River-class frigates used vertical triple-expansion machinery rated at 5,500 indicated horsepower, while Black Swan sloops used turbines rated at 4,300 shaft horsepower for the same official 20-knot speed. Indicated and shaft horsepower measure power at different points in the system. Some loss between engine output and the propeller shaft is therefore expected, although the direct-drive losses would not plausibly consume the entire 1,200-horsepower difference. The comparison is a useful warning against reading machinery figures before checking what each rating measures.
Chapter · 25:37-27:35
How indicated and shaft horsepower compare
The River and Black Swan power figures introduce the difference between indicated and shaft horsepower before any comparison of their 20-knot performance.