A piston-engined Whirlwind provides the scenario's 95-knot benchmark, approximately 109 miles per hour. At that speed it can cover ten miles in a little under six minutes, allowing aircraft to converge rapidly on a submarine near a convoy. The Swordfish comparison makes the same point: an aircraft need not be fast by fighter standards to threaten ships and submarines. Helicopter speed matters relative to the target and the distance between patrols.
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Helicopter speed relative to a submarine
The Whirlwind's modest speed by aircraft standards still allows a rapid response to submarines near a convoy.