As sloops, the ships had to remain below the London Naval Treaty's 20-knot threshold. Enchantress was officially rated at 18.75 knots, driven through two shafts by geared turbines producing about 3,300 shaft horsepower. The class displaced roughly 1,200 tons, carried about 125 people, and measured 81 metres long with an 11-metre beam. The closely related Egrets and Black Swans were longer and had about 300 additional horsepower, yet their published maximum speeds were only 19.25 and 19 knots respectively. One proposed explanation is that gearing or a governor enforced a treaty-compliant rating that a well-worked-up crew could alter in wartime. The comparison does not prove a hidden maximum, but it gives reason to treat official treaty-era speeds with care.
Chapter · 8:03-13:18
Treaty speed limits shaped the Bitterns' machinery
The sloops' official sub-20-knot speed kept them within the London treaty definition, while comparisons with later classes raise questions about how much performance their machinery could actually yield.