Half tide docks are particularly useful in areas with a large tidal range.
Obviously this was easier to achieve with small ships of shallow draught, or in areas with a large tidal range.
The importance of the sill, and the tide's height above it, is reflected by these dock sills becoming an important local datum level and for tide tables being calculated for heights above it (i.e. clear draught in and out of the dock).
Operation is usually hydraulic, with automatic control; and warning lights or alarms will indicate when the gate-flap is about to be lowered or raised.
Such gate-flap systems are in place in Swansea, Padstow, Douglas, and Peel (the latter two in the Isle of Man).