An accomplished sailor is missing at sea, days after his empty 50-foot sailboat was found off the San Diego coast, authorities said Saturday. John P. Buser, in his early 70s, went for a solo sail on Wednesday but failed to return by late afternoon, prompting a massive search. Buser was last heard from about 1:30 p.m. by phone when he reported his position to be about a half nautical mile from Point Loma, where he lives, said Petty Officer 1st Class Patrick Kelley, a Coast Guard spokesman. Around 4:30 p.m., the Coast Guard put out an emergency alert to all nearby mariners to be on the lookout for the boat, a K-50 named The Rendezvous. A Coast Guard cutter in the area found the boat within a few hours, with no sign of Buser. A search from the sea and air went through the night, with help from a Coast Guard plane and helicopter, a Navy helicopter and Harbor Police. The Mexican Navy was also alerted. The search continued at first light Thursday but was soon suspended, as the chances of surviving in the 61-degree ocean lessened. “They searched for three hours longer than it was calculated he could survive,” said family friend Steve Wright, who also joined in the search. Buser is a Navy pilot veteran who is an avid sailor and racer, as well as past commodore of the Ancient Mariner Sailing Society in San Diego.