Know-how: Modify a Blackwater System

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The V-berth holding tank re-plumbed with rigid PVC pipe and the manual discharge pump in the foreground

The V-berth holding tank re-plumbed with rigid PVC pipe and the manual discharge pump in the foreground

My dissatisfaction with the head and holding tank plumbing arrangement on our 1987 Sabre 38 had grown as we cruised the boat away from the comforts of a marina for longer periods of time. When we are tied up at a marina, the use of regular bathrooms generally trumps the onboard system, and one can get rather complacent. Once untethered, though, the dynamic quickly changes. There is only one option inside the three-mile limit, and that’s to use the holding tank.

We inherited a fairly typical setup, with one glaring exception. For reasons unknown to me, the previous owner had converted the original 34 gal holding tank into a water tank. Before your mind gets too wrapped up in that thought, I should point out that he bought the boat brand-new and had never pumped waste into it. To compensate, he plumbed in a modest 14.5 gal bladder tank that rested on top of the original rigid plastic factory-installed tank, which is in a typical location forward under the V-berth.

Unfortunately, we often have four people onboard for our cruises and found that the small bladder tank became our primary reason for seeking out a marina instead of an anchorage after just a few days of use. On more than one occasion, and before we were aware of what was going on, the bladder became full enough to push up the access panel and V-berth cushions above it. There were never any ruptures, but it was a scary sight, to say the least.

Two of the 45-degree elbows used to cross the tank top

Two of the 45-degree elbows used to cross the tank top

Beyond that, the original hoses were now long past their useful lifespan and leaching odors, and I was also unhappy with the double Y-valve setup we had, which allowed for direct overboard discharge straight from the head line and also from the tank through a macerator when outside the three-mile limit. Botton line: we needed a change.

We are fortunate in that Sabre is still in business—albeit not building sailboats anymore, only powerboats—and the company has not only kept meticulous records but has someone on staff tasked with helping the legacy fleet when questions arise. Thinking that the configuration I had was a factory build, I, therefore, reached out to Glen Chaplin to ask if I could convert the forward water tank back to a holding tank. He told me that, of course, I could, as that’s what it was originally intended to be. This was excellent news. Glen then forwarded a schematic of how the tank was originally plumbed, and I got busy drawing my new plan.

One of two PVC ball valves plumbed into the tank exit points

One of two PVC ball valves plumbed into the tank exit points

My background is in residential construction, where the coin of the realm in terms of sanitary sewer systems is rigid PVC pipe, and I reasoned that if it could be engineered, PVC would be an excellent choice for a boat’s sanitary waste system as well. Clearly, I would need some flexible hose to make the necessary transitions and to route in difficult areas. However, I also reasoned that a small percentage of flexible hose would still be preferable to a system that was 100 percent hose, due to the fact that flexible hose will inevitably allow odor to permeate through after it reaches a certain age. Also, while hose clamps rarely fail, they are notoriously difficult to use and adjust, in contrast to a properly assembled and glued PVC joint, which is also permanent. Yet another positive was the fact that PVC pipe is flat-out cheap, while high-quality marine sanitary hose is decidedly not. Who among us does not like to save money on a boat project? The 1 1/2in PVC pipe and all fittings, including two ball valves, set me back less than $30.

The manual blackwater pump and a length of flexible sanitation hose in the foreground; the rigid PVC inlet line in the background

The manual blackwater pump and a length of flexible sanitation hose in the foreground; the rigid PVC inlet line in the background

In terms of design, the path for the discharge pipe from our head compartment, which sits on the port side forward of the salon, is routed through the head vanity cabinet and then past the accessible area of the hull where the overboard discharge seacock and raw water intake seacock are placed. From there it goes through the V-berth bulkhead into the tank space as described above. There are two ports and an inspection hatch on the tank’s top surface. The larger port is the waste inlet; the smaller, the vent outlet. There are also two outlets at the bottom of the tank facing the stern. One can be plumbed for overboard discharge, the other for pumpout use through a deck fitting. The tools needed for a project like this one are basic: a drill and hole saw of the proper diameter; a hacksaw, hand saw or Sawzall to cut the pipe and hose; and a screwdriver or socket for the hose clamps. For the flexible sanitary hose, I wanted the best quality, regardless of price as an odorless system was a top priority. After some online research, I chose Shields Poly-X Premium Sanitation hose, which has a lifetime warranty against odor permeation.

Back aboard the boat, the demolition work went slowly but smoothly. The old hose had stiffened from age and interior vitrification, and there were some tight bends that gave me trouble. However, with patience, the work was completed in about two hours. After that, it was time to start installing the new system. The good news was that most of the existing holes for the original hose would be re-usable. I had also planned a route that was more direct than the old path, which included the aforementioned Y-valves and included several 90-degree turns.

Of course, when using flexible hose, one can change elevation and bend around corners with relative ease. Not so with rigid PVC. And while the exit from the head through the vanity cabinet was a straight run, once past the head/V-berth bulkhead and into the space where the tank discharge and head inlet thru-hulls were located, I would need to route the pipe at close to a 45 degree angle to reach the top of the tank about 5 feet away. The other challenge stemmed from the fact that the hull was getting ever narrower toward the bow, thus requiring some compound pipe angles. Further complicating matters was the relative inaccessibility of the area, the kind of hard-to-reach, tight-fitting situation that we as boat owners consider a given.

A closeup of the 3/4in to 1in reducing coupling used to enlarge the tank vent line

A closeup of the 3/4in to 1in reducing coupling used to enlarge the tank vent line

Ultimately, two 45-degree elbow fittings would be needed to gain the appropriate elevation to end up on top of the tank. The only way to accomplish this was through trial and error: cutting straight pipe and then dry fitting; moving the elbows around so as to gain the elevation as well as angling the pipe inward to follow the ever-narrowing hull shape. Not surprisingly, there were several false starts as straight pipe sections had to be re-cut to different lengths to get the proportions right.

Once I had a workable dry-fit, I used a permanent black marker to place arrows at each point where a straight pipe went into an elbow, knowing I would have to disassemble the entire run and glue each joint separately, letting it set before continuing on the next one. The arrows, when lined up, produced the correct alignment and also pointed toward the tank, helping me to keep from inadvertently reversing an elbow when re-assembling. Fortunately, the glue-up went as planned. The re-route from the head to the holding tank was complete. Time spent was in the neighborhood of six hours.

After that, the job was much more straightforward. For the pumpout and the overboard discharge, I used rigid 1 1/2in PVC and installed a ball valve down low to isolate the lines when not in use, since when sailing and heeled, these lines would likely end up holding a significant amount of tank waste. It, therefore, seemed logical to keep the lines as empty as possible. I also switched to flexible sanitary hose to reach the deck fitting and through-hull respectively. For the pumpout line, I used a grade of hose that was not as pricey as the Shields Poly-X, reasoning that it would only hold waste briefly and therefore would not be subject to the same rigors as the head waste line. I used Shields Poly-X for the overboard discharge line simply because I had enough leftover from the small amount needed to exit the head.

Finally, I re-plumbed the vent line hose, stepping it up from the 3/4in threaded outlet on the tank to 1in. (I reasoned the larger size could only help the overall functionality of the vent system.) Creating a larger threaded outlet was out of the question, so I used a PVC reducer—not a perfect solution, but one that worked.

As for the rest of my new rigid PVC piping system, I was very happy with the results, and it has worked well to date. Of course, only through long term use will I be able to tell if it was truly worth the extra time and effort involved. 

Photos by David Popken

November 2019

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