The bar dries 0.8 m. Your yacht draws 1.5 m and you want 0.5 m under the keel. What height of tide do you need to cross?
A cross-section at low tide: a yacht afloat in deep water beside a sand bar. The top of the bar stands above the sea surface and above the dashed chart datum line. Labels: bar dries 0.8 m, draught 1.5 m, wanted under the keel over the bar 0.5 m.
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2.8 m. A drying height is above chart datum, so the tide has to cover it before you gain any depth. You need 0.8 m to reach the top of the bar, plus 1.5 m of draught, plus 0.5 m of clearance: 2.8 m.
