How did rear admiral john dahlgren of the union blockade of charleston attempt to prevent attacks underwater such as the hunley

"Admiral John Dahlgren, the head of the Union blockading fleet, learned of the diving submarine from Confederate deserters. In response, Dahlgren ordered his blockading squadron to anchor in shallow water...**hang ropes and chains over their sides as defensive measures, and deploy picket craft to keep torpedo-bearing boats away"

I think this means they prevented the armed boats(torpedoes) from firing at the submarines

Why not which it you