Too large a propeller overloads an engine, requiring more power than the engine can produce. The correct propeller selection is probably the most important factor in any marine engine installation. Here is a quick method to see how your propeller relates to your engine, reduction gear and hull. Generally, a larger propeller diameter produces greater thrust, and will offer better slow-speed handling characteristics. Owners of higher-speed vessels will find smaller prop diameters more efficient.
Propeller pitch affects the engine rpm at a given hull speed through the water.
Before you haul out the boat and measure the propellers, you'll want to collect some data on the existing propellers and how they operate under way. Get the boat under way in smooth water, and determine the maximum rpm that the engine will run at with the throttle open all the way. It is helpful to have a fairly clean bottom for this test. As a rule of thumb, use the pitch ratio to check the existing propeller installation. The diameter and pitch should be stamped on the propeller hub.