Basically, early submarines were surface craft that had the ability to dive for a limited period. The standard design had the vessel spending the vast majority of her time on the surface, using the diesels. When weather gets too bad, attacking an enemy by surprise, or finally to evade enemies, the batteries will allow maneuverability underwater for a limited time - but eventually you HAVE to surface, due to either the air getting bad or the batteries running out - and they can only be recharged by the diesels. Key to this is the diesels need a constant input of air to run, thus only work on the surface. Later in the war, when der Schnorchel is mounted, you can recharge the batteries while still submerged - but you are VERY shallow and detectable.

thanks drake! exactly the answer i was looking for:salute:

The snorkel is easily spotted by radar by the time it becomes available. Also don't need to be moving while the snorkel is up to recharge the batteries, depending on where you are you might get lucky and have time for a full recharge, or only get a few minutes till an aircraft spots you and goes after you.