please help with parsing of 18 down.Clue is: Ship on water only held by chain across harbour.6 letters.I have ?O?T?M.I am wodernig if the answer is BOTTOM, but can't see why.Many thanks.
//A boom or a chain (also boom defence, harbour chain, river chain, chain boom, boom chain or variants) is an obstacle strung across a navigable stretch of water to control or block navigation.//