If you have the IXB type sub I also presume, that you have the C/30 3,7cm heavy flak. It is slowly firing weapon, especially if you compare it with the C/30 2cm Flakzwilling. You also need to have at least one skilled flakgunner manning the cannon, because the reloading times are much faster, but even then you can shoot just a few shots at a plane attacking you. But a well placed shot can cripple any of the early war planes.
It's quite hard if not impossible to hit a hurricane with it, but it can be quite deadly against Sunderlands. When they do an attack run, they are flying straight at you and you can hit them quite easily. Also the AA ammo explodes and do some damage even if you don't hit the plane.
When the M42 comes into action, it's quite a different story - you can aim with them just as with a light flak, pointing tracers at a plane.
The different story is, that you can use 3,7cm AP ammo to sink smaller ships. It's quite useful when you meet a lone coastal merchant, which surely isn't worth a torpedo. Or have that last T3 left sitting after a convoy run and are out of any other ammo... Especially with IX's low cannon ammo storage.
That's why I go directly from turm IX/I to IX/III, to not to lose heavy flak
