COPPER HARBOR, Mich. (AP) _ A Coast Guard cutter ran aground today in 6-foot waves off Michigan in Lake Superior, injuring at least three crewmen and causing extensive damage, authorities reported.

Two helicopters and three cutters helped rescue the 50 crew members who were forced to abandon the 180-foot Mesquite, said Petty Officer Frank Jennings, a spokesman at the Coast Guard's Great Lakes district office in Cleveland.

Those flown to a hospital included a crew member with a broken arm and one who apparently suffered a heart attack, according to Jennings, who said the nature of the third injury was not immediately available.

The other crew members were put aboard a commercial vessel, the Mengal Desai, Jennings said.

The World War II-vintage cutter, a buoy tender based in Charlevoix, Mich., hit a submerged ledge at 2:30 a.m. about three-fourths of a mile off Keweenaw Point near Copper Harbor in Michigan's Upper Peninsula, Jennings said.

''She started taking on water and that flooded a number of compartments,'' said Coast Guard Lt. John Andrzejewski in Sault Ste. Marie. Andrzejewski said the ship came to rest on a sloping rock ledge but appeared stable.