About 30 tugboats work San Francisco Bay, helping cargo ships and tankers navigate challenges of wind, tides, currents, fog and even parallel parking. A tugboat’s job isn’t only to tug large vessels: the tugs also help steer and brake the bigger boats.

FROM HUMBLE BEGINNINGS

In the late 1800s, rowboats charged out to ships anchored outside the Bay. The faster ones landed the job of ferrying crew members and supplies back and forth to land; the strongest rescued ships in distress. Today’s local tugs, some equipped with 7,000-horsepower engines, assist up to 30 vessels daily.

TUGGING BY THE NUMBERS

Tugboat companies deal in large sums: the bill to dock a tanker can run from $15,000 to more than $50,000 depending on a number of variables. Captains may earn six-figure salaries, and a new tugboat can cost more than $10 million.

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NOT (YET) IN COMMAND

When large vessels reach the Bay, experienced sailors, known as bar pilots, get on board to navigate as they dock with assistance from the tugs. “We’re their helping hands,” says Drue Kasper, 32, a tug captain who, like many others, aspires to become a bar pilot.

KEEPING UP WITH THE JONES

The Jones Act, a law passed in 1920, requires all vessels working in United States ports to be American-built and to have crews made up of at least 75 percent United States citizens.

QUICK TURNAROUND?

A tug captain operates dual joysticks that direct the propellers — the size of Mini Coopers. It can easily take 10 years to advance from deckhand to captain; crews work seven consecutive days, often staying in comfortable below-deck quarters.

CHARACTERS FOR SURE

“Tugboat Annie,” a 1933 movie about a pair of quarrelsome tugboat operators, was based on the life of Thea Foss, a Norwegian immigrant who founded the Foss Maritime Company in Tacoma, Wash., in 1889. The business is still operating.

JOB OPENINGS?

According to Allen Haig-Brown, a maritime expert, the qualifications for a good tug captain include respect for the sea and nerves of steel. Captains generally fall into two types, he said, screamers and Cool Hand Lukes. “In this work you can’t afford to make mistakes,” Kasper, one of the Lukes, said. “If you do, if won’t be long before you’re out of a job.”

This article also appears in the Bay Area edition of The New York Times.