This is the small driving cap for small jobs. We make a super driving cap also available in our dock tool section.
It is used to protect the pipe from the sledge hammer and provide an even surface to drive the pipe in.
This part is expected to wear out eventually as it is beaten repeatedly between the dock pipe and the sledge hammer.
There is a hole to tie a rope so it does not bounce off and land in the water. Rope should loop around the pipe being driven.
To avoid confusion when buying pipe that fits in this, the industry standard calls the pipe size as 1.5″ schedule 40 but the O.D. is actually 1 7/8″.