Hmmm ... For staying long submerge u need a good, not-skeleton Planesman. Get some Elite Potential (+16 Pot), boost it and make IJN Planesman. I don't think problem is amount of air, problem of IJN SS's is their low speed (or weak engine sailors on them) ... EBVE Planesman and EBVE Engine Sailors are keys to good SS hunt ... if 1 Planesman doesn't satisfy maybe try make another one - SS5 IJN has 7 Support Sailor Slots, so 2 Planesmans will fit (they won't give more air but u can 'recover' it faster under some smoke/cloud) ... ;]

Planesmen only increase the speet the air is refilled when surfaced.
And i don't think planesmen stack (and if htey do it's probably not worth it anyway) but i'm no expert in this field.

Planesmen do not stack.

As for having "low air" I cannot find anything where the air capacity of subs are listed.  Sorry.

I have decided to go MN. because of the faster speed. That way i can actually reach the enemy bb line. Thanks for your help guys.

Hi,
all subs have -compared among tier (so compairing just ss1, or ss2, etc.)- the same amount of air (pick randomly 2 ss5s and make the test). The only thing that varies is the recovering of the air, which depends of the planesman.

You just need 1 planesman! More won´t work better, so it´s a waste of support slot, money and time to have a second. 

The advantage of IJN subs are the torps, and in case of the ss5 the torpedo bombers you can use. (Not to speak of the 8 front tubes of the ss4). 

Aderezo,

    I don't know what you are smoking, but I hope you are smoking it when I play against you.

    Every sub's air time is different.  To prove it to myself, I just ran a test with the KM SS5 and the IJN SS5.  KM stays under for 217 seconds, the IJN for 229 seconds.  Not a lot of difference, but the IJN is under for  5.5% longer.  Subs of the same level will have similar air times.  
   For recovery, the planesman is the most important aspect, but different subs have different recovery times.  

      Traditionally, the slower the sub, the longer the air time.  So, for everything I know, the IJN subs stay under the longest.  I am sure there are posts out there (maybe on the old website) which tell you exact numbers -- I don't have the time to run them all down.  

   Which brings up a fun point about IJN SS5's with good crews.  They are the best for a HA decoy fleet.  You first launch 3 TBs to scout the other team, then you wait for them to get near, go under and run in OH as far as you can.  The TBs fool the other team into thinking a CV is in the area, and the long underwater time is hated if they don't have ASW.  One game, I kept the room open for like 30 minutes because they couldn't find me!  Another one, my TBs found the BB and CV in the room and me and another sub killed them.