“Yay! Another article about blackwater holding tanks!” Said no one ever. This is nobody’s favorite subject, but like it or not, responsible waste management is a part of boating and is here to stay. If you are just sitting down to read this with a sandwich on your lunch break, you may want to put this article aside until later. I’m sure there is a nice non-gross review of mop handles elsewhere in this publication.
To continue reading this article or issue you must be a paid member. Sign in
Join Practical Sailor
Get full access to Practical-Sailor.com – more than 4,000 articles – for just $34.
Join today and save 42% off our full price.
I have gravity fed discharge overboard but after the first time it clogged I went with an electric head and have never had another problem with it plugging.
Does the electric head still discharge into the gravity drain tank? I’m unclear how this would prevent clogging. Most electric heads just motorize the pumping action.
I had a 88 Pearson 27 with a macerator. I’m not sure if it was original or someone put in in. I loved it. When I was out to sea and wanted to drain the tank, I would use the macerator and it would pump the tank out. Even if you have a tank which would allow for gravity to drain the waste, the pump speeds up the process and helps get rid of sediment. Usually, if I was running the macerator I would get into the head and pump in saltwater like a mad man. This would reduce the sediment. I would finally add some kill odor. I would fill the tank with the kill odor fluid and pump a cup or two into the now empty tank. I just aquired a Sabre 38 MK II which doesn’t have a macerator and uses only gravity to empty into the ocean. Is there a reason I wouldn’t add a pump to really make sure the tank was empty?
It would depend how you plumbed the macerator. If you put the pump in the gravity discharge line between the tank and the seacock, it would no longer be a gravity drain system as the pump would block the discharge. I would also worry that the pump, being at the low point of the system, would load up with sediment and do bad things to the diaphragm. I don’t think the inside of the pump should be “wet” in its idle state. You could plumb the macerator pump so it sucks out of a down-tube from the top of the tank, essentially putting it in parallel with the gravity drain, but keeping the diaphragm dry. However this would leave you with no way to clear clogs in the gravity side (other than the disgusting swim of death) and down-tube discharge always leaves an inch or so of liquid in the tank. For my money, once you have a way to clear the occasional clog, the gravity system is pretty foolproof.
Two vents, port and starboard. Big, say at least 5/8″. No need for screens. This allows air flow either way, depending on wind pressure. Totally passive, just be sure there are not loops and water traps in hose. This keeps enough oxygen in the tank to eliminate, or reduce, the anaerobic decay – which causes the smell. Aerobic decay is not so smelly. Worked for us for decades. Also, used macerator pump out at sea. Worked fine and kept tank low in boat. Next boat may have a composting head to eliminate all the space lost to tanks, plumbing, pumps, three thru-hulls.
I was hoping someone would bring up composting toilets. They may well be the future, but I have zero experience with them. YouTubers Acorn to Arabella felt the need to install a powered vent fan in the head to deal with odors from their composting bog. I’d be interested to hear people’s experience with them. You are absolutely correct about keeping air moving through the tank to promote aerobic decay. I worried that the flapper-check-valves in my system would shut down airflow, but rocking motion seems to keep enough air moving to keep the microbes happy.
There are now five or more composting heads on boats in our small yacht club. I asked one owner of his assessment after a few years. I summarize: If you like older British sports cars with manual shift, you’ll be fine with a composting head. If you like modern German cars with paddle shifters, not so much.
Composting heads require a vent to deck. Most have installed solar vent fans, or 12v computer pancake fans, to assist the aerobic composting.