An ozone generator can help rid a boat of mould and the accompanying bad odour.

PRACTICAL - Elminating boat odours

To understand how to eliminate noxious smells you first have to understand where they come from. Smells are actually small amounts of the parent material “gassing” and sensed by “dendrites” inside our nose. So if a material does not gas (such as steel) it has no smell. We actually have to inhale some of the parent material’s gas to smell it.


What causes bad boat smell?

Environmental factors such as temperature and humidity affect the gassing process. Ever notice how much stronger a boat smells on a hot humid day? While the smell may be disturbing, it also gives you a hint about eliminating it — and explains why air-fresheners are not the answer.

Smell strength in the air is measured in parts per million. Eliminating a smell is two-sided — we can reduce the “parts” (cleaning) as well as increase the “millions” by adding airflow.

To eliminate a smell, we have to remove the source and dilute the remaining concentration with more air.


Removing a smell typically means identifying, locating and removing its source. Air-fresheners simply cover the existing odour or deaden your smell sensation.

Airflow is a must in removing stale smells, so open the hatches, install fans, and put up a wind-scoop.


The most common boat-odours come from:

* Dirty bilges;
* Holding tanks and head systems;
* Bacteria and mould growing inside the boat;
* Gassing from leaked petroleum products on a hot engine; and,
* Fish residue.

Dirty bilges
Bilges eventually collect everything lost, dripped, spilled, or regurgitated on the boat. Fibreglass, epoxy, enamel paint, and everything stored in the bilge can absorb a small amount of the original odour-producing material. Forever more, a slow release of the smell will waft from the bilge. Since most bilges don’t have much airflow the “millions” side of the equation works against us.

“The simple trick is to fill the bilge with hot, fresh, soapy water,” says Martin Craig, base manager for The Moorings charter company in Tonga, where yacht interiors are thoroughly tested. “Use plenty of detergent, and let the mix sit for a while before clearing the bilges.”

Be sure to remove all materials stored in the bilge that can hold smell (e.g. cardboard, paper, and cloth).

PRO TIP: The best time to wash the bilges is in a rolling anchorage or during a day’s sailing or cruising; letting the motion of the boat combine with soap to agitate the offending substances.


Head odours
The odour of holding tanks is identifiable to anyone who has spent time aboard boats. The odour may be resident in the toilet, the hoses, in the holding tank, or even spills left over from past clogs.

The first step in eliminating the smell is a simple acid flush. Use a mixture of muriatic acid and freshwater. Muriatic acid can be purchased at any marine store that deals with LectraSan holding tank systems.

A mild solution of one litre of acid mixed into a 20lt bucket of water and slowly pumped through the head hoses will dissolve the thick, “clogged artery” residue inside the hoses.

When dealing with acid flushes the trick is time, not concentration. Follow the mixing instructions on the bottle and leave the mix sit inside the hoses overnight. Remember to pump a little acid then change the outlet valve configuration, pumping again to work the flushing acid into every hose, and finally into the holding tank.


Hose smell

Head hoses will last years if the head is pumped till the hoses are completely clear. The problem comes when some residual waste is left sitting in the low spot of the hose. This concentration can saturate the hose with waste over time causing a larger project of hose replacement.

To check if a hose is odour-impregnated, wrap a rag soaked in hot water around the hose at its lowest point. Be sure to check the toilet outlet hose, the vent and the holding tank outlet hose.

After the rags cool, give them the sniff test. If the unpleasant odour has transferred to the rag it’s time to change the hoses.

PRO TIP: Consider adding six full strokes to every flush to help keep the hoses clear. Flushing with freshwater (and occasionally with acid) can make head hoses last nearly forever. A less-aggressive method is to flush a litre of vinegar down the head every month. Let the vinegar sit in the lines, breaking up deposits.


Head intake hose

The intake hose of any saltwater system is prone to a build-up of micro-organisms that give off a distinct “rotten” odour when the head is flushed. Over time, this organism can infect the complete head system.

The trick to removing this smell is to kill the micro-organisms.
Pull off the intake hose at the hull and suck up a bleach/fresh water mix (10:1 ratio). Continue to pump till the bleach mix has completely filled the inlet hose and the smell of bleach is present in the toilet. If the head has a strainer, this may take 10 or even 20lt of water.

Once the bleach is inside the inlet hose, let it sit for a few hours before pumping out. This should eliminate the source of the smell.

PRO TIP: Before leaving the boat for an extended period of time, flush the inlet hose using a freshwater feed. This will prevent the micro-organism from gaining a foothold.


Vent hose

The vent from the holding tank is often led directly to deck. This can be a steady source of “floating” or wafting odours through the cabin. On larger vessels the vent can be led overhead to the radar arch, or up a mizzen mast.

Carbon filters can be added to the head vent-hose to catch the odour before it reaches deck. Vacuflush toilets from Dometic, which are found on most Rivieras, usually have an inline carbon-filter that might last up to two years. We recommend you carry one in your spares.

Unfortunately, it’s possible for the head odour to end up back in the boat due to pressure differences between the deck and cabin. For example, a good strong gust of wind over the deck with an open main hatch can create a negative pressure in the boat compared to on deck. This can cause galley sinks and heads to pull a small amount of tank air (thus smell), into the cabin. One solution is to hook a small fan to the vent hose that keeps a slight negative pressure in the tank.


Sink and gray water smells

Gray water has a distinct stale grease smell. The source may be the grey-water tank, the shower sump or often the sink trap.


