MILWAUKEE (AP) _ A woman may inherit her boyfriend's estate because he wrote a will on the hatch cover of a friend's sailboat.

Michael J. Gudlin's will, written on the 18-by-24-inch hatch cover of a 35- foot trimaran sailboat, was entered Thursday in Milwaukee Probate Court.

If the will is ruled legitimate, Mary Varichak of Greenfield will inherit his entire estate. The couple had lived together for the past nine years.

Gudlin, 36, died May 3 of a heart attack. He drew up the will in January 1985 while he was vacationing in the Virgin Islands. During the trip he fell overboard from a sailboat owned by a friend, Joyce Ayers.

''He wasn't in danger of drowning,'' said Herbert Hiller, a lawyer for Varichak.

''After he got back in the boat, Joyce Ayers jokingly told him, 'If you don't have a will, you're not going sailing with us anymore,''' Hiller said.

Gudlin dictated a will to Ayers that she wrote on the boat's hatch cover. Gudlin signed the will, and Ayers and her daughter signed as witnesses.

Varichak said Gudlin never told her about the will. Ayers brought the hatch cover back when she recently made a business trip to the Virgin Islands, where the sailboat is kept, Varichak said.

Hiller said Gudlin had a half-interest in a firm that rents storage vehicles and his estate was estimated at $100,000.

Judge David Jennings set a July 1 hearing on the will.