Shortstop Cal Ripken booted a potential inning-ending ground ball by Al Cowens in the eighth inning, allowing Spike Owen to score the winning run and give Chuck Cottier a victory in his managerial debut tonight as the Seattle Mariners defeated the Baltimore Orioles, 10-9.

Cottier, the Mariners' third-base coach and a former Washington Senator, took over as interim manger for the rest of the season after Del Crandall was fired early today. The Mariners are in last place in the American League West.

With Baltimore leading, 9-7, in the eighth, Jim Presley started the winning rally with his second homer, a two-out solo shot off Sammy Stewart. Jack Perconte walked, bringing on Tippy Martinez, who gave up a single to Owen and walked Al Davis to load the bases. Pinch hitter Barry Bonnell walked to force in Perconte with the tying run, and Owen scored the winning run on Ripken's error.

Ed Vande Berg (8-12), who entered in the seventh inning, was the winner. Ed Nunez retired the Orioles in the ninth for his fifth save.

The Orioles had taken an 8-7 lead in the seventh with five runs, the key hit being pinch hitter Ken Singleton's grand slam homer.

John Shelby singled to chase starter Matt Young. Mike Stanton walked Ripken, Eddie Murray and John Lowenstein to force in a run. Vande Berg replaced Stanton, and Singleton hit his second pitch over the wall.

Ripken, who had three hits and two walks, gave the Orioles a 1-0 lead in the first inning with his 23rd homer. Ripken has reached base 11 consecutive times in his last three games.

The firing of Crandall, the Mariners' fourth manager in their eight seasons, came less than two weeks after George Argyros, the team owner, had endorsed him. Argyros emphasized at the time that team President Chuck Armstrong and General Manager Hal Keller were running his operation.

"I was informed at 10:30 a.m. (PDT)," Crandall said in a radio interview today. "It is a disappointment when things don't work out because I enjoy managing.

" . . . This is the way baseball works. Obviously, there were some differences. This is the thing they felt they had to do."