1915 season, the Tigers sent Heilmann to the San Francisco Seals of the Pacific Coast League (PCL). Heilmann missed the latter portion of the season after suffering from dizzy spells. Before the health problems sidelined him, Heilmann appeared in 98 games for the Seals and compiled a .364 batting average and .544 slugging percentage with 23 doubles and 12 home runs. His .364 batting average led the PCL for the 1915 season. Heilmann returned to the Tigers in 1916 and appeared in 136 games, including 30 games at first base and 77 as an outfielder. Although his .282 batting average was low by the standards he would set later, he ranked among the American League leaders with 30 doubles (seventh best), 77 RBIs (sixth best) and 43 extra base hits (eighth). He also became a favorite in Detroit for his actions off the field. On July 25, 1916, he saved a woman whose vehicle had rolled into the Detroit River, diving into the water to rescue her, although two others died in the accident. His heroic act was reported in the national press, and he was loudly applauded at the next day's game. He continued to improve at the plate in 1917, once again ranking among the American League leaders with 85 RBIs (fourth), five home runs (seventh) and 11 triples (eighth); but he was called "Slug" due to his notoriously slow running and difficulties in the field. In 1917, the Tigers tried playing him in right field (95 games), center field (28) and first base (27), but he was not particularly good at any of those positions. In 1918, Heilmann appeared in 79 games for the Tigers, 40 in right field and 37 at first base, compiling a .276 batting average. In mid-July 1918, he left the Tigers to enlist in the United States Navy during World War I. He served as a quartermaster on submarines off the West Coast of the United States. He returned to the Tigers in 1919 and had his best season to date, ranking among the American League leaders with a .320 batting average (10th best), .477 slugging percentage (seventh), 92 RBIs (fourth), 53 extra base hits (fourth again), 15 triples (second), 256 total bases (fifth), 172 hits (sixth) and eight home runs (eighth). He had another strong year in 1920 with a .309 average, 41 extra base hits and 89 RBIs, but he