SAN DIEGO -- An 18-year-old man was severely injured Monday when the motorboat he was piloting on Mission Bay collided at high speed with one being driven by his father, authorities reported.

The boating incident occurred about 2:40 p.m., when the younger man's vessel suddenly slowed as his father was following close behind, lifeguard Lt. Andy Lerum said.

The older man's 25-foot cabin cruiser went "up and over" the son's 18-foot outboard-motor boat, landing on the teenager and crushing him against the steering wheel, according to Lerum.

The impact knocked out the son, and his boat continued on at high speed until it went ashore on Ski Beach, sliding about 20 feet up the sloping sand before coming to a stop.

Lifeguards had to free the teen from the badly damaged boat, and medics took him, still unconscious, to UCSD Medical Center for treatment of serious injuries to his head, neck and torso.

His father, who told authorities he'd gotten distracted and looked away from his son's boat moments before the collision, was unhurt, the lieutenant said.

Boaters are required to stay at least 100 feet from other vessels if traveling at more than 5 mph, a regulation the father and son apparently were violating, according to Lerum.

The two men's identities were not disclosed.