During his tour on the guided missile frigate USS Gridley, Kerry requested duty in South Vietnam, listing as his first preference a position as the commander of a Fast Patrol Craft (PCF), also known as a "Swift boat." These 50-foot (15 m) boats have aluminum hulls and have little or no armor, but are heavily armed and rely on speed. "I didn't really want to get involved in the war", Kerry said in a book of Vietnam reminiscences published in 1986. "When I signed up for the swift boats, they had very little to do with the war. They were engaged in coastal patrolling and that's what I thought I was going to be doing." However, his second choice of billet was on a river patrol boat, or "PBR", which at the time was serving a more dangerous duty on the rivers of Vietnam.
What kind of ship was the USS Gridley? (If the question is unanswerable, say "unanswerable")
guided missile frigate