I get calls all the time--especially about 2 weeks into the first summer heat wave--from people who've torn out most of their sanitation system trying to get rid of what they thought was "head" odor, but without success. It's just never occurred to them that there are LOTS of things besides the toilet or holding tank that can make a boat smell like a swamp, or even a sewer, especially in hot weather. Let's take them one at a time, starting with the bilge: A wet bilge is a dark stagnant pond, and it behaves like one--grows a variety of molds, fungi and bacteria that thrive in dark stagnant water...add dead and decaying sea water micro-organisms, and you have a real primordial soup! Cleaning a bilge doesn't consist of just throwing in some bilge cleaner/and or bleach and calling it done. You wouldn't clean a bathtub that way...you wouldn't skip the rinse cycle in the clothes washer...and you can't expect to have a clean bilge unless you rinse all the dirty water out of it either. And if you really want to do it right, you need to dry it out completely too... use a handpump and a sponge to get what the bilge pump leaves behind, and leave the hatches open for at least a day so that plenty of fresh air can circulate in it. Another culprit is a dirty sump. Somehow it never occurs to most people that shower sumps and drains need regular cleaning...but a wet dirty sump can smell like a sewer even faster than a wet dirty bilge. Think about what's in it--soap scum, hair, body oils, and dirt...same things you'd find in a dirty bathtub. Don't clean the tub for a year, and it will prob'ly stink too! The cure: like most things, prevention. Put a healthy squirt of Raritan C.P. (available from the online store here) down it once a week. The most overlooked culprit is the chain locker, especially in salt water. Those same critters that can make a sea water head intake stink are all over your wet anchor rode...fermenting in the summer heat. Once or twice a year, take the entire rode out, lay it on the dock,rinse it thoroughly with clean fresh water, and let it dry. Clean the locker the same way you'll clean the bilge--with a good detergent and a thorough rinse, and--if possible--a good airing out...open the cabin access and aim a fan at it. Remember: odors are always strongest at their source. So when your boat stinks like a sewer, don't just jump to the conclusion that it IS the sewer...follow your nose to source. And sometimes it IS the sanitation system. Odor out the tank vent line and odor inside the boat are two entirely separate issues. Odor out the tank vent line comes from inside the holding tank. Neither replacing hoses, rebuilding or replacing the toilet, nor anything else you do to anything except the tank will cure odor out the vent line. Conversely, unless you have a leak somewhere in the system, no amount of increased ventilation to the inside of the tank and nothing you put in the holding tank will cure odor inside the boat. So let's go through the system, starting from the head intake. Toilets that use sea water have more odor problems than toilets that use fresh water--whether that water is supplied by the onboard fresh water system or comes out of a lake or river. Sea water, especially coastal sea water, is alive with animal and vegetable organisms...when they become trapped in toilet intake lines they die, decay and stink. If you aren't sure whether that's at least part of your problem, there are a couple of indicators: the odor is concentrated in the head and not all pervasive throughout the cabin--or at least is much worse in the head than in the rest of the boat...and is worse after the boat has been sitting for a few days than after you've flushed the toilet a few times. To be sure, close the seacock and flush the head as dry as possible. Disconnect the hose from the thru-hull and stick it in a bucket of water that's liberally laced with white vinegar. Pump/flush the whole bucketful through the system. You can reconnect the hose to the thru-hull now, but don't open the seacock...use cups of fresh water from the sink to flush the head for the entire weekend. If the odor is gone when you come back to the boat next weekend...bingo--you found the source. How to cure it depends upon what kind of toilet you have and several other factors. However, I do not recommend using the "inline" chemical devices...the chemicals in them can be harmful to rubber and neoprene parts in the toilet. If you have a Lectra/San, the chemicals in them can damage the electrodes. Permeated hoses are a major cause of odor inside the boat, there's no predicting how long it will take for a hose to permeate--brand new top quality sanitation hose permeated on my own boat in less than 3 months...the same brand and type of hose had been on my previous boat for nearly 7 years without a trace of odor. What causes hose to permeate? Sewage left to stand in it...because 9 people out of 10 stop flushing as soon as the bowl is empty instead of flushing till the bowl contents have been washed all the way through the system...so each flush sits in the discharge line, pushed another foot or two by the next flush and the next... The head discharge hose is an anaerobic environment, so whatever is in it WILL putrify and stink. Leaving sewage in the head discharge hose also causes another problem: urine crystals that build up in the hose...and as they do, they create a rough surface that traps bits of solids and paper. Over time--sometimes an incredibly short period of time--the combination of urine crystals, salt water calcium carbonates or hard water minerals, and those trapped bits can reduce a 1.5" diameter hose to less than a 1/2". VacuFlush toilets (I have one, btw) are the worst offdenders when it comes to leaving sewage in the hoses because the suction splatters waste all over the inside of the hose and the vacuum tank and the miniscule amount of flush water that's a major selling point for VacuFlush isn't enough to fill the hose to rinse it out. It's the real reason SeaLand developed a "bullet proof" hose...they had no choice. There is no cure for permeated hose, only prevention. I've given up trying to convince people to flush enough every time to rinse out the discharge hose, because the inevitable reply is, "But that fills up my holding tank too fast!!" So I had to come up with an alternative, and this does work: once a day--last thing before you go to bed is a good time because that's when the system is likely to be unused for the longest period of time--close the seacock, flush the head as dry as possible, and flush a couple of quarts of fresh water through the system (VacuFlush owners fill the bowl to the rim and flush). Be especially careful to do that when you close up the boat at the end of the weekend, adding one more step...after you've flushed the fresh water through, follow it with a cup of white vinegar. Don't rinse the vinegar out. NEVER use bleach...nor any chemical bowl cleaners, drain openers, ammonia, Lysol, pine oil cleaners, solvents or anything containing alcohol or any petroleum products. These not only are harmful to rubber, neoprene and Lectra/San electrodes, they break down hoses. When was the last time you even thought about, much less replaced, the joker valve in your head discharge? After enough paper and solids--just flush water--have been forced through a joker valve, the "duckbill" can no longer close tightly...and if you ignore it long enough, it starts to resemble a large mouth bass instead of duck's bill. When it can close completely, it blocks any odor coming back into the head from the discharge hose...but when it can no longer close completely, there's nothing to stop that odor from rolling out of your toilet. Change the joker valve at least once a year. Leaky overboard discharge pumps--manual or electric--and y-valves are another source of odor from the sanitation system. Repair, rebuild or even replace if necessary. Remember: any odor is always strongest at its source...the ONLY way to eliminate odor is to eliminate the source...and more often than not, finding the source becomes a process of elimination. The bad news is, odor is worse in hot weather...but the good news is, that makes it easier to find the source. And finally, prevention is always a better solution to odor problems than curing them.