What Black Swan's 20-knot rating concealed

Should the Black Swan sloops have been faster, and how fast could they actually go?

Their 20-knot figure was an all-conditions rating; the design likely had appreciably more speed available when pushed.

The quoted figures pair roughly 3,600 to 4,300 horsepower with geared turbines, two shafts, and a 20-knot rating. British rating practice is interpreted here as a speed the ship should achieve across weather and operating conditions. That makes the nominal figure different from the maximum attainable when machinery is pressed. Speeds well into the twenties are suggested as possible, but no trial result or specific maximum is supplied.