Double Bottom
Twin layer of shell plating with the inner-bottom forming the cargo hold or tank floor; required on most cargo vessels for damage stability and ballast.
Regulatory detail & full definition
A double bottom is a hull configuration in which the ship's bottom is built as two watertight layers — the outer shell plating and an inner bottom (tank top) above it — with the space between divided into tanks used for fuel, ballast or fresh water. The void protects cargo spaces and machinery from flooding if the outer bottom is holed by grounding. MARPOL Annex I, Regulation 19 makes double bottoms (as part of a double-hull arrangement) mandatory on oil tankers above 5,000 DWT, so that a grounding does not breach the cargo tanks directly and cause an oil spill.
Authoritative source: MARPOL Annex I (Oil) ↗
Related
Source: MARPOL Annex I Reg. 19
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