Who commanded Royal Navy trawlers

What was the usual rank of an armed trawler's commanding officer?

Typically a lieutenant, often from the Royal Naval Volunteer Reserve; some civilian trawler skippers received wartime commissions and retained command of their vessels.

Small escort craft did not require the rank structure of a destroyer or cruiser. The Royal Navy could commission experienced civilian skippers while also assigning RNVR lieutenants, drawing on both naval organization and trade knowledge. This was one way rapid wartime expansion converted commercial vessels into an effective patrol force.