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Old 10-05-2001
Contributing Authors
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Grey-water Thru-hulls

Why are thru-hulls for grey water, like the sink drains and in some cases even the cockpit drains, below the water line on a sailboat but above the water line on many power boats? Wouldn’t it be safer for as many thru-hulls as possible to be above the water line because then even if the line broke or the seacock leaked, the boat wouldn’t sink? Is there some reason I am unaware of for placing the boats more at risk of sinking?

Sue and Larry Respond:

Yes, it does make good sense to minimize the number of seacocks whenever possible. The problem arises as the boat begins to heel. On sailboats, grey water and particularly cockpit drains are designed so that they will continue to function as drains at what can sometimes be extreme angles of heel. Depending upon the height and location of your sinks and the depth of your cockpit, a particular drain may or may not work well if it exits above the waterline. As far as sink drains are concerned, there is probably a small element of aesthetics involved as well as no one wants to see spittle or toothpaste drool running down the side of their boat.

As a compromise, some manufacturers have tried different arrangements. Amel positions their sink drains right at the water line, and their cockpit drains lead straight down through a tube fiberglassed into the hull eliminating a hose and seacock. Others have employed holding tanks to contain the grey water and discharge it at a single location above the waterline usually via a pump.

For the majority of us with numerous seacocks below the waterline, it’s important to establish and follow a regular maintenance schedule. Hose clamps need to be checked for tightness, hoses inspected for wear, handles on seacocks activated regularly, and ball valves need to be kept lubricated. It’s all part of what we call "Out of sight, but not out of mind maintenance."

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