F100 prop

I have an f100 on a 19ft ranieri at full throttle it rev was 5500 30 knots,i changed the prop to a 13*21 s/s and full throttle rev was 5000 27 knots it gave me a crusing spead with less rpm.Is there a minimum rpm? Does it harm the motor givig it more load?It is more economical with a bigger prop?

[boats.net]

The operating range on that motor is 5000-6000 rpm's. Its better for motor and fuel economy to prop to high end of range.