Hull form helps explain the power comparison. A finer Black Swan hull could rise and reduce the area dragging through the water as speed increased. The broader River form retained more wetted surface and therefore more friction. That resistance helps explain why the frigate needed greater rated power to reach the same official 20 knots. The penalty was accepted in exchange for stability, strength, and construction in a much wider range of yards.
Notable Point · 28:53-30:36
Why the River hull needed more power at speed
The River class traded the Black Swan's finer hydrodynamic form for a broad, stable hull whose wetted area remained comparatively high as speed increased.