Calculating safe water depth, under-keel clearance and grounding

When calculating the under-keel reserve, take draught plus half the wave height plus half a metre:

that is the minimum depth of water you need.

With a one-metre wave, the keel dips half a metre deeper in the trough than it does on flat water. Add to that chart inaccuracies, low water driven by persistent wind and shifting sediment in river mouths. That half metre in reserve is not over-caution, it is experience.

Running aground is the most frequently reported cause of loss in boating. The consequences range from a buckled rudder blade through a damaged propeller shaft and gearbox to hull damage with water ingress, and the salvage costs come on top of that.

Grounding is usually a matter for comprehensive hull cover; check whether consequential damage to the drive train and the hull is included. Some policies also pay for craning the boat out for inspection where grounding is suspected, with limits that vary by tariff. Anyone who touches the bottom and carries on without checking risks being accused of making the damage worse.

Worked out for your type of boat

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