OM: Transfers

I play SH3 GWX3, but I've been enjoying SH4 w/ OM lately, and have a question. What are the restrictions on transfers, if any? I've noticed that the transfer screen says that no transfers are available. I have only finished three patrols so far (1 promotion), but am curious about when this might become available.

While I am currently using a Type VIIB, I may eventually wish to transfer to the 2nd Flotilla with hopes of getting a Type IXB in time for Operation Drumbeat. I have been doing well, getting medals on each patrol, and I hear that it is performance that influences whether you get a new boat offered.