BOAT-COMPARATOR Guide
Going Through a Lock on a Rental Boat: Tides, Wet Basins and Technique
Saint-Malo, Deauville, Honfleur, Arzal: on the Channel and Atlantic coasts, locks are part of the voyage. Tide-based schedules, hand-held lines, turbulence: the manual.
On the Channel-Atlantic seaboard the tidal range exceeds ten metres in places: without locks, half the harbours would be mudflats at low water. The wet basins of Saint-Malo and Deauville, Honfleur’s old dock, the Arzal dam at the mouth of the Vilaine: for a charterer, knowing how to transit a lock is part of the licence to cruise these waters. The manoeuvre is simple — provided you know its rules.
Why locks? A story of tidal range
A lock keeps a basin at constant level while the sea outside rises and falls. The direct consequence: it only opens around high water, at times published by the harbour office and set by the day’s tidal coefficient. First rule of lock transits: they are planned by the tide, not by whim — our guide to Atlantic tides lays the groundwork, and the one on bar entrances completes the picture of sensitive approaches.
Before the chamber: VHF, lights and waiting
Call the harbour office on the VHF channel listed in the ship’s papers or posted at the entrance, and take your place without overtaking anyone: the waiting order is law. Red and green lights command the entrance — you only move on green, never while the gates are moving. Use the wait to prepare the boat: fenders on both sides (you rarely know in advance which side you will lie), one line forward, one aft, crew briefed and boathook ready.
Inside the chamber: lines held, never made fast
This is THE golden rule: in a lock the water level changes — sometimes by several metres. Lines are turned around a cleat or led around a bollard but stay held by hand, taken in or eased continuously; a line made fast during a descent can leave the boat hanging by its cleat. During filling, the incoming water creates turbulence that genuinely moves the boat: engineers dimension locks precisely around these hawser forces, as detailed in De Mulder’s synthesis presented at the 2009 PIANC international workshop on navigation lock design (see the study on Google Scholar). Engine ready to engage, hands away from the edge, and never a limb between hull and chamber wall.
The great classics of the French west
The Naye lock opens Saint-Malo’s basins at the foot of the ramparts; Deauville and Honfleur work on the same principle around high water; the Arzal lock separates the tidal Vilaine from its permanent freshwater lake — a textbook case where you pass from sea to lake in minutes. At all these sites, rafting up is common in season: our guide to rafting up describes the manoeuvre, and the panorama northern Brittany and Normandy places these harbours within an itinerary. One search on Boat-Comparator compares the platforms’ real offers on Saint-Malo, Granville and Deauville — lock included in the scenery.
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