German estimates called for sinking 300,000 tons of shipping each month before the United States entered the war, then 700,000 tons after American entry. Liberty and Victory ship production, rationing, and greater self-sufficiency raised the scale of the challenge. Tankers were tempting targets, yet some much smaller cargoes could be more critical than oil, petrol, or aviation fuel. The Allies spread essential material across many ships, which made it difficult for unrestricted submarine warfare to destroy one indispensable shipment. Britain kept those supplies moving more effectively than Germany did under Allied blockade. A complete interruption lasting two or three weeks might have been decisive, but the U-boats never achieved it.
Chapter · 3:30-5:18
Shipping tonnage, rationing, and British resilience
The campaign depended on whether U-boats could sink shipping faster than the Allies could replace it and sustain essential imports.