But with rowing there is one extra wrinkle. Fox-hunting, which also answers to the above description (add in "dangerous") has never passed muster as a spectator sport, unless you count the saboteurs. But once a year, and this year it's at 4.10 this afternoon, rowing ceases to be the inscrutable preoccupation of the hard, strong, silent and personality-deficient and becomes instead a matter of intense national interest, at least for the 18-odd minutes the course from Putney to Mortlake takes the Oxford and Cambridge crews to complete.
We are watching the (not "a") Boat Race. We are crowding to the bank of the Thames for a quick gander as the crews snort past. We (and "400 million others around the world", as commentators continue implausibly to boast) are following it on the box, and observing with, let's face it, sadistic enjoyment that even at Hammersmith Bridge they look as if they have swallowed hemlock, by Chiswick Steps they are writhing in agony and threatening to faint, by Mortlake and the finish they are throwing up, they are dead to the world, they wish they had never been born - and that's just the winners.
When one dips in and out of television snooker, or dressage, or sheep dog trials, the truth is borne home that one man's sport is another man's bafflement. But rowing repels boarders more strenuously than most: it is totally demanding for those who have been sucked in, but once the spell is broken, even successful and accomplished rowers can be left asking themselves what on earth it was all about.
"Rowers are the most boring people on earth," declares a colleague who rowed only a couple of years ago for his university. "All they ever talk about is rowing technique. The rowing itself absolutely kills you to buggery. And with outings every morning before tutorials and training every evening, it totally takes over your life."
For a top sports journalist whose interests straddle baseball and wrestling and just about everything else in between, it is the repetitiveness of rowing that he cannot forgive. "The reason football is so fascinating is that it's never the same; no two situations are identical. Rowing is deadly dull because it is always the same, that movement of the oars repeated endlessly. It's impossible to sustain interest in it ... You notice that rowers themselves always talk about the pleasure of their sport in the past tense - they're relieved and delighted that it's all over."
In fact this is not strictly true: there is pleasure to be had in rowing beside the simple fact that one has stopped, and all the rowers I've spoken to are in agreement as to what that pleasure is, though they use different words to describe it.
Dan Topolski, director of coaching for Oxford's team this year, says, "What's thrilling is making the boat sing. It's quite the most exhilarating thing: you're going fast, you can hear the bubbles under the boat, you're on top of the water, all moving harmoniously, and suddenly it feels effortless. Only an afternoon making love can equal that feeling. It's very sensuous. It's the sense of going beyond the edge."
Phoebe Mason, an American who has rowed and coached at high level on both sides of the Atlantic, is more down to earth. "People either like it or they don't," she says, "regardless of their physical strength or endurance. Even true beginners can get a feedback from the boat when it's going well: it feels less heavy, everything seems lighter, goes better, is more steady." That "feedback", once sensed and enjoyed, can return in ever more refined and exalted form as rowers improve. "It's a very technical sport; there are lots of ways of doing it wrong, but people go on getting better and better, training for years and years."
There is an argument, readily advanced by Rosemary Knapp, national manager of the Amateur Rowing Association, that rowing is Britain's most successful sport: our only medals in Atlanta's Olympics last summer, including our only gold, were in rowing. And anyone who clings to the view that today's race is a tired, elitist anachronism would hesitate to apply that judgement to the sport as a whole if they had seen last Saturday's Heads of the River race over the same course, when 420 eights - the maximum permitted by the Port of London Authority - crowded at the starting-line.
The Oxford and Cambridge race retains its prominence despite a waning of interest in comparable events such as the Oxford and Cambridge cricket fixture, because the universities remain among the best rowing sides in the world, with about one-third of the English national side coming from the crews of one or other of them. And although the public schools that have traditionally filled the boats continue to supply their share - Eton has two old boys in the Oxford boat and one in the Cambridge - the social and ethnic mix has widened to include not only Americans, an Italian and a Croatian, but also a growing complement of state school boys. Oxford's star rower, for example, Tim Foster, the stroke, is a product of Bedford Modern and the University of London. Olympic gold medallist Steven Redgrave's partner in the pairs, Matthew Pinsent, is a product of Eton and Oxford, but Redgrave himself, the best rower Britain has ever produced, was a state school boy. (The two men's relationship is famously semi-detached. "You don't have to like the man you're rowing with," Redgrave has said.)
The main thing holding back greater democratisation of the sport - which is also played by more and more women and girls - is that the facilities are expensive. Here, traditional rowing schools have a huge built-in advantage. Lottery-derived grants for building or improving club houses among the nation's 250 rowing clubs will in time do plenty to remedy that. After all, as Rosemary Knapp points out, unlike riding, say, rowing is not an intrinsically expensive sport.
It is, however, an excessively rugged one, when done in the traditional English way. I went down to the river earlier this week to watch them at it. If one of the blonde Amazons supplied by Beefeater Gin, the sponsors, had not lent me her padded anorak, I doubt whether I'd have made it back.
Like most traditional English activities, rowing is a cabal of fanatics: the grizzled rowing journalists huddled together in the bow of the sleek launch, Panache, bound by the fellowship of decades; Dan Topolski, at the helm of the Oxford coaches' dinghy, who has been intimately involved with his side's fortunes since being in the winning Oxford crew in 1967; the other aficionados in the launch Bosporus keeping a beady eye on the crew's performance.
Even our own helmsman in Panache was a member of the club: Bobby Prentice by name, a Thames lighterman by trade, in 1973 he won the Doggets Coat and Badge Race between London and Chelsea Bridges, the longest and oldest sculling race in the world, started in 1715. He, too, is signed up to the sport for life.
The day was chilly, the water choppy; the unmistakable Thames smell came at us out of the filthy murk, compounded of ancient drains and mud and petrol and much else besides. We followed the Oxford eight along the course past Lord Rogers's architectural office on one side and the Hitchcockian bulk of the Harrods Furniture Depository on the other. At Hammersmith two fans with cameras gave us the thumbs-up. Beyond the bridge the weather worsened; the surface of the river was dotted with white horses, the rain slashed down in our faces and the Oxford team fought their way forward. And this was nearly at low water; similar conditions at 4 this afternoon, when the tide is in its spring flood, will give the crews a nightmare ride.
The oars went up and down, in and out. I can' t tell you much more about it. Sometimes they went faster, sometimes slower, sometimes stopped outright when Rene Mijnders, the Dutch head coach, bellowed at them through his megaphone. But watching them row - and at one point, past Hammersmith Bridge again on the return leg, they suddenly picked up their skirts and flew along - various points about the sport became clear to me.
The joy of rowing is dynamic harmony. Such a state involving eight (plus one) men or women is so inherently improbable that when achieved it takes on the complexion of ecstasy. More than that: as Phoebe Mason told me, the better your collective technique, the more intense that rare but real ecstasy is likely to be.
These peculiarities of rowing help to explain others: the intimacy between crew members, which is different from, and may even exclude (witness Redgrave and Pinsent) friendship; the fanatical dedication to training; the compulsive, addictive nature of the sport.
Rowing, then, is not like other sports. It has much more in common with oriental techniques of meditation, whose goal is ecstasy, enlightenment, liberation.
Such a state must be the only true goal of rowing. It is not, however, a very manly one, conventionally viewed, so Oxford and Cambridge continue to have us believe that their only object is to destroy their opponents, who, just to underline the intensity of their hatred, they refer to as "filth" and other horrid terms. But they protest too much. How long before we find them out?