The London Naval Treaty left qualifying surface combatants between 600 and 2,000 tons outside overall tonnage limits. To qualify, a ship could not carry a gun above 6.1 inches, more than four guns above 3 inches, torpedo launchers, or a design speed over 20 knots. This allowed the Royal Navy to build patrol and escort hulls without consuming restricted cruiser or destroyer tonnage. A four-inch main battery also gave the small ships useful anti-aircraft fire for convoy protection. Third Sea Lord Reginald Henderson used this opening from 1933 to develop long-range escorts and other small warships.
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London Treaty rules created free small-ship tonnage
The 1930 London Naval Treaty let navies build unlimited qualifying ships below 2,000 tons, opening a useful procurement space for sloops.