B777 Fuselage drain ports

[STBYRUD]

Hello there,
I've been looking for some time for an explanation as to what the six (or was it eight) wedge shaped, roughly transponder antenna sized drain ports centered along the bottom of the aft 777 fuselage are for - the forward fuselage has the standard small, round drains that I remember from other types such as the 737, but the aft ones look different. Are they standard bilge drains or do they have a special function? I looked in the AMM, didn't find anything useful there unfortunately.
http://abload.de/img/wroql940_0u0g3q-g9evkq7scm.jpg

[halas]

Just guessing, but l suspect there is difference in Pr between the fwd and aft drains. The fwd are ok with delta P to work whilst the aft need a bit more assistance with the aft facing mast.

Landed in Riyadh once and the fwd GMC was all dark. So asked engineer to check it out.
Turns out it was covered in prawn juice. We had 11 tonnes of prawns that were supposed to be frozen in the fwd hold. They were not. They had been left on the tarmac in 45 degrees for many hours prior to loading. :ugh:
The juice came out through the drains and on to the camera lens, obliterating the view.
The aircraft was pretty high when we got back to base. Total mess!

[TURIN]

Not sure about the deltaP theory. The pressure in the rear and aft holds should be very similar. Besides, don't the drain valves automatically close when the cabin pressure is above ambient?

Boundary layer along the rear fuselage perhaps.