house floors

•Ship floors are not equal to house floors

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Floors

• ship floors are– vertical– transverse– extend across ship– from bilge to bilge– usually at every frame (rib)

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Formation of the “Strength Ring”

• floors along with deck beams, beam brackets, and frames (ribs) complete the “strength ring”– The entire vessel hull forms a structure

similar to a duct keel– This structure is referred to as the hull

girder

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Three Types of Floors

• closed floors

• solid floors

• open or bracket floors

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Closed Floors

• constructed from plates of steel

• tank end members– make tank boundaries

• watertight

• oil tight

• pierced only by piping, then seal welded

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Solid Floors

• like closed floors in that they are fabricated from steel plate

• biggest difference is ...

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Solid Floors

• biggest difference is that solid floors are not watertight or oil-tight

– solid floors have…• lightening holes

• air holes

• limber holes

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Solid Floors

• lightening holes– allow access• for maintenance

• for inspection

– reduce weight– cut along neutral axis

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Solid Floors

• air holes– located at top of floor– allows air, to escape or enter , via vent pipes

(goose neck vents) when loading or discharging or when ballasting or de-ballasting

– air holes allow equalization of pressure • atmosphere escapes through goose neck vents

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Solid Floors

• limber holes– drainage holes

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Open Floors

• constructed mainly from plain angle

• 25% lighter then solid floors

• not allowed under machinery spaces

• not recommended under areas subjected to heavy pounding (from loading or discharging)

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Double Bottom

• compartment at the bottom of the ship between the inner bottom (tank top) and the outer bottom

• cellular (with many “bays”)– think...• ice cube tray

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Double Bottoms

Double bottoms may be Transversely or Longitudinally framed

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Double Bottom

• advantages over single-bottom construction– results in stronger bottom

– tanks can be used to carry fuel oil and ballast

– results in a structure which can withstand bottom damage from grounding, without flooding the holds or machinery spaces, so long as the inner bottom remains intact

– better resists pollution

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