Why horsepower figures need their test context

Should early motor-boat engine figures be read as brake horsepower or shaft horsepower?

It depends on the test and period; some early trials used brake horsepower, while British practice increasingly standardized figures as shaft horsepower for comparison.

Early motor-boat sources did not always use brake horsepower and shaft horsepower consistently. Some trials reported brake horsepower, while British constructors could convert figures to shaft horsepower so different ship designs could be compared. By World War II, shaft horsepower had become the standard measure for output delivered through gearing to the propeller. The discussion also notes that some early motor boats used direct drive, which helps explain why older figures require attention to the original test and machinery arrangement.