Anyone who has rebuilt a boat knows the gut-wrenching terror of the first launch. Will it sink? Have we closed all the cocks? Will the exhaust fittings seal?

Ron Cain of Alexandria is trailering his 28-foot open-class ocean racer down to Miami this week and on Saturday it will get a test that will leave no doubt about its sea worthiness. He and two friends will cross the starting line with about a dozen boats for a race around Miami, West Palm Beach and Bimini.

Since Cain's boat, the "Can Do," can do 75 miles per hour, three hours is all it should take to zoom around the 200-mile course.

"If it holds together, that is," said Cain, 36. "But I'm hopeful; I did all the restoration work on the boat and it's as good as I can make it. When I found it, it was a piece of garbage."

That was seven years ago. Cain, who works for Washington Security, said that $15,000 and 1,600 hours have made it worth $60,000 and put it in shape for its first try at high-speed, high-seas racing.

Cain and his crew will also be racing on the ocean for the frist time, but the skipper has competed in smaller power boats and has watched ocean racing. He is pretty sure he will be using a successful strategy.

"There are two appraches," Cain said. "The rabbits like to put the hammer down and let it run until it breaks or wins. The more conservaive teams like to drive at about 93 per cent of full power most of the way and pour it on only near the finish. We'll do that; it's a lot less risky."

Not that there aren't risks aplenty, even under reduced power. Each racer will wear $300 worth of special clothing that includes a kidney belt, wet suit, corset, face mask, life vest and crash helmet. Because sitting would lead to an intolerable body beating, all three stand erect and are held in place by padded, wrap-around back rests.

As helmanan, Cain's job is the most physical. He must musics a steering wheel that is responding to two engines putting out 950 horse-power while simultaneously making the delicate adjustments needed to keep the craft precisely on course. He's the one who decides at what run the engines should be set and if and when adjustment's should be made. Cain must also steer the boat close enough to the turning marks so that the boat's number can be read by the racing judges.

"But not too close. If I'm closer than 50 yards we could be disqualified."

His ultimate responsibility, of course, is to keep the boat under control.

"If we come to an unexpected wave condition and I'm not holding onto the wheel good, the boat could flip."

Avoiding such conditions is the job of Don Loranger, 51, of McLean, a retired Navy captain who flew jets in the service. As navigator, Loranger will get a forecast of the wind and the Gulf Stream current on the day of the race and plot a course for the ride. A compass will be his sole navigation tool.

"There's no way I can do any plotting while we're racing. It's too bouncy," he said. "I'll plan courses for the three legs before we start and we'll all memorize them."

But course corrections might be made during the race if Loranger sees that that the wind strenth and direction vary significantly from the forecast.

"I've looked at a lot of water through the years and I guess I'm fairly good at understanding what's happening to it," Loranger said.

"The navigation can be critical. Our range of vision for a turning mark is only about 3 1/2 miles and if our course is off by only a couple of degrees we could miss it."

The man who built Cain's house will be responsible for looking after the engines. Neal Fridenstine, 35, of Lancaster, Va., is a general contractor. As codriver he controls the throt tles and sets the engine rpms according to hand signals from Cain. When the seas are calm the engines run at a steady rate, but when they get choppy Fridenstine has his hands full.

"If we become airbone I want to reduce the power right away - and advance it when we're waterborne again."

Besides throttle-jockeying, Fridenstine must keep his eyes moving steadily over the 12 dash-board gauges that tell him the condition of the engines.

Ocean racing is a form of toil that pays very little. Cain said the team will invest $2,000 in entry fees, fuel, boat transportation and incidentals and can expect about $5,000 in prize money if they should come in first.

"I'm compelled," said Cain. "I got into fast boats about seven years ago and the exhilaration of it all keeps me going."

Can they win?

Cain puts it this way: "If the boat holds together, if Neal can keep the power plant undamaged, if Dona can find the course and if I can give it a competitive ride - well, those will be victories enough. We're taking ocean racing step by step - and the giant steps are still ahead of us."