It looks like a standard stern packing gland as fitted to propshafts.
If it is like the rudder gland on my 321 it was just a matter of releasing the locking nut and then tightening up the main nut.
I used one big stilsons and also some very large plumbers waterpump pliers.
Presumably if very old the packing material may need replacing.
Probably best to ask Nauticat directly what size the packing is.