on an ss1, you'll want to maximize speed. At ss2, you can start adding armor (i prefer
bulk head over bulge, some may differ), however, if you add armor you will lose valuable
speed. At ss3, I load bulk head until i drop speed.
The reason I prefer bulk head over bulge is that once you have a high SD, you can take
more hits, and often times, those hit will not be bulge hits, but dmg from shells. Because
of this, bulge is not very usefull (unless you want to foolishly tank through a torp wall
or get in a fight with a km ss). Also, bulk head gives you the ability to hold overheat
and higher speeds even when you are crippled*(not crippled, but where you would normally
start to lose speed). As a submarine, this is valuable as you may lose half your DP in the
first few shots if you are unlucky.

don't bother putting armor on ss it will just slow you down,

make your bo reduced weight (lowering the number of recruits)
torpedo man need to be 2 or more level so changing from ss1 to ss2 or ss2 to ss3 or s3 to
ss4 with easy
dont bother putting gunner on the guns if it has one, instead put rep
for the extra support slot put an engy

while in battle, dont dive to early from the start cause you will ran out of air since
your ss1, i know your to slow, you need to have patience, avoid high tier sub in face to
face battle especially a km sub, if your figthing enemy sub always watch out if they got
rear tubes, avoid small ships such as ff, dd and cl they can see subs clearly with sonar
(ff,dd can see even without one but short range) if you can't avoid them sink them if they
bring a hh, crit dive when they fire their nasty hh if its a dc you can surface it have
small damage in surface but you'll be attraction to others...