The ship is a paddle-boat that the pilots peddle from within. The "mouth" of the ship has three cannons that can create quite some damage. It was stated by Patty that it is the ultimate weapon of Baratie.
The Sabagashira I was used against the Krieg Pirates. Patty and Carne tried to use the cannons to strike down Don Krieg, but his armour protected him from the blasts, and he lifted the ship with ease, throwing it onto the rooftop of the restaurant.
Saba (鯖) means "mackerel", and "gashira" (頭) means "head". This makes the literal translation of Sabagashira "Mackerel Head".