The metaphor of a ship holed below the waterline is an engaging one. Above the surface nothing much seems wrong - perhaps a minor list. But down below the water is pouring in; barring extreme good fortune the ship is doomed. I remember using it in 2007, when the world's interbank markets froze over; the ensuing collapse of the banking system did not happen until over a year later, when Lehman Brothers failed, though a surprising number of people did not see it coming. I think the metaphor is just as appropriate for Labour's new leader: Jeremy Corbyn.
Many wrote Mr Corbyn off from the start, as a far-leftist, backed by trade unions promoting fantasy economics. These were my instincts, but I tried to give him the benefit of the doubt. He sparked real enthusiasm amongst hundreds of thousands of activists. He came over as an engaging, anti-politician - a nice chap. With the public so tired of the usual type of politician, might he not spark enthusiasm amongst the wider public? Hearing him occasionally on the radio, he seemed to be talking a lot of sense. He had real momentum. You could put a positive spin on most of what he said.
Of course there were doubts. His Shadow Chancellor John McDonell's inept handling of the government's fiscal plans did not bode well. There is something chaotic about the party under his leadership, with no coherence across the Shadow Cabinet. But that sort of thing happened to Mrs Thatcher in her early days too - and look what happened to her. And I did not take much notice of those putting it about that next year's elections, in Scotland, Wales, London and local councils, would be a critical test. Mr Corbyn was enjoying his new job, and his backers were determined to consolidate their hold on the party. I thought he could weather one set of bad results at least. Politicians are expert at finding somebody else to blame; no doubt the left would simply blame die-hards from the old order.
And then came the Paris attacks. The public regards these things as a critical test of political leadership. There is a lot of fear out there. Could something like that happen here? Can we not holiday in safety in European capital cities? We want leaders who can express our outrage, provide reassurance, and take charge of stopping the bad guys. The FT's Janan Ganesh suggests that the public's insecurity might make them seek older, more experienced politicians, especially ex-soldiers - in place of the callow think-tankers, PR types or charity workers that currently dominate the political ranks.
What they do not want are the intellectual prevarications we have had from Mr Corbyn in the last few days. He suggested that the killing of British terrorist Jihadi John by drone attack was not as good as bringing him to justice in a court. He failed to dissociate himself from Stop the War Coalition, which he used to chair, when it suggested the the French were reaping what they had sowed. He professed his nervousness about a police shoot-to-kill policy, when the public carried mental pictures of gunmen in suicide vests firing indescriminately. He seemed to rule out attacks on Islamic State in Syria (or Iraq come to that) by the British military.
The point isn't that these views are without validity. Extra-judicial killing makes many feel queasy - and making martyrs, with all the risk of killing innocents alongside them, is no substitute for the grinding humiliation inflicted by judicial process and punishment. Our politicians have often suggested that the country's interventions in the Middle East are designed to make our streets safer; that is open to challenge, to put it mildly. Trigger-happy police kill innocent civilians - as Londoners well know from 2005. It isn't clear how bombing IS target in Syria will help.
But now was the wrong time to raise these concerns. They smack not just of qualified outrage, but of indecisive leadership that will be no match for the hard men (and women) that want to kill us. Unless Britain becomes an unexpectedly more secure and optimistic place, the vast majority of the British public will take fright at the idea that Mr Corbyn could be Prime Minister. The best that could be said of him is that he is too nice for the job. It was bad enough in May when Ed Miliband didn't look Prime Ministerial enough; this is infinitely worse. In the full heat of a General Election campaign, Labour would be lucky to hold onto seats it had previously considered safe. What happened in Scotland could be repeated across England and Wales. The Conservatives, Ukip, or even the Liberal Democrats could clean up.
And yet SS Labour sails serenely on. Labour MPs know they are in trouble, but apart from an outburst at an MPs' meeting on Monday they don't feel they can do much about it- Mr Corbyn's support amongst grassroots activists is too strong. Those activists are in denial, dismissing these difficulties as a bit of a wobble. They can't possibly admit they have been so wrong only a month or two ago.
But the party is in serious trouble. I think there will be three key arenas in which this drama will play out: Scotland, London and the old industrial heartlands of England.
Labour must win back Scotland from the SNP in order to regain power in Westminster. Mr Corbyn's supporters claimed that he was the right man to do this, interpreting the SNP's rise as a backlash against austerity, rather than against rampant incompetence. But the SNP leader, Nicola Sturgeon's handling of the Paris outrages has been assured, even promising an open mind on bombing Syria, in spite of her party's conference resolution against it. Labour do not look like a serious government in waiting in Scotland, and they are sure to be routed in next May's elections to the Scottish parliament. That will a a hard failure for the left to gloss over.
London holds its Mayoral and Assembly elections next May too. The interest here is that London is the biggest stronghold of Mr Corbyn's activists. If there is to be a pro-Corbyn surge, it will start here. But Zack Goldsmith, the Conservative candidate for Mayor is well-funded, and has hired top-rate campaign advisers, fresh from the Tory General Election victory. He wants to win and will pull no punches - even if other Tories would happily give Labour a run here to keep Mr Corbyn in place. If Labour do badly it will be devastating for their future prospects - though Mr Goldsmith has weaknesses of his own, and it wouldn't do write off Labour's Sadiq Khan just yet. He certainly gets my second preference over Mr Goldsmith.
But perhaps the most interesting battleground will be in England's northern and central heartlands: Manchester, Leeds, Birmingham and so on. Many of these cities are virtually Labour one-party states. But interestingly a number of local leaders are taking a pragmatic and enterprising line to power. They support local devolution and are prepared to work with the government on that basis. And yet Labour's working class voters will be amongst the most distrustful of Mr Corbyn's metropolitan ways - and outraged at his recent prevarications on security. An early test of their feelings will come in a by election in Oldham in December. Ukip, anxious to overcome their disappointments in May, scent blood. Even the Lib Dems, with their Lancastrian leader, Tim Farron, are putting in an effort. This will be an interesting election to watch. If Labour fare badly in these heartlands, an anti-Corbyn coup is surely only a matter of time.
And what will happen when the good ship Corbyn finally keels over and sinks? That's another matter, but Labour's problems would hardly end then.