The Outboard Motor 2 Stroke manufacturer of your outboard likely outfitted it with a decent, middle-of-the-road propeller. If not, the dealer likely did. But if either of them set up the perfect prop for you, it was likely dumb luck: The right prop is dependent on how you're load your boat, and how you'll use it.
First, some propeller fundamentals. Props are quantified in diameter and pitch. You recall diameter from high-school geometry, naturally, however pitch? That is the theoretical distance the propeller would travel through one complete revolution when there was no slippage.
Moving off out of the brace itself for a minute, consider that 15 Hp Outboard Motor manufacturers rate their merchandise horsepower at a specific rpm, setting rpm ranges for top-end operation. It can only hit that top-end bracket when it's powering the right propeller. If the brace allows the engine to over-rev, wear and friction can happen. If the propeller restrains the motor from attaining that prime selection, it becomes bad mileage and you risk other harm. You may protect your engine and boost its performance by choosing the perfect pitch.
Picturer from https://yamahaoutboards.com/en-us/#