Hello MattyBoy
Looks like that nobody likes to give you an answer. So I will try my best english to gave you at least When the Hatchcovers are opened, fixed Hydraulicstamps are activated by one cental lever and the Hatchcovers will be lift for the rolling out position. Rolling out to the side will be made with the help of Hydraulic Motors or Air driven motors. This motors will pull a chain and the Hatch covers will roll out on the rails to the resting position. I worked on different Bulk Carries until 1998. The vessels where I sailed on was build between 1972 and 1981 and in this time the system worked as I described, but I do not think that it have changed alot in the last decade.
May some additional info. Sometimes it was also necassary to open the hatch covers on navigation for better access for cleaning work or to dry the cargo (Grain Products). In this case it has to be sure that the vessel is not more rolling as 3 degrees and the Hatch covers had to be secured by lashing equipment.
Best regards
Capesize
here one link to some of the Bulkies I sailed on http://www.test.swiss-ships.ch/schiffe-ausland/metrofin/akron_D5FV/frame_akron_D5FV.html
http://www.test.swiss-ships.ch/schiffe-ausland/metrofin/theodore-a_D5DY/frame_theodore-a_D5DY.html
http://www.test.swiss-ships.ch/schiffe-ausland/metrofin/alfios_ELBJ7/frame_alfios_ELBJ7.html

How do they open those hatches? Is it done by hand or is it electric?