Is my propeller too small?

Too small a propeller will produce very low torque, poor thrust and low speed at the stated RPM.  The engine’s hp cannot get transmitted into the water and the propeller becomes an “egg beater”.  Because the blade loading is far too high, the blades slip and water boils off the blade, which is cavitation.   The situation is called “under propping”.  Speed is compromised particularly in headwinds, head sea or current.