When do you switch from s-class to bigger boat

[Von Hinten]

Okay, so Seafarer got me thinking with this remark:

Originally Posted by seafarer

Of course, if you prefer to toot around in your kiddie boat, that's okay with me When you're ready to join the real submariners, we might let you have one of the big boy boats

Now I did some searching but was unable to find an answer on this. Say I am ready and would want one of the big boy boats ... Does anyone know when you're presented the opportunity to switch to a bigger boat?

Two patrols ago I did get one 'good job lad, it's time you quit and started training our new officers' kind of request which I kindly declined so I could keep on sailing, but I'm actually curious as to what I have to achieve to be allowed a bigger boat.

I started my career with the Asiatic Fleet and got assigned my current sugar boat, the SS-138 and still have that same boat whereas when I first began playing SHIV, I started at Pearl and seem to remember being given a new boat every three or four patrols. (And no, not because I sank the previous one ... )