The propeller of a speed boat is $1 \mathrm{ft}$ in diameter and is $1 \mathrm{ft}$ below the surface of the water. At what speed (rpm) will cavitation occur at the propeller? Water density $=64 \mathrm{lb}_{\mathrm{m}} / \mathrm{ft}^{3}$, vapor pressure of water, $P_{\mathrm{v}}=18.65 \mathrm{mmHg}$.