What is "flatbottom"

Usage examples of "flatbottom".

So the flatbottomed boats were built at every little fishing-village along the coast of France, and they crept from their pieces of origin towards the great military camp of Boulogne, keeping to the shallows, moving by oar more than by sail, sheltering when necessary under the coastal batteries, each boat manned by fifty soldiers and a couple of seamen.