[ETR3(SS)]

You're right when you say the Damage Control situation in SH3 is much easier to understand, at least in my opinion. For SH4 it sounds like you've pretty much figured out the system. The red bar next to the damaged item is the damage to that item in a percent format. 1 being completely destroyed and 0 being no damage. As for estimates on repair times, the only estimate you will ever get is for the bulkheads if they take damage. I assume that at this point you may have seen some flooding occur in game and saw the water level rising in your DC screen. If you mouse over that you get a time to repair the bulkhead and a time the compartment will be completely flooded in. Something to keep in mind if something is completely destroyed is you can refit at a friendly port and repair that item good as new. But this does not repair Hull Damage.

A Dive Plane Transmission is what controls your Dive Planes, break that and your bow or stern planes won't function anymore. The Compressor is an Air Compressor. You need compressed air to surface the boat, important thing to have! The Hydraulic Pump supplies hydraulics to your Dive Planes Transmission and the Rudder Transmission so that these control surfaces can move. The Main Pump is what you use to dewater your boat should you start flooding. Hope that answers some of your questions.