Guys the best way to do this is to select crew with mechanical skill and assign them to DC slots in port (don't worry they are not always on duty and will sleep)
I usually fill up all the DC slots with men, when at sea after taking damage you just have to activate the DC icon or if you already were at battle stations they will already be on duty, of course when repairs are completed you have to manually de-activate them or they will stay on duty and fatigue.
Hope this helps
