chain booms
... that chain booms were used to block rivers and harbours in order to keep enemy ships out or collect tolls?
... that <b id="mwD3A"><a href="./Boom_(navigational_barrier)" id="mwD3E" rel="mw:WikiLink" title="Boom (navigational barrier)">chain booms</a></b> <i id="mwD3I">(pictured)</i> were used to block rivers and harbours in order to keep enemy ships out or collect tolls?