The two basic crew hullmods, mutually exclusive.
(For the sake for not complicating things too much, the two hullmods do not do anything if crew understrength is happening)
Unskilled crew - Costs OP depending on ship class (fri/des/cru/cap), crew pay for (skeleton crew req of ship) numbers of crew reduced by 30%
Skilled crew - Gives OP depending on ship class (fri/des/cru/cap), crew pay for (skeleton crew req of ship) numbers of crew increased by 30%.
Specialized crew hullmods can be mounted if ship has skilled crew hullmod, all specialized crew cost of the same amount of OP as given by skilled crew hullmod. Only one specialized crew hullmod can be installed at a time. Some (example) specialized crew mods:
Expert Gunnery: Increases the accuracy and range of all weapons.
Damage Control: Increased hull integrity and ship repair speed.
Ace pilots: Increases speed and armor of strike craft.
Marine Armory: Increases combat effectiveness of 25/50/75/100 marines like Ground Support Package.