The 23-foot (7-metre) wooden vessel was unearthed earlier this week as crews shoveled beneath the roadway of Folsom Street in the city's South of Market district to prepare for the development of residential towers.
After workers noticed the boat's outline, they dug around it by hand to minimize damage, and a conservationist was called in for further evaluation, said Lynn Cullivan, management assistant at the San Francisco Maritime National Historical Park.
"It's a totally lost piece of history that's interesting to keep alive," Cullivan said, adding the boat had some bad planks, "but basically it's intact, and that's really unusual."
It is not uncommon to find pieces of ships buried below street level in San Francisco, but it is extremely rare to find a boat in as good shape as this one, he said.
Cullivan said he believed the flat-bottomed cargo boat, called a lighter, came from the California Gold Rush period starting in the late 1840s when hundreds of ships landed in San Francisco Bay with passengers in search of new lives and precious metals.
"It's interesting to think that people in that day went down to the financial district of San Francisco, and they'd get in one of those little boats, and they'd row out," Cullivan said. "Today, no one would imagine doing that."
The lighter, which was powered by ores or towed, carried food and other supplies to land from large ships that could not get close to San Francisco's shallow shore. It was deemed obsolete in about 1860, when the city had developed piers that allowed big ships to pull up and unload.
If the lighter's wooden body is found to be strong enough to be transported and preserved, it will be taken by flatbed truck to the maritime agency's warehouse east of San Francisco in Livermore, where it will be kept safe and likely be put on display for public viewing in the near future, Cullivan said.