You can theoretically do the math and come close to a proposed top speed, but even with that, you have to know how fast your engine can spin any prop. So a tachometer is a must to determine if you are getting everything you can from any marine engine and prop combination.
A prop pitch is basically stating that from ever revolution, you will move forward X inches with the prop pitch. In your case you stated you have a 21 pitch prop. So for ever revolution, your hull should move forward 21 inches. However, there is always slip with any prop. And the theoretically is never going to match the actual.
So knowing your engine's WOT (wide open throttle) RPMs will tell you how fast your engine is turning your prop. Then you have to also know the gear reduction ratio in the drive to the prop, and then do the math. But if your engine isn't riving up to what it should be turning, you need to reduce the prop pitch to get you to that WOT RPM. JMHO
