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Keel block 
Summary
A keel block, in marine terms, is a concrete or dense wood cuboid that rests under a ship during a time of repair, construction, or in the event of a dock being drained. The block rests under the keel of a ship.[1]
Purpose edit
The purpose of a keel block is to prevent the ship from sitting directly on the ground and to prevent damage or instability that sitting on the ground may cause.[2]