Or, better, have them weighted down under the surface, so that even at low tide these shallow boats can pass over them, but deep-draught ships can not.

One of these, the Bull Wall, along with associated projects, led to the creation of Bull Island in Dublin Bay, and enabled deep-draught ships to use the port for the first time.

The only difficulty is the Capelan bank, to the south of Logonna-Daoulas, where the depth is less than 5m - this bank has to be passed to reach the base and thus prevents very deep-draught vessels from reaching it.