Boat motor cooling systems work by drawing water into the lower unit of the motor using its impeller pump. Water is circulated through a system of ports and passages and then out the water exhaust port carrying with it excess heat. Boat motors can be damaged if salt and silt are not flushed from the cooling system. Flushing a boat motor can be accomplished hooking a garden hose to a flushing device which directs fresh water into the motor's cooling system intake. The motor's impeller pump circulates the fresh water through the cooling system and out the water exhaust port along with harmful salt and silt. The most common flushing device, known as 'ear muffs', consists of two rubber cuffs connected by a U-shaped soft steel bracket which can be bent for a better fit. One of the cuffs has a fitting to which an ordinary garden hose can be attached. Some newer motors have a flushing attachment built in. Refer to your owner's manual for instructions.