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Sill
A raised threshold across a harbour, marina, lock or dock entrance that limits or retains water depth.
A marina sill is commonly an underwater barrier designed to retain a minimum water level inside a basin as the tide falls. A sill also forms the lower threshold beneath lock or dock gates and can determine the controlling depth at an entrance.
Safe passage depends on the height of water above the sill, not simply the depth within the basin. Its charted level or drying height, tidal prediction, weather effects, vessel draught and required under-keel clearance must therefore be considered before crossing.
