Any doubts about the identity of the Kentucky Derby favorite were eradicated on the stretch turn at Gulfstream Park this afternoon. When Timely Writer caught the pacesetting Star Gallant and drew away to win the $250,000 Florida Derby, he demonstrated that he has all the necessary qualities to win at Churchill Downs four weeks hence.

Timely Writer seemed to be facing a difficult task today. One of the principal contenders, Distinctive Pro, was scratched five hours before post time, and his defection left the unbeaten Star Gallant as the only speed horse in the field. Able to dictate the early pace, he could not possibly have met Timely Writer under more favorable circumstances.

But it didn't matter. Timely Writer, the ill-bred colt who started his career in a $30,000 claiming race, scored his two-length victory with consummate ease. And none of the other well-regarded horses in the field were close to him. Third-place finisher Our Escapade was 10 lengths behind.

When trainer Jimmy Croll scratched Distinctive Pro because of a shin injury that will keep him out of the Triple Crown series, Timely Writer's co-owner, Francis Martin, said, "We were shocked. We were concerned." Trainer Dominick Imprescia originally had wanted his colt to make the same kind of big stretch run with which he had won the Flamingo Stakes last month, but now he had to instruct jockey Jeffrey Fell, "If the pace is too slow, don't lay too far off it."

Timely Writer did break alertly, but he doesn't have Star Gallant's kind of speed and the unbeaten colt took a clear lead as he raced into the first turn. "He relaxed on the lead," jockey Sandy Hawley said, and was able to set relatively slow fractions: a quarter mile in 23 2/5, a half in 47 1/5, three-quarters of a mile in 1:11 1/5. The horses stalking him were longshots without much of a chance; Timely Writer had five lengths to make up. The circumstances were perfect for Star Gallant.

But as soon as Fell asked Timely Writer to accelerate, he said, "I was confident I could get him." He made up the whole deficit running around the turn, and when the two horses swung into the stretch abreast of each other, the outcome was a foregone conclusion.

"I never really set my horse down throughout the race," Fell said. "I let him finish the way he was comfortable. Since I've been riding him it was his easiest victory." That may account for his relatively slow time of 1:49 3/5 for the mile and one-eighth.

Timely Writer earned $150,000 for his owners and returned his backers $3.60, $2.40 and $2.40. Star Gallant paid $2.60 and $2.40 and Our Escapade paid $4.60 to show.

Star Gallant was just the latest in a series of unbeaten, highly regarded horses whose reputations have been smashed by Timely Writer. He upset Out of Hock in the Hopeful Stakes at Saratoga last year. He upset Before Dawn and Deputy Minister in the Champagne Stakes at Belmont. But this was perhaps his most notable achievement, for Star Gallant had looked like a brilliant racehorse compiling his four-for-four record. The ease with which Timely Writer beat him suggests that Star Gallant is not a true distance horse; he is certainly not a Derby horse.

If Star Gallant's reputation was tarnished today, the rest of the field was thoroughly humiliated. Royal Roberto had been considered a plausible Derby candidate before today; he finished a distant fourth. Laser Light came into this race with a good reputation; he finished last by 20 lengths.