Barnacles are small sea creatures that attach themselves to the hulls and propellers of boats. A colony of barnacles forms an encrustation of hard, sharp shells that can foul propellers, block drains and greatly reduce engine efficiency. Left unchecked, enough barnacles can build up on a propeller to render an engine useless. Removing them is hard work, and it must be done with some care to avoid scratching the propeller. Gouging the propeller with metal tools will speed up corrosion. Methods for barnacle removal differ depending on whether your boat is in or out of the water.
Shut down your boat's engine and wait for the propeller to stop spinning. If possible, haul your boat out of the water.
Wedge and scrape the barnacles from the prop blades using a plastic putty knife. Although metal blades are stronger, they will damage the propeller. This will be easiest when the boat is recently removed from the water and the barnacles are still soft. You can do this while your boat is in the water by diving down to the propeller.
Apply barnacle-removing chemicals to the residue left behind after you scrape the barnacles off. These chemicals (available from boating suppliers) contain acids that will dissolve the leftover bits of barnacle. Your boat must be out of the water to do this.
Put on gloves and scrub the prop blades with a cleaning brush once the chemicals start to foam.
Wash the cleaning chemicals off your propeller with soapy water.
Haul your boat out of the water using whatever truck or winch you normally would to bring your boat in and out of a marina. If you have a large boat, you may have to take it to a dockyard with a dry dock for this.
Run a hose from the nearest water pipe to a pressure washer set up behind your boat.
Plug the pressure washer in and turn it on.
Turn the hose on.
Put on safety goggles. Squeeze the trigger of the pressure washer to start blasting barnacles off of your prop.
Remove as many barnacles as you can with the pressure washer, then shut it off and scrape the rest by hand. Finish the job by treating the propeller with barnacle-removing chemicals.
You can have your propeller coated with slippery treatments that make it much more difficult for barnacles to adhere to it. Enquire with local boat maintenance businesses.