Definition
Required height of tide equals draught plus desired under-keel clearance minus charted depth. A yacht drawing 1.8 m wanting 0.5 m clearance over a 1.2 m sounding needs a tide of at least 1.1 m. Add allowance for squat, for negative surge in high pressure or offshore wind, for swell reducing instantaneous depth by roughly half the wave height, and for survey age in poorly charted waters.
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