BOAT-COMPARATOR Guide

Draft, Under-Keel Clearance, Air Draft: Three Heights to Know Before You Rent

How much water under the keel, how much safety margin, how much height under bridges: three simple notions that prevent groundings and bad surprises on a rental boat.

Three numbers describe the space a boat occupies vertically — and all three appear in the listing or the rental contract. Knowing them avoids boating’s most expensive mistake: touching the bottom.

Draft: what sits below the waterline

The draft is the immersed height of the boat, from the waterline to its lowest point (keel, drives, propellers). Orders of magnitude: around 50 cm for a RIB, 80 cm to 1.20 m for a motorboat, 1.80 m to 2.20 m for a cruising sailboat, 1.10 m to 1.50 m for a catamaran. First reflex at handover: note that figure — it decides which coves you can enter and where you cut the engine for a swim.

Under-keel clearance: the margin that forgives

The pilot’s foot — the under-keel clearance you deliberately add to the draft — is traditionally 30 cm to 1 m depending on the seabed: generous over rock, more tolerant over sand or mud. That margin absorbs three things the chart does not show: sounding inaccuracies, the trough of the swell that drops the hull between two crests, and dynamic sinkage (“squat”): a boat under way in shallow water sinks deeper the faster it goes. The phenomenon is serious enough that port engineers devote entire formulas to it — the reference synthesis by Briggs, Vantorre and Uliczka on predicting squat for under-keel clearance shows how thoroughly (see the study on Google Scholar). At rental-boat scale, the fix is simple: in shallow water, slow down.

Actual water height: sounding + tide

Chart depths are measured from chart datum, set in France at the lowest astronomical tide: the real height is almost always greater, provided you add the tidal height of the moment — decisive on the Atlantic, where our guides to the tides and to chart reading give the method. In the Mediterranean the tide is small but the bottom shoals fast near beaches: the depth sounder remains your best friend.

Air draft: the height everyone forgets

The air draft is the boat’s height above the water — mast included on a sailboat. You rarely think of it at sea, and always too late in front of a bridge: river passages, canals, power lines and marina gantries all have posted clearances. A 12-metre sailboat carries a mast 15 to 18 metres above the water: coastal road bridges are only crossed where designed for it. When in doubt, the harbour office gives the limiting air draft of each passage.

And if you touch anyway?

Any contact with the bottom, however light, must be reported to the owner — our guide to running aground details the right reflexes and the deposit stakes. But the simplest is never to get there: known draft, generous clearance, reduced speed in the shallows. On Boat-Comparator, the draft appears in each offer’s specifications — a filter worth using if your plans aim for the coves.

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Also worth reading

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• License-Free Boat Rental in France: Rules, Prices & Where to Book
• Renting with or without a skipper: how to choose