When someone uses the head and evacuates the bowl there is an immediate "head smell" both inside and outside of the boat. We were told it was possibly a clogged vent but we couldn't detect any blockage.
What was your "detection" method? And yes, the tank vent thru-hull does have to be well above the waterline.
Odor inside the boat and odor out the tank vent have to be from different sources...the odor
outside the boat is from the inside of the tank, pushed out the vent. Unless the tank is leaking, it cannot be the source of odor
inside the boat because odor from inside the tank only has one place to go: out the tank vent. That is, unless the toilet joker valve has become so worn that the slit has become a hole, allowing odor from the tank to escape via the toilet bowl (only one of several reasons why joker valves should be replaced at least annually).
There are a number of possible sources for odor inside the boat:
If it's emanating from the toilet bowl, is confined to the head, the source is most likely to be dead and decaying animal or vegetable sea life trapped in the intake and/or channel in the rim of the bowl. If it's throughout the boat, permeated sanitation hoses are the most common culprit...the pressure of the flush inside the hose pushes odor out. However, some odor from permeated hoses will be present all the time, not just when the head is flushed.
A leaking y-valve or hose connection is another possible source. A little red food coloring flushed down the toilet is the quickest way to find any leaks. If you pump out, rinse and fill the tank with clean water before flushing the dye, it'll reveal any cracks in the tank too.
Odor aside, your yalve may have failed...the handle may be aimed to direct flushes to the tank, but the valve is remaining in the overboard position.
...I took a quick glance overboard and to my horror, bits of toilet paper and waste had been ejected directly into the water. I'm used to this happening if the tank is chock full...
A strong indication that you need a tank level indicator because that should never be allowed to happen. The contents of an over-full tank
should only have
one escape route: out the tank vent...and that never should be allowed to happen either because the vent is NOT an "overflow"...air is the only thing that should be allowed to pass through it unless you want vent blockages. If tank contents are getting into the water any other way, there are serious problems with your sanitation plumbing!
Since you don't seem to be very familiar with the way a marine sanitation system should have been installed and how to maintain it to prevent all these problems and most others too, I recommend that you get my book (see link in my signature below). Its title (my publisher's idea) is a bit misleading...'cuz although it does deal with every source of odor on a boat and how to cure, or better yet PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the US laws, describes all the types of systems and how they work, and will help you learn how to operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is! And I'm always glad to answer any questions it doesn't.
--Peggie