You can test your 2 Stroke 15hp Outboard Motor by running your boat at wide-open throttle. (If you're over the maximum rpm, throttle down until the needle is within the manufacturer-suggested range.) Ashore, inspect the propeller to learn its pitch. Figuring that you get about a 200 rpm drop for each increase in pitch, try a different prop. A cupped propeller will also reduce rpm by about 200. Some dealers will even let you test a prop, provided you return it undamaged and promptly.
As long as it hits but doesn't exceed the 15hp Outboard Motor Impeller Assy's top-end range, a prop delivering wide-open rpms toward the bottom of the range provides stronger hole shots, while those toward the top generate slightly higher top speeds.
Prop condition can be more important than type: A damaged prop can destroy the engine spinning it.
• Stainless-steel props run truest and are five times as strong, but aluminum is cheaper and sacrifices itself to protect the lower unit.
• A four-blade prop beats a three-blade in time to plane, midrange speed and low-speed handling.
Picturer from https://yamahaoutboards.com/en-us/#