It goes without saying, boats are not connected to the sewer or water system like we experience at home. Instead, boats store, collect, and take in water for important onboard amenities. Boats have three types or “water systems”, fresh water, grey water, and black water. Let’s discuss how all three systems work.

Fresh Water- Fresh water is stored in plastic tanks built into the boat, commonly these tanks are under a bed or settee and can be anywhere from 10-100 gallons. On a modern sailboat, tanks are +/- 50 gallons and boats above 40’ have two or sometimes three tanks. Since the water tanks are at or below sea level, a 12 volt freshwater pump is used to pressurize the system and deliver water to the faucets as needed. This pump will come on when it feels the pressure drop below a certain level and turn off once the appropriate pressure is achieved. Without an onboard water maker or access to dock city water, these tanks are the only source of freshwater for extended trip.

Grey Water- Grey water refers to water after it has been used. The most common grey waters are from the sink and shower but any water source that is used in some fashion onboard is now considered grey water. For now, grey water is allowed to be drained directly overboard via the boats plumbing. As an example, sink water drains through a P trap and then exists via a thru-hull located below the drain. Sometimes two drains, like the sink and refrigerator, lead to a single thru-hull for reduced build cost and ongoing maintenance.

The other example of common grey water comes from the shower, but this water typically takes a slightly different path. Considering most shower drains are at or below sea level, it would be impossible for the water to use gravity to exit the thru-hull like the example above. Instead, most boats will have a dedicated shower pump the turns on and evacuates the water collecting at the bottom of the shower. Some boats will have an automatic pump with a sensor while others may have a manual switch.

It’s good to keep in mind that everything you put down the drain leads directly to the ocean.

Black Water- Black water comes from the head and simply put… is sewage. Unlike grey water, black water can only be discharged from the boat in certain situations. When the head is flushed, it goes through a hose and eventually makes its way to the black water tank or holding tank. Traditionally, the waste would first pass through a Y-value or diverter value that would send the waste to the holding tank or to the direct overboard thru-hull. That said, it’s common to now see gravity holding tanks that do not require a Y-valve. Explanation of both coming below.

In the US, it is legal to flush black water into the sea given you are more than three miles from any point of land. In this case, one could simple direct the Y-valve to the overboard position, open the head overboard thru-hull, and use the head. If your boat has a gravity holding tank, you simple open the tank outlet thru-hull and the waste will go from the head, straight thru the holding tank and overboard.

Since you are allowed to pump directly overboard while three miles offshore, you are also allowed to empty your holding tank of all waste. If equipped with a gravity tank, you simple open the valve at the bottom of the tank and gravity will take the waste and empty it overboard. If you are equipped with a traditional Y-valve and macerator, it works a little different. In this set up, the holding tank will have a hose that leads to a macerator pump and then to an overboard thru-hull. One would open the overboard thru-hull and engage the macerator pump to empty the tank.


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