Seventy-five years ago, a transportation revolution was happening on Maine’s coast.

In January 1936 at Snow’s Shipyard in Rockland, the keel was laid for a new type of boat to be used on Penobscot Bay: a drive-through ferry to transport vehicles from the mainland to the island of Islesboro.

Christened the Gov. Brann, the diesel-powered ferry was launched in May. By the end of the first season, some 6,500 automobiles had been carried, eight at a time. That winter, the ferry returned to the yard for the addition of a section amidships, which increased the per-trip capacity to 12.

The ferry marked a significant change for the Islesboro summer colony, which, back in 1913, had actually had the state legislature pass a law banning automobiles, to preserve the peace and quiet of the quaint island. This meant no Model Ts for the year-rounders and the summer people had to leave their Packards at home and arrive by steamboat or yacht. In 1933, the year-round residents got the ban rescinded and began to raise money for a ferry.

By this time, transportation was already well on its way to shifting from an integrated “intermodal” system to one dominated by the automobile. In the new model, waterways were viewed as barriers rather than highways, which gave rise to an era of bridge construction.

The 1927 Carleton Bridge across the Kennebec River made rail and road access to the Midcoast much faster than crossing the river by ferry. The opening of the Waldo-Hancock Bridge in 1931 did the same for West Penobscot Bay and Mount Desert. The Deer Isle-Sedgwick Bridge, a Works Project Administration project begun in 1936, provided a lasting legacy of the Great Depression that still serves us today.

Land-based travel replaced the Eastern Steamship Company’s Boston-to-Bangor overnight steamer service, which made stops at Rockland, Camden, Belfast, and towns north to Bangor. Its last run was in 1935. Another casualty was the sophisticated network of electric inter-urban railcars that carried passengers and freight, with service from Thomaston to Camden and down the St. George peninsula. Truck transport and private cars forced the railroad’s shut down in 1931.

The inter-island steamboat network on Penobscot Bay persisted into the 1940s. From the Rockland hub, boats traveled to islands like North Haven and Vinalhaven, to Stonington and Deer Island, and around to Blue Hill and Mount Desert. Steamers also served smaller harbors along the Brooklin Peninsula, which were hard to reach by road.

After the war, new automobile-oriented motor vessels replaced the ferries to North Haven and Vinalhaven. Yet in 1936, it was still possible to get on a train in Boston, disembark in Rockland to board a steamboat, and venture to any of a dozen towns in Penobscot Bay… all without a car.

Ben Fuller is curator at the Penobscot Marine Museum on Route 1 in Searsport, where the “75 for 75” exhibit is on display through Oct. 23, 2011. The Maine Boats, Homes & Harbors Show, held Aug. 12-14, will include exhibits about the coast circa 1936 as part of an annual exploration of how “Tradition Shapes Innovation.”