A Frenchman once jabbed, “Why do you stupid Americans store your toilet waste under your bed on your boats?! That’s such a disgusting habit!”
Exactly what constitutes “disgusting” may vary among cultures, but his observation about plumbing systems within boats flying a U.S. flag is spot-on. Indeed, holding tank use is strictly an American preoccupation, and the infrastructure for pumping out waste tanks is virtually nonexistent outside the U.S. The majority of sailors from the rest of the world have no tank and instead pump their toilet waste directly overboard. As an American, I think our habits around holding tank use, and the regulations and pump-out systems that support those habits, are something to be proud of and emulated, not ridiculed.
That said, my French friend has a point. Holding tanks are, by nature, pretty gross and often the source of bad boat smells. To keep your boat smelling fresh—and to ensure your good environmental stewardship practices don’t turn the insides of your boat into a hazmat zone—you need to adhere to an effective tank care routine and build your marine sanitation systems out of the best possible supplies.
To monitor the situation, let your nose be the judge. Unwanted odors can be addressed by evaluating the following possible causes:
Stagnant seawater. When a seawater toilet hasn’t been used in a few days, it will produce a pungent odor when it is first flushed again. It’s helpful to know why. About a gallon of seawater fills the hose between the intake seacock and the bowl. When that water sits for several days, all the microorganisms that live in that seawater die. Their microscopic little corpses then rot, and that rot is what you smell coming from the toilet bowl when you flush. Somehow, it’s comforting to know that the smell is from dead seawater and has nothing at all to do with human waste. If you flush your toilet at least once a day, the water doesn’t have time to stagnate and won’t produce any odor. If the boat sits unused for a few days, the stinky water will be removed from the boat after the first flush.
A simple psychological trick can be a remarkably effective solution for addressing this particular odor: Now that you know what the smell is, try to associate it with happy thoughts about the beach and oysters. Suddenly the smell is a welcome natural perfume of summer and not something to make you grimace in disgust. If you can’t make the mental gymnastics work, another way to mitigate the stale seawater smell is to add a splash of holding tank deodorant to the first flush after the boat has been sitting for a few days. As a bonus, this dose of deodorant will advance down the hose to fight odor in the tank.
Switching to a freshwater toilet will solve the stagnant seawater smell problem completely. Unfortunately, this choice comes with significant downsides. It feels wasteful to tap into your limited supply of freshwater to flush a toilet while surrounded by an infinite supply of seawater that could do the job just as well. Freshwater toilets are more complicated than seawater toilets, and as such are more prone to breakdowns. Freshwater toilets use more precious battery power than seawater toilets. Lastly, if a back-flow preventer valve fails, it is possible that wastewater from a freshwater toilet can migrate backwards and contaminate your freshwater supply in the water tank—not something you want to think about when making lemonade or brushing teeth. All factors considered, I believe it is smart to stick with seawater flush for all onboard toilets.
Smelly waste in the tank. Frequent pump-outs coupled with copious use of holding tank deodorant is the solution here. And I mean copious amounts! An eighth of a gallon jug per flush, or more, is often needed. My favorite brand of holding tank deodorant is “K.O.” by Raritan. The stuff is expensive, but it works well if you use a lot of it.
Tank or plumbing failure due to overfilling. Holding tanks and their fittings will sometimes break when someone tries to get one last flush in there that won’t fit. Odors and waste easily escape through these types of failures. Tank gauges should be monitored regularly, and all crew aboard must be careful not to overfill the holding tank. If the odor problem is new and acute, a physical failure of the sanitation system is a likely cause. In such cases, you can usually identify the point of failure by visually noting the waste leaking out of it.
Missing or abused holding tank vent filter. Every time you flush waste into your holding tank, air is expelled from that tank’s vent to make room. As you might imagine, that air smells bad. The vent hose exhausts outside the boat, but on a calm day, or with the wind blowing from the wrong direction, that fetid air can linger around the boat something fierce. The solution here is a holding tank vent filter. Hopefully the boatbuilder included one of these filters when they built your boat. If not, adding one can be a relatively easy DIY project. You simply cut the ¾-inch holding tank vent hose at an opportune location and install an inline vent filter with a pair of hose clamps. If access to the vent hose is good, this project is easy. If access is poor, the project gets harder.
Tank vents (with or without filters) will clog with sewage if the tank is overfilled. A clogged vent will make it impossible to flush anything in regardless of the waste level in the tank. To remedy this situation, clear the clog and change the filter. Unfortunately, vent filters lose their efficacy over time and need to be changed regularly even if they are never subjected to an overfill clog. Let your nose be the judge for the timing on this project.
Old and/or low-quality sanitation hose. Sanitation hoses start to smell bad as they age. This is especially true for a hose that wasn’t specifically built to prevent odor permeation. The solution is to remove older hoses and replace them with new, high-quality marine sanitation hoses. I find the Shields Poly X sanitation hose easy to work with and the best hose on the market for running clean to the nose.
A bad bilge. Your sanitation system is far and away the most likely cause of unwanted odors aboard, but it’s not the only possible offender. Your bilge can also smell bad, especially if leaking engine fluids manage to migrate down and mix with bilge water in that space. Repairing engine fluid leaks followed by a thorough bilge clean should address that problem effectively. A stem-to-stern assault with a hose and soapy bilge cleaner every six months, even if you have no engine fluid leaks, will also go a long way towards keeping your boat smelling fresh. Smells love to squat down there, and it’s smart to evict them on the regular.
Mold and mildew. These can contribute to a smell problem on a boat, both in and above the bilge. Vigorous cleaning with vinegar and/or bleach-based products tends to work best. A detailed interior cleaning including all storage lockers and the contents thereof is a good work item to add to your spring commissioning and fall winterization checklists if it’s not on there already.
Increasing air flow in the boat with additional dorades and/or solar vents will help mitigate against mildew-breeding condensation, while also effectively flushing out foul odors. Prompt repair of any deck leaks will go a long way towards prevention of mold and mildew, because maintaining arid storage lockers and cabin spaces is an essential part of the fight.
The chain locker. An unwanted stench may be found lurking way up in the bow in the anchor rode. To a great extent, your chain stink risk depends on where you anchor your boat. If she’s exclusively tethered above a sandy bottom, the chain should come up clean with little or no smells. But if you anchor in stinky mud, failure to adequately clean your chain as it comes up will make your whole boat reek. A robust saltwater wash down system on the foredeck will ensure that most of the offending mud and weed never gets aboard. Regular cleaning of the chain locker will take care of whatever still sneaks in. This cleaning task is especially important if your chain locker drains through a limber hole into your bilge instead of out to sea.
I hope this article didn’t catch you in the middle of breakfast. Foul odor mitigation is a necessary but disgusting business. (When I’m honest with myself and acknowledge how central this discussion was to my 9-5 for 36 years, I start to question my career choices.) Unpleasant as it may be, it’s essential to get the stink gone because, as I told my French friend, Un bateau propre est un bateau heureux. Et un océan propre est un océan heureux.
A clean boat is a happy boat. And a clean ocean is a happy ocean.
January/February 2025
