The center hubs used on outboard motors and sterndrive boat propellers are separate from the boat propeller itself. There are two types of hub systems used. The oldest type has been around for over sixty years and is made of rubber that surrounds a metal spindle that goes on the propeller shaft. The rubber hub is larger in diameter than the hub cavity of the boat prop and is compressed under intense pressure then put into the propeller. Once installed the rubber tries to expand thus creating an extremely tight fit that will not move under normal boat operation. The rubber is suppose to break away from the inner wall of the prop hub and spin inside the hub if a sudden propeller hard impact with an underwater object occurs. This hub system is installed at the factory and is not meant to let the boat owner take it out or put it in themselves. Only a propeller repair shop should do that.
The second type of break away hub system is called the hub kit system. This system has been around for about twenty years now. Boat props that use this system are empty in the hub area of the prop and the proper hub kit designed for your drive system must be put in. These hub kits can be easily installed and removed by the boat owner without having to use any special tools.There is a hard composite drive sleeve that slides into the boat prop hub then a metal prop shaft spindle that is included in the hub kit slides into the drive sleeve. Attaching hardware pieces that come in the kit like spacers and nuts are used once the boat prop in on the prop shaft to secure the prop on to the shaft. The composite drive sleeve is designed to break away from the inside of the prop hub if an impact happens like the rubber hub system is designed to do.
The question is do these systems work and save you from damaging your drive system if you hit something underwater with your prop? The answer is not always. Only about half the time. Why? Because if you hit something real hard and real fast there is no time for these systems to break away and spin inside the boat propeller. The full impact pressure is taken and the most common damage received is a bent propeller shaft. Not much of a chance of that occurring if you are using aluminum boat props because you will most likely have the aluminum blades get destroyed before any other damage happens. Cast aluminum is a brittle metal and will give way easily when hit hard. Stainless steel propellers are a different story. They are about five to seven times stronger than an aluminum prop is so they have very little give to them. You have to be more careful using stainless steel boat propellers in shallow waters.