A coal trimmer or trimmer is a position within the engineering department of a coal fired ship which involves all coal handling tasks starting with the loading of coal into the ship and ending with the delivery of the coal to the stoker.
The trimmers worked inside the coal bunkers located on top of and between the boilers. Trimmers used shovels and wheelbarrows to move coal around the bunkers in order to keep the coal level, and to shovel the coal down the coal chute to the firemen below, who shoveled it into the furnaces. If too much coal built up on one side of a coal bunker, the ship would actually list to that side.
Trimmers were also involved in extinguishing fires in the coal bunkers. Fires occurred frequently due to spontaneous combustion of the coal. The fires had to be extinguished with fire hoses and by removing the burning coal by feeding it into the furnace. 
Of the engineering crew, the trimmers were paid the least. The working conditions of a trimmer were poor, primarily as a result of their environment: the inside of a coal bunker was poorly lighted, full of coal dust, and extremely hot due to residual heat emanating from the boilers. 
- Titanic Research & Modeling Association: Coal Bunker Fire
- Lee W. Merideth (2002). Titanic Names: A Complete List of the Passengers and Crew. Rocklin Press.