There are a few different ways you can tell if you need a new propeller for your boat. One way is to look at the blades. If they are bent, chipped, or cracked, then it's time for a new one. Another way to tell is by how your boat is performing. If you notice a decrease in speed or fuel efficiency, then it's probably time to get a new prop. Lastly, if you hear strange noises coming from the engine area, it could be an indication that the prop is damaged and needs to be replaced.