I learned that sonar spotting distance is determined by the FCS space (not the FCS
system) of a particular ship and then that distance is lengthened based upon the true
potential ability of the sonar sailor (if any).

I had been taking a Kagero as sonar escort (i.e. a kagero with the smallest possible
guns and crew and ammo but with a sonar sailor) but that is a fail in IJN. A much much
better choice although more expensive is a Yubari with 66 (the largest FCS space along
with Kita and Dhonburi of any CL in any nation). You must use a sonar sailor with the
Yubari since technically it is a CL.

Get a sonar and tow one of those behind your bb.

If you are strapped for cash, the Akitsuki-Kai is just a tad more than the Kagero but has
FCS of 45 vs 30 of the Kagero.