As a point of reference, my boat has a 27K BTU unit on the bridge; in the central Florida summer it doesn't do really anything but chisel it from 120 to 95 degrees. It has two large diffusers that blow from the helm forward and three round adjustable ones blowing towards the seats at the helm. Just my experience and the 52DB's bridge is quite a bit larger but I'm thinking you are wasting money on those small AC units.I am planning on putting a seperate self-contained A/C up top. Originally, I had planned on putting it in the space port and aft of the trash can.
After some time, I have shifted my thoughts and plan on putting under the seat just starboard of helm. My thought on the change is that with the ice maker already under the steering wheel that is enough excess heat in that space (I think it killed my autopilot computer, since replaced).
I am trying to decide between an 8k and 10K unit, knowing that really the cool air is only going to blow on folks and not drop the temp up top. I do also want it for heat for Christmas parades and other "winter" excursions in Northwest Florida.
I am thinking two small vents for the helmsman, a vent for the forward starboard seat, and vents blowing aft towards the big couch across the aft of the bridge. Maybe, a small vent to cool under the helm to save electronics.
As hot as it gets here, I can't imagine "stealing" from an 8k BTU mid-ship is going to give the relief required for a full day hanging out on the bridge. Other flybridge boats have hard windshields and other ways to allow folks to close off the bridge, not my boat. It is going to be about airflow to blow cold air on people giving some respite from heat/humidity
Going to run cooling water from the manifold in the aft state room up (yes, will increase pump size), and then back down to the "main drain" where the salon A/C dumps cooling water. The cooling feed and return all will go through the starboard area behind the washer dryer (3 SR model). Condensate will go out the helm sink drain which shares the washer/dryer drain.