Name this "light-ship" or one of its sister-ships

[Staga]

And also tell why name "light-ship"  

[Arlo]

Väinämöinen and Illmarinen? Finnish coastal defense ships. They had shallow drafts and were ill-suited for operations in the open sea. That would be my guess about the "light ship" thing.

 ShruG ... if I'm even close.

[Staga]

Ships were Ilmarinen and Väinämöinen (Names are from Finnish mythology).

Story tells that in Spithead in coronation of King George VI some old Admiral said when he saw Väinämöinen "A bold little navy indeed. Even the lightships are carrying big guns".

Ships were build for coastal defence (movable shore batteries) and main armament were 4x254mm(10") with range of +30km with quite sophisticated fire control system and 8x105mm Bofors cannons. Ships needed only 5 meter water depth.