Before today, jockey Jack Kaenel had always said that his greatest thrill was winning the Watermelon Derby in Rocky Ford, Colo., where he triumphantly carried a large green watermelon into the winner's circle.

But this afternoon the 16-year-old product of America's bush-league race tracks was carrying a blanket of black-eyed Susans after a performance that neither he nor the racing world will soon forget.

By directing Aloma's Ruler to a half-length victory over Linkage in the Preakness Stakes, Cowboy Jack became the youngest jockey ever to win a Triple Crown event. What's more, he did it by outsmarting and outriding the most illustrious member of his profession, Bill Shoemaker.

Kaenel got Aloma's Ruler to the rail--the most favorable part of the Pimlico track--set a slow pace and opened a commanding lead while Shoemaker lingered behind him and played into his hands. But the colt's winning time of 1:55 2/5 for 1 3/16 miles over a deep track indicated that his victory was no fluke; it was a superb effort.

The first thing jockeys learn at the minor-league tracks of the Midwest, where Kaenel learned the trade from his father, is the importance of speed. Forget the subtleties: just get to the front. At Pimlico, where horses who got to the lead on the rail have been virtually invincible the last two days, Kaenel had the ideal place to put his childhood lessons into practice.

Trainer Butch Lenzini, a Maryland regular, and owner Nathan Scherr, a Baltimorean, recognized the bias at Pimlico, too. So Lenzini told his jockey, "Break out of the gate, get to the wood (the rail) and slow it down."

To get to the wood, Kaenel would have to get past Linkage, the 1-to-2 favorite, who was breaking from the stall inside him. But Shoemaker is a patient rider, and he intended to bring his mount from off the pace.

So when the gate opened, Shoemaker permitted Kaenel to take command. Only Leroy Moyers, another Marylander who was riding Bold Style, tried to do what Kaenel wanted, but Aloma's Ruler was too fast.

He dropped over to the rail at the first turn, covered the first quarter in 23 4/5 seconds and the half in a slow 48 seconds. (Half an hour later, an $8,500 horse set the pace of a route race in :47 flat.)

The Preakness pace was so slow that Laser Light, a notorious plodder, was running within a length of Linkage on the backstretch, but still Shoemaker was content to sit a couple of lengths behind the leader, on the outside, and wait.

"I thought I was in good position," Shoemaker said. "I thought I could take him any time."

He was wrong. When they turned into the stretch, Shoemaker now asked Linkage for a serious response, but Aloma's Ruler was still a strong horse. Although the favorite was gaining ground slowly, he never seriously threatened to overtake the leader, and finished a half-length behind. Longshot Cut Away finished third, another 6 3/4 lengths behind.

Aloma's Ruler, a bay by Iron Ruler-Aloma, went off at 7 to 1 and earned $209,900 in winning his third race in four starts. He paid $15.80, $4.60 and $3.60 to his backers in the crowd of 80,724. Linkage returned $2.60 and $2.60, and Cut Away, a 40-to-1 choice, paid $6 to show.

Just a week ago, Aloma's Ruler had not been considered even a probable Preakness entrant. Although he had shown great promise earlier this season, he had been sidelined by an ankle injury and then came back to lose an allowance race at Pimlico.

But Scherr still was entertaining visions of the Preakness, and so Aloma's Ruler went to Aqueduct for the Withers Stakes. He miraculously recaptured his old form and won in an impressive fashion.

Kaenel got the mount that day because Angel Cordero Jr. had another commitment, and both Scherr and Lenzini decided to stick with him in the Preakness.

That was only his first stroke of luck this week. On Tuesday Kaenel was driving to the track when a station wagon crashed into his 1982 Cadillac, smashing his head into the windshield. For some reason, Kaenel was wearing his riding helmet instead of his ever-present cowboy hat; he emerged with minor injuries.

"Somebody up there must have been looking out for me," Kaenel said.

But his luck that day was only a harbinger of the good fortune to come this afternoon when the condition of the track at Pimlico and the bad judgment of Shoemaker gave Kaenel the opportunity to take off the riding helmet, put on his cowboy hat and be led to the Pimlico winner's circle, triumphant.