I'm currently listening to the Recorded Books edition of "Dark Voyage," by Alan Furst. It's about a Dutch tramp being used as a spy ship in WW2. 

Anyway . . .

Ship's Captain, in response to a question from the officer commanding the British commandos the ship is carrying to the site of a planned raid: "This is a tramp steamer. The only promise I can make regarding our itinerary is that we'll never be early."