The police thwarted the would-be assassin.
a) A brace, perpendicular to the keel, that helps maintain the beam (breadth) of a marine vessel against external water pressure and that may serve to support the rail.
A well made doughout canoe rarely needs a thwart.
b) A seat across a boat on which a rower may sit.
The fisherman sat on the aft thwart to row.
See Also: beam
Wikipedia foundation.