What type of license is necessary for unols or research ships in general?

I’m an engineer thinking about transitioning to the wheelhouse. Most of my work has been on unlimited hp ships so I’m familiar with the unlimited license structure, but I’ve been wondering what kinds of ships can you work on with a 1600 ton mates license? In particular research ships. I know that the big ones like the Thomas Thompson require an unlimited license, but I’m unsure about the rest of the fleet. Also, tangentially related, does unols have a “relief pool” system for the fleet? Or are you directly hired and locked into a rotation? Thanks.

Vsl tonnage will dictate what license is required. I did relief work for several UNOLS institutions. It was an informal arrangement. Their are relief but not a pool. Usually the permanents work as long as they want and as a relief with knowledge of a specific class of vessel. I would just rotate through the big three. I did this for 5 years. I sailed on the TGT, Knoor, Oceanus, Revelle.

Thanks. Were most of the ships unlimited?