Around 330 crews take part in the 21 mile race from Docklands to Ham.
Traditional boats, or traditional-style replicas, powered by a minimum of four oars or paddles may enter.
From 12.45pm boats leave Millwall drawback on a 'slowest first, fastest last' handicap basis, giving all crews an equal chance.
The first boats reach Tower Bridge about 1.15pm (when a bridge lift is due) and Lambeth Bridge about 20 minutes later.
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