Neither vessel knew the other was there.
That stark fact condemned fifty-one people to death, in a river tragedy in the heart of the capital that should have been rendered impossible by official safety standards. But one of those standards was the age-old requirement to post an adequate lookout. And neither captain had fulfilled it.
The two boats were about as different as you could find, and the tragedy was much more than a clash of hardware. It was a fatal mix of two cultures. The Marchioness party-boat was all hippie-casual, a style that would clearly work against sound captaincy and good order. The sand-dredger Bowbelle operated in a very different atmosphere, with an experienced crew tainted by the deeply-embedded corruption of London's river.
Many were shocked to hear that the Bowbelle's captain Henderson and one of his crew had drunk thirteen pints of lager between them. Although this was (rather oddly) ruled irrelevant, it did reflect a dubious history. Another recent collision and a near-miss by the Bowbelle. Several similar incidents in co-owned vessels. Henderson forging his own qualifications. And two hung juries over whether he was guilty of failing to post a proper lookout. You could smell the corruption.
As for the Captain of the Marchioness, he was dead but not beyond critical reach. He seems to have been heading for the same point under the central arch of Cannon Street railway bridge as the Bowbelle, when he should have steered further to starboard (south). Whether or not he too might have been drinking through the day, there had been no wild debauchery on-board to blame for the disaster, as some people wanted to think. Because as the shock wore off, a rather nasty strand of public opinion began to portray the Marchioness partygoers as decadent yuppies who somehow deserved their fate. They were the envy-targets, mostly successful young bachelors working up at the sharp end of the creative/hi-tech world, with even suggestions of a gay coterie.
No doubt the Marchioness was managed in a sloppy way. For example, the passenger-numbers hadn't been logged, but were slightly over the limit, and beyond the capacity of the life-rafts. At that rate, there may have been other oversights that will never be known.
But Henderson is the one who should have known better than to leave his duty-lookout unbriefed, when a heavy load at the stern had raised the prow, blocking half the river from view. And that was the man who had drunk seven pints, to Henderson's six! Irrelevant? Hmm...