Any boat with a permanently installed toilet is required to have a marine sanitation device (MSD) in U.S. waters, able to either store waste until pumped out on shore or adequately treat it for overboard discharge.
What’s Your Type?
Manual systems pump water into the toilet bowl, and sewage out to a holding tank, via hand (typically piston-rod) pump. Choose horizontal-throw handles; they take less effort than verticals do. Electric motors eliminate the manual pumping. Look for a large “joker valve” to avoid clogs.
Vacuum systems rely on an electric pump to create a vacuum, which literally pulls fresh water into the bowl and waste out. They require little preventive maintenance and are considered the easiest for landlubbers to operate.
Electric macerating systems feature an impeller pump to draw water in and a discharge pump to pull contents out. The middleman in the equation is essentially a garbage disposal, which grinds waste before passing it on to the holding tank. Foreign objects can damage the motor, so guest education is essential.
Three of the most popular brands are Jabsco, Thetford and Raritan.
Heads that use raw water stink, especially in salt water. Salt water also causes scale deposits to build in the discharge channels and hoses, reducing flushing efficiency and potentially causing a blockage. An on-board freshwater tank is better, but both supply and holding tank capacities are a concern when at anchor.
Vacuum heads use the least water, as little as one to three pints with each flush. Macerators require the most — a minimum of one gallon to rinse clear urine and three gallons to clear solids and paper.
A holding tank with through-deck fitting may suffice in areas with convenient pump-out facilities. An optional overboard discharge via a Y-valve installed in the line between toilet and holding tank provides a way to pump overboard when beyond the three-mile limit, but accidental discharge is a possibility. We like a direct path from toilet to holding tank, followed by multiple discharge options (pump-out or overboard discharge) downstream. Make sure the seacock for any overboard discharge is within easy reach; it has to be locked inshore (zip ties work great) to satisfy legal requirements.
Hoses get a bad rap for odor. Inadequate flushing (rinsing) is more often the cause. Use only the hose recommended for marine sanitation. Avoid corrugated hose; it traps sewage. Likewise avoid low spots where waste can collect. A vented loop must pass above the waterline in any raw-water intake hoses, as well as head discharge hoses. This prevents siphon-created backflow, which could lead to stinking. A three-quarter-inch through-hull and intake seacock is common for seawater flushing, 1½ inches for overboard discharge. Choose bronze or Marelon for durability.
Avoid metal holding tanks, since urine is corrosive. High-density polyethylene, at least one-quarter-inch thick, is best. Wall thickness should increase with tank size to prevent bulging.