Tramp steamers

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Tramp steamers are ships without a fixed schedule. Instead of pre-arranged routes, they trade on the spot market picking up new cargo upon the delivery of whatever they are hauling. Although they can be any type of ship: freighters, tankers, container ships, etc., in popular imagination they are smaller, older, rust buckets.

There is a sub-genre of movies and literature set on tramp streamers. They feature a cast of colorful passengers, most with dubious motives and questionable pasts, as well as crews full of sketchy characters that are likely up to no good. Of course, there will also be a hot dame or two mixed in to roil the pot. They're almost always entertaining in a cheesy sort of a way.  

I thought insurance issues would have prevented modern freighters from carrying passengers, but apparently you can still book passage on some tramp steamers. Sounds kind of dreadful to me, but who knows?