BoatWorks: Holding Tank Set-up

So, given that holding tanks are a regrettable fact of cruising life, why not make them as easy to deal with as possible?

By Peter Nielsen

June 8, 2015

If there is one thing that sailors of any stripe—racer, cruiser, anything in between—can agree upon, it is that they universally detest holding tanks. There is no upside whatsoever to sailing around with a tankful of poop in your boat, and the number of holding tank horror stories bobbing around in boating circles underscores that fact.

On the other hand, no one likes swimming in anyone’s effluent, so unless you have a composting toilet or a porta-potty on board you’ll likely have a holding tank too. There are no-discharge zones in 27 states where you can’t discharge your blackwater overboard, and federal laws prohibit “vessels with installed toilets” from emptying their sewage into freshwater lakes or reservoirs.

So, given that holding tanks are a regrettable fact of cruising life, why not make them as easy to deal with as possible? I have seen many holding tank installations, some in much greater detail than I’d have liked, which to my mind were needlessly complicated, hoses going every which way and bristling with diverter valves that had to be opened and shut in certain sequences; get it wrong and the consequences were too awful to contemplate.

FLUSHED WITH SUCCESS

I decided to get rid of just such a system on our project boat, and replace it with the simplest set-up I could imagine: a gravity-drain holding tank. There are various permutations of such a system, but in its most uncomplicated form it is merely a tank installed with its lowest point above the waterline; the waste hose from the toilet is plumbed into the top of this tank, and a discharge hose is plumbed into its bottom and taken to a ball valve directly below it.

Waste is pumped directly into the tank, and disappears into the depths with a satisfying gurgle when you open the seacock. You can sail with the seacock open, and discharge directly to the sea; or with it closed, and save the goodies for later disposal. No need to pump anything out, no need for expensive and unreliable macerator pumps, no diverter valves, a minimum of plumbing joints: add a breather hose from the top of the tank vented outside the boat, and it acts as a vented loop on the discharge hose. You have achieved holding tank nirvana.

THE DOWN SIDE

This all sounds too good to be true, and if you sail in a no-discharge zone, it is. You will have to add a deck fitting so you can have the tank pumped out in a marina, and plumb that into your system. I was fortunate in that an old water-fill deck fitting was located just above the heads compartment. I ran a hose from that directly to the head discharge through-hull, and installed a T-fitting to take the discharge hose from the tank.

Depending on how your boat is laid out, you’ll have to make some kind of a sacrifice when you install a gravity drain tank. In my case, I had to surrender most of the boat’s only hanging locker. I located the tank as high as possible under the side deck, partly to retain useable space underneath it, partly to help gravity do its work. It stands to reason that the bigger your boat, the easier you’ll find it to install such a tank. The good news is that since you will most likely have to have the tank made to measure, you can make use of odd-shaped spaces that are no good for anything else.

THE UPSIDE

In a word, or rather, two words: user-friendliness. It is hard to make anything foolproof on a boat, given that fools can be very ingenious, but this is as close as it gets. There are, however, a couple of quirks to the system. Depending on the type of toilet you have, you may have to pump more water through it to ensure all the waste is moved past the bend where the discharge hose enters the tank; you don’t want waste water draining back into the bowl.

If there are any right-angle bends in the discharge hose—for instance if it tees into the standpipe for the deck discharge, as mine does—it is also possible for blockages to form. You can get past this by sailing with the discharge seacock open, so seawater flushes any residue out of the tank and pipes. If you want to be sure you’ll never have a blockage, practice the nothing-you-haven’t-eaten rule and don’t put paper in your toilet. When we leave the boat, we pump a bowlful of clean water through the plumbing to clean out the discharge hose. Every couple of weekends, we also pump a gallon of white vinegar through the system and let it stand with the seacock closed. This cleans the scale off the insides of the hose. We have never had a blockage or any other issues.

In the years since I removed our boat’s ancient holding tank and its snake’s wedding of seeping plumbing from its malodorous lair beneath the V-berth and replaced it with a gravity tank, I have seen the world’s production boatbuilders go the same way. Why? Two reasons—with its minimum of moving parts, such a system is easy and inexpensive to install, and it is simple to use.

CHOOSING A TANK

To my mind the best material for a holding tank is polyethylene (PE). It’s light, clean, easily worked, non-permeable and won’t stink, unlike the bladder tank that our boat was originally fitted with. I’m a fan of bladder tanks for water, but not for sewage. They must be carefully installed and protected from chafe; if one springs a leak I’d rather it was filled with water than the other. There was a definite stink about the one I removed from our boat. Don’t use metal—you’ll be amazed and possibly horrified at the potency of urine as a corrosive liquid. Fiberglass is a good option, if you have a mind to make your own tank.

Several companies offer ready-made PE tanks in various shapes and sizes. It will be up to you to work out where you want the holes for inlet and outlet and vent installed. If you have an oddly shaped space to fill, you will have to carefully take measurements to supply to the manufacturer; I would recommend building a cardboard mockup just to make sure your measurements are correct. I had my PE tank built by Raritan; it holds 15 gallons, and I almost immediately regretted not having it made a little larger.

I installed it on a plywood shelf supported by hefty aluminum angle. The tank is retained by smaller aluminum angle strips and secured by a pair of tie-downs. In eight years of sailing, it hasn’t budged at all.

I used 11/2in ID Shields sanitation hose, which has remained odor-free. I located the tee from the tank to the pump-out standpipe above the waterline in case of hose or clamp failure, but have had no such issues. I made sure there were no low spots in the plumbing where effluent could pool; and since the hoses and tank are effectively rinsed out by the mere action of sailing, odor has never been a problem.

Note that I have a Lavac toilet—I favor these for their simplicity and because they break up waste like no other head—with its own Henderson diaphragm pump. You can use just about any sea toilet with your gravity drain system.

RESOURCES

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