A 'boat tram' will soon sail San Francisco's streets. Here's how to ride it.

If you spy an object that appears better-suited to the open seas than the Embarcadero in the coming weeks, hesitate before calling 311 or the U.S. Coast Guard.

Muni's Blackpool Boat Tram will begin regular service on Tuesdays and Wednesdays, beginning May 28. It will run through Labor Day, and possibly longer, from the Steuart Street stop by the Ferry Building to Pier 39, between the hours of 11 a.m. to about 5 p.m. Regular Muni fares apply — even though you are riding a noticeably irregular vehicle.

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The tram will run as a "summer gift to San Francisco" courtesy of SFMTA's Director of Transit Julie Kirschbaum, according to the Market Street Railway, a nonprofit that preserves historic transit in the city.

The Blackpool Boat Tram has a unique history as far as historic trams go. Just 12 of the open-topped streetcars were manufactured in Blackpool, England, in 1934. They originally ran along the Irish Sea Promenade and eventually fell into disuse. Three continue to operate, including the two purchased by the Market Street Railway.

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The first boat tram operated in San Francisco on-lease from a museum between 1983 and 1984. Market Street Railway purchased its own car — called No. 228 — in 1984 and, 30 years later, purchased a second (No. 233). The former tram will be in-service this season, while No. 233 gets new wheels.

Michelle Robertson is an SFGATE producer. Email: michelle.robertson@sfgate.com | Twitter: @mrobertsonsf