This paper examines the relative effectiveness of different vessels used as convoy escorts in the North Atlantic by the Allied navies during the Second World War. A data set is created that matches convoy movements to German U-boat locations in order to examine the escort composition and the number of merchant ships lost when an engagement occurred. I find that larger escort vessels, such as destroyers and frigates, were between 1.4 and 2.25 more effective than smaller escort vessels, such as corvettes, at preventing the loss of a merchant ship. As an application of these results, I examine the cost faced by the Allies from Canada failing to develop domestic naval manufacturing capabilities prior to World War II. By constructing a counterfactual scenario, I find that developing a Canadian domestic shipbuilding industry would have given the Allies a net benefit of 23.2 million 1940 Canadian dollars.