At what RPM is the maximum speed achieved?

• A. 5000 RPM
• B. 6000 RPM
• C. 6500 RPM
• D. 7000 RPM

Answer: (B)

Maximum speed comes from balancing how much power the engine can deliver with how efficiently that power is turned into forward motion. As you increase RPM, the boat generally goes faster because more thrust is produced. But this doesn’t continue indefinitely. At some point, the propeller’s efficiency and the hull’s drag determine a peak: beyond that RPM, adding more engine speed adds power that mostly overcomes drag only a little, and propeller losses (like slip and cavitation) plus increasing hydrodynamic drag keep the speed from rising much or even reduce it.

In this boat setup, six thousand RPM sits at that optimal point where the propeller and hull convert the available engine power into forward motion most effectively. At lower RPM, there isn’t enough power to reach the peak, and at higher RPM, the gains in thrust are offset by reduced propulsive efficiency and greater drag, so speed stops increasing significantly.