Grey water

Grey-water tanks need complete cleaning every few years. This is because oil and grease washed from the body can combine with soap to form a thick, gooey substance that sinks and coats the bottom of the holding tank with a thick slime. This slime is organic and will eventually rot, giving off a repulsive odour. This slime is difficult to pump and detergent won’t break it up.

Scooping the slim from the bottom of the tank is one easy solution. Alternatively, the evacuation pump can be left on while a powerful hose breaks up the slim allowing it to be sucked out when in motion.


Sink traps

Sink traps hold heavy grease, hair, and other contaminants. The typically recommended method of cleaning the trap is to take it apart and clean the elbows individually. This has the disadvantage of opening new leaks in the drain hose, possibly turning a quick project into an all-day ordeal and a trip to the local hardware store.

Another solution is to pour hot water down the drain then use a plunger to break up the held material. Most drains with a trap will have a small air vent in the sink. This vent must be sealed with a rag so the plunger force will be directed into the drain.



Mould has a purpose in the ecosystem of breaking down organic matter into new soil. While this may be good in the bush, it can be dangerous onboard. Mould not only makes a boat smell stale, it can cause respiratory problems, headaches and even long-term illness.

Removing the mould is not so easy. Mould spores, or the seeds, are so robust some scientists have theorised the organisms could have arrived on earth from outer space.

Start a de-moulding project by protecting yourself. This means wearing a respirator and long gloves during the de-moulding process. Cleaning will be disturbing the mould and you don’t want the spores taking root in your lungs.

Use a mixture of vinegar with a capful of bleach in a bucket of water. Wipe all exposed surfaces and let the mixture dry. Remember, bleach does not kill till it dries, so wipe the bleach on and leave it alone.

Pull all sheets, towels and anything else that can be washed from the boat. Run it through the laundry adding a small amount of bleach and hang in the sun to dry.


Impregnated mould

Removing mould that has gotten into the wood or the interior can be a real challenge. Soft overheads and leather are particularly difficult to de-mould. A pan filled with a strong bleach solution and left open in the cabin with all the hatches closed can help as a shock treatment to the living mould.


Ozone generators

Ozone generators are a better but more expensive solution to killing mould. Ozone is a type of oxygen you often smell after a lightning strike. There are many Ozone generators on the market including some that run on 12V. Put the generator inside the cabin, close the hatches and let it run.

It’s best to leave the boat unattended during an ozone-generation cycle as ozone is not good to breath. If the ozone is noticeable as a smell the concentration is too high to be safe. Leave the area till the treatment is complete.

Let the boat air for a day or so before reoccupying. Resilient mould may continue on as spores and thus repeated ozone treatments are often needed to kill the mould.


Chain locker

Larger boats often have a chain locker that is part of the forward section of the boat leading into the forward cabin. Water dripping from the anchor chain contains bottom muck, and small amounts of micro-organisms that will begin to decay once inside the boat.

To remove the smell use a deckwash down-hose to clean the chain as it comes aboard. Next, separate the chain locker from the main bilge to contain the messy water, where it can be easily pumped overboard.

To clean dirty chain lay out it out on the dock, scrub out the interior of the chain locker and wipe it down with a mild bleach solution. Let the bleach completely dry before replacing the clean chain.


Random boat smell

The most difficult smells to eliminate are those that only show up randomly. I worked aboard one boat for five years, where about once a month a nasty head odour would flow through the boat. To find the source of the odour we took careful notes. Every time the odour was noticed we took observations of:

* Boat speed;
* Wind speed;
* Wind direction;
* Number of passengers;
* Holding tank level; and
* Grey-water level.

Eventually, we found someone had linked a shower-drain vent to a head vent. With just the right wind conditions head odour was sucked through a shower drain into the boat. Without careful notes we would not have succeed in locating the source of the problem.


Engine odours

Engines and generators can produce an array of odours. Many come from petroleum products that have spilled or leaked onto the engine, “gassing” once the engine has reached operating temperature.

The easy way to deal with these is to clean the engine completely. Locate any new leaks and fix them. From that point on there should not be any engine odours.

Wherever you can, use an electric engineroom blower while the engine is running and continue to check for new engine leaks.

PRO TIP: Try a toilet brush to scrub the engine and bilge area. The short handle and wide bristles are about right for a quick clean without dirtying your hands.


Fish small on a boat

Fish is one of the most delicate foods to preserve. It begins decaying moments after dying, giving off the classic fishy smell. Fish also contain a high percentage of oil that helps the odour to impregnate into any substance it touches, making it one of the most challenging smells to remove.

Start with a good cleaning of the affected area with a detergent and bleach mixture. Let the area soak, and then rinse. The idea is to let the detergent lift the fish oils, and the bleach to kill the offensive bacteria.

After the area has dried sprinkle baking soda and moisten with a fine spray of water. Leave to soak for at least 15 minutes, followed by a good scrub of the baking soda. Leave to soak again, then rinse.

PRO TIP: In severe cases a rinse with hydrogen peroxide has been known to work wonders.

To eliminate smells from killbins, try rinsing with bleach water. Leave to dry, and then scrub with moist baking soda. Rinse with hot water. Still smelly? Scrub with lemon juice or vinegar, and leave a scoop of baking soda in the bottom of the bin as a preservative.

And what if the Shorepower has tripped out or your fridge has been left off? That’s a major clean, with disinfectant, fridge wipes and then vanilla essence. And if you’ve been through that nightmare, you’ll know the value in storing food in air-tight containers to help save on future fridge cleans.