This film addresses the hard lot that ex-servicemen had to face in nigh-on bankrupt post-war Britain; many of them felt of as little use as their now redundant equipment, and some sought excitement and ill-gotten gains by whatever means available.
The Ship in question is a Royal Navy Motor Gun Boat and appears to be (I think) a Fairmile type C. These boats were fast, heavily armed and themselves somewhat vulnerable to attack having little or no armour and containing hundreds of gallons of fuel. They were powered by three large engines and sported between ~2000 and 4000hp depending on the specification.
The idea of using a craft of this sort for smuggling is a bit of a stretch; they use several gallons of fuel per mile and are ruinously expensive to operate, so much so that few of this type survive today. Any smuggling operation using one would have to pay out handsomely just to meet the fuel bills.
Anyway the film is a little darker and has less overt sympathies than many of this period. The cinematography and model work is pretty good and the cast is excellent.
This film is well worth watching, almost a forgotten classic.