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Height of tide needed to berth with the company's under-keel clearance

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Height of tide needed to berth: deepest draught plus the required clearance, less the charted depthSide profile alongside the berth, vertical to scale. Deepest draught 12.46 m plus the company's required clearance 1.1 m means she needs 13.56 m of water (the mean draught, 12.24 m, is not used). The chart gives 11.2 m below chart datum, so the tide must supply 13.56 − 11.2 = 2.36 m above chart datum before she can berth.tide required2.36 mcharted depth11.2 mwater level neededchart datumdeepest draught12.46 mnot the mean12.24 mrequiredUKC 1.1 malongside: no squatwater needed = 12.46 + 1.1 = 13.56 mtide = 13.56 − 11.2 = 2.36 m

The rule

  • Water needed = deepest draught + the clearance the company requires.
  • The chart already gives the charted depth below chart datum; the tide has to find the rest.
  • Use the deepest draught, not the mean: she arrives trimmed and the deepest end touches first.
  • Lay the required height against the predicted tide curve to get the window when she can berth; keep a margin for wind and barometer.

Formula

  • Height of tide required = deepest draught + required UKC − charted depth

Worked example

Charted depth alongside the berth11.2 m
Deepest arrival draught12.46 m
Mean arrival draught12.24 m
Minimum under-keel clearance required by the company1.1 m
  1. Water needed = deepest draught 12.46 + required clearance 1.1 = 13.56 m (not the mean 12.24 m)
  2. The chart gives 11.2 m below chart datum
  3. Height of tide required = 13.56 − 11.2 = 2.36 m

Answer2.36 m

Common mistake

Leaving the company's clearance out of the sum.

1.26 m of tide puts her keel exactly on the seabed alongside: worth knowing as the absolute floor, never a figure to plan an arrival on.