There is something about a boat that makes a man shout at his woman. It's reached the point in sailing circles that the hottest item on the accessory market is a T-shirt for ladies with the slogan, "Don't Shout at Me!"

This phenomenon makes no concessions to size of craft or nature of outing. The same boor who howls at his mate for life on a yacht in the deep Atlantic will thrash and posture from the stern of a canoe on the Potomac. Never mind there is no danger. It's a matter of control. If the boat is out of control the man is out of control. And it's all the woman's fault.

The folks from Mensa, a self-styled "selection agency" for IQ whizzes, bent their exclusive rules last week and allowed four nonmembers to join a shitewater canoe outing near Harper's Ferry.

The Mensas, who accept only applicants who prove to be in the top 2 per cent in intelligence in the nation, were predictably thorough in their preparations.

Leader Sue King horrified the 15 beginners's-level paddlers with vivid tales of smashed boats, half-drowned canoeists and daring rescues.

The canoes were loaded over a dam upstream of the stepping stones, a stretch of fast water, and the paddlers climbed aboard, two to a boat.

The first hurdle had not been crossed before the two male non-Mensas were at the throats of the two female non-Mensas.

"Pull, dammit, pull. Can't you see that rock. O.K., which way now, left? Did you say left? Well holler it out, I can't see anything from here. You're in the bow. Oh God, can't you do anything right? Watch out!" Crash.

For all its lofty pretensions, Mensa is a social club and most of the men and women were singles out for recreation. The non-Mensas were couples such intimately related couples that carry on so crassly. Casual friends would never treat each other that way.

The miserable scenario replayed itself downstream until lunch break was called at a cove.The two harried women exchanged long-suffering glances, then feigned sleep to avoid more arguing.

Somehow the men gathered that something was wrong, and while the Mensas broke out sandwiches and diet snacks they devised a brilliant scheme to patch thing up. The men would take one canoe, the women the other.

"We'll go ahead, so we can keep an eye out and make sure you don't get in any trouble," they advised haughtily.

And would you believe it? The women managed to shoot, every rapid without a spill, they were not the last one in and they went back for a second run and beat the men to the take-out point.

"You know," said one on the ride back home, "it's amazing, but I think today showed me that I just might be able to do something right if I was left alone. Amazing."

The Mensa group had the right idea about introducing paddlers to whitewater. They picked a mellow stretch of the Potomac with just enough action to keep it from being dull.

Sue King, for all her overstatement, did a decent job of teaching the basic strokes. And there were a couple of old hands along to bail everybody out when they spilled.

There was only one disaster, at White Horse Rapids near the old town of Harper's Ferry, when three boats went tumbling over in the fast water; much laughter no harm done.

But canoeing with a group has its drawbacks. Everything takes about five times as long as it should when you have seven boats. And every time someone gets in a jam the whole convoy is in a jam.

The Mensas themselves are a rather serious-minded bunch to whom humor is a rare commodity. The exception is Angus McKellar, a raffish Scot with a grey beard and quick laugh, who kept things light and moving.

And the sport itself has great possibilities. It's a thoroughly satisfying experience to wind your way with muscle and instinct past boulders in quick water. And the gentle scrape of a rock beneath you, the slow bulge of the hull as you drive through, puts you close to nature, which is what it's all about.