Lusitania would never have been a Q-ship. Q-ships were much smaller merchant ships which were intended to bring U-boats to the surface to sink them with gunfire (rather than using a much more expensive torpedo). They had concealed weapons.

Passenger liners were converted into Armed Merchant Cruisers by adding (usually) 6 to 8 6 inch guns - wich were not concealed. Most countries gave a grant to the shipping companies owning large fast passenger vessels to have strongpoints built in so that these guns could be added in case of war. The other main use of these vessels was, of course, as troopships.

Les