It was built very early in Stampy's Lovely World. At first it was called Stampy's Ark, but one day he changed his mind and decided to call it the S.S. Stumpy. Because of it's original name, some of Stampy's minecraft helpers built animals aboard it such as a giraffe (which is still there now).
In Sinking Feeling, It was used to sail out to Hit the Target's ship to destroy it. Stampy, and L for Leeeee x went after Hit the Target and blew up his ship. They made Hit the Target walk the plank. After the battle they sailed home.
At Episode 214, Beat The Heat, a fire is seen on the port side of SS Stumpy.