On New Moon's trailer I had a jack with the flat plate bottom, as did Dr Bob. Much sturdier than the swiveling wheeled types. Worked for 15 years on my ~12,000 lb boat/trailer. Every fall I would run some fresh water through the innards and add some new grease to the bevel gears. Here's one like mine:
https://www.etrailer.com/dept-pg-Traile ... ulton.aspx
https://www.etrailer.com/dept-pg-Traile ... ulton.aspx