Kristjan, I'm not a "specialist" on this matters but I got the idea that trimmers were the ones who carried the coal from the deposits to near the firemen. I think this was their main job, although they might had some other duties.
Sorry for not to be of greater help but if you research carefully you'll find threads about this issues.
>> I think this was their main job, although they might had some other duties.<<
That was part of it. They would also be responsible for making sure the coal was distributed in such a manner throughout the bunkers so that it wouldn't have a negetive influance on weight and balance of the ship...say having an unusual list to one side or another.