William S. Hart Jr., who championed the legacy of his silent film star father despite losing a court battle for the movie cowboy's Horseshoe Ranch that is now a Los Angeles County park, has died. He was 81.

Hart, a former Santa Monica real estate appraiser who was his father's only child, died of unspecified causes May 13 at Swedish Hospital in Seattle. He had lived on nearby Bainbridge Island since his retirement in 1989.

Hart graduated from UCLA, and later returned for a master's degree in environmental planning. During World War II, he worked in Washington, D.C., as a cryptographer for the Office of Strategic Services, forerunner of the CIA.

In addition to his real estate and development business, Hart taught appraisal, business and finance classes for 40 years at UCLA, USC and Santa Monica College.

But throughout his life he remained a child and namesake of old Hollywood. His father was William Surrey Hart, a successful Broadway actor who became the silent screen's first western hero and one of the biggest box office stars of the 1920s. The larger-than-life actor made more than 50 silent films between 1914 and 1925 and went on to write western novels, as well as the 1929 autobiography "My Life — East and West."

The elder Hart met actress Winifred Westover while making the silent cowboy picture "John Petticoats" and married her in 1921. Hart Sr. was 58 and his bride was 21.

They separated five months later and formally divorced in 1927.

As the elder Hart's health deteriorated in 1946, friends went to court to seek guardianship of his person and his estate. Hart Jr., nearing his mid-20s, sued for the job himself.

A judge ultimately named the son co-guardian of the father's person along with tax consultant and friend George H. Frost, and appointed Frost sole guardian of the estate.

Hart Sr. died June 24, 1946, a few days after the guardianship ruling, leaving a will that sent his son right back to court. The cowboy, who had established a $100,000 trust fund for the boy before he was born, had stated clearly: "I have made no provision in this will for my son for the reason that I have amply provided for him during my lifetime."

The actor left a few relatively small bequests to individuals and charities, including $50,000 each to a surviving sister and to the Los Angeles Society for the Prevention of Cruelty to Animals.

But the bulk of his nearly $1.2-million estate, including the 220-acre Horseshoe Ranch and 27-room villa near Newhall in the Santa Clarita Valley, was willed to Los Angeles County for a public park.

In lengthy litigation over several years, Hart Jr. was unable to convince a jury or appellate courts that his father had been unduly influenced by others to exclude him from the will.

The trial, along with court hearings over the guardianship, revealed a close relationship between father and son, despite their ages and physical separation.

The younger Hart spoke movingly of how his father had a bronze bust of the son's head made to keep at the ranch and testified in 1950:

"I remember him showing me the ranch, outside and inside, and he said, 'All this I am building for you.' He was affectionate — kissed me and said I was his son."

Although the son was unsuccessful in acquiring his father's ranch, he maintained a lifelong interest in its management as a park. The William S. Hart County Park and Museum has been curated by the Los Angeles County Museum of Natural History and is under the jurisdiction of the Department of Parks and Recreation.

Hart Jr. also spoke out to preserve another property his father had given the public: his De Longpre Avenue mansion. In 1944, Hart Sr. donated the mansion, an acre of ground and $50,000 to the city of Los Angeles for a public park — even though it lay outside Los Angeles city limits.

In 1989, partly through Hart Jr.'s efforts, Los Angeles turned over the parcel, also known as William S. Hart Park, and the trust fund then estimated at $284,000 to West Hollywood for 30 years.

William S. Hart Jr. is survived by his wife, Rosalind; three daughters, Vanessa, Karina and Christina; and two grandchildren.

Local services are planned June 12 at Pierce Bros. Westwood Village Mortuary.