How to Build a One-Off Holding Tank
by: Mark Parker, Bristol built Triton #516 "All Ways"
Photo's by:  Kim Parker

As seen in Good Old Boat magazine, November/December 1999, http://www.goodoldboat.com

     Like most good old boats, All Ways, my 28-foot Pearson Triton, was built with an overboard discharge marine head. Since my favorite cruising area was recently declared a No Discharge Zone (NDZ), installing a holding tank became an important priority in my refit. The previous owner had installed a 2-gallon plastic "tank" that fulfilled the law but was of little real use. (He bragged that he never had an odor problem since the tank had never had sewage in it!) I wanted a tank that was large enough for at least several days for two of us.
     My first step was to research the proper design and installation of a marine sanitation device (MSD). That search led me directly to Peggie Hall of Peal Products and her publication, Marine Sanitation: Fact vs. Folklore. For a free copy, contact peghall@worldnet.att.net. It is easily the clearest, most concise discourse I have ever seen on the subject. Peggie is also a good source of information. She's a frequent contributor to the rec.boats.building newsgroup and answers email questions. Her company sells a huge variety of rotomolded polyethylene tanks to fit many installations.

Location

     Unfortunately, even after Peggie faxed me several pages of dimensions, we could not find one that fit perfectly in the V-area of my forward berth. A review of options available from Defender, West Marine, and BOAT/U.S. yielded far fewer choices and still nothing that really fit. I decided to build my own one-off fiberglass holding tank. Here is the thought process that went into my final choice. I wanted:
       As large a tank as practical. (Initial calculations suggested 35 gallons, but I finally decided on 20 gallons as a minimum.)
       To keep the head-to-tank hoses as short as possible. (To aid in odor prevention, you should pump the discharge line dry after each flush. The longer the hose, the more water it takes to flush the line, and the quicker your tank fills up.)
       To have as little negative effect from the weight as possible on vessel trim. (Ideally this would put the holding tank in the bilge, however midline, and not in the bow, was a reasonable compromise.)
       To avoid odor at all costs. (Following Peggie's recommendations, this meant two large-diameter vent lines, one forward to the bow and one aft by the discharge line to create cross ventilation.)
       To make use of the V-berth area. I had already decided to make the V-berth into a large double bed. Therefore the area underneath was available. The area was a rectangular trapezoid, 31 inches wide at the base, 20 inches at the top, 21 inches long, and 20 inches high. That calculates to 35 gal. Allowing for space at the top for hose access drops the height to 16 inches which, allowing for wall thickness, yields a 32-gallon capacity.

Other choices considered and rejected included:
       Behind the head (An excellent article in BoatBuilder, XIV:6 details such an installation) - too small.
       In the forecastle - too far forward for weight, difficult to work in.
       Under both sides of the V using flexible bladders - plumbing complex (two tanks) and concerns about odor in flex tanks.
       In the hanging locker opposite the head - plumbing complex (hoses need to cross midline of boat ‹ no place to do that).
       Under the port berth ‹ unbalanced location, marginal size.

Plumbing and materials

     Having decided on the location, the next issue was plumbing. The West Marine catalog has a nice set of diagrams showing the various options for plumbing a marine head. Given that most of my cruising would be in an NDZ, I chose to route all discharge into the holding tank but to maintain the option of emptying the tank overboard when beyond the three-mile limit. That yielded a relatively simple design with only one Y-valve.
      The final decision was construction of the tank. I elected to use a pure fiberglass construction with polyester resin (refer to article regarding the safe use of polyester resin that follows). I rejected epoxy over plywood because I just did not trust it to remain laminated. (Polyester over plywood was out of the question.) I also rejected epoxy/fiberglass as much more expensive. Plus, I had some resin, mat, and roving left from my deck repair and was comfortable working with it. The only real drawback to polyester compared to epoxy for one-off construction is that you cannot use foam to construct your plug, as the resin will dissolve the foam. Since this was a truly one-off construction with no thought of ever making a second, there was no need to construct a durable plug. I decided to make the plug out of drywall (also known as Sheetrock). It is cheap, easy to work with, and easy to destroy. The corners can be nicely rounded using joint compound (mud) and tape.

The plug

     To build the plug, first determine the outside dimensions of the final tank (allow clearance inside your space). Before proceeding, be sure the finished tank will fit through the companionway and any doorways necessary to install it. In getting the angles right for the trapezoid, I first cut a pattern from cardboard. Using a rectangle and two triangles, I maneuvered them into place and taped them together fixing the final shape. Using this as a starting point, I calculated the dimensions of the plug. The following calculation derives the inside dimensions of the panels. I subtracted 1/2 inch from each outside dimension of the cardboard mockup to allow for 1/4 inch wall thickness all around. Because I was using 1/2-inch drywall, I subtracted another inch for the thickness of the panels making the plug.
      I did this to all dimensions because I did not want the sides to overlap the top.  I added 3 inches to the height. This allowed me to cut the finished tank apart, remove the plug, taper the edges, and reassemble the tank. I cut out the panels and assembled the plug using softwood nailing blocks on all corners. If you are making a trapezoid like mine, this will require ripping some of the blocks to the proper angle. This is easily done on a table saw (set the top on the table and tilt the blade to match), but could also be done on a band saw or even with a hand plane - great accuracy is not needed. I used a Sureform rasp to round all edges, eliminating the corner at each edge.
  The resulting radius should be covered with "mud" and tape, smoothing it with your hand. Remember, this is the inside of the tank. No one will see it; it just has to be smooth enough to release well. Test fit your plug before proceeding, remembering that the actual tank will be roughly 1/2 inch larger overall. Paint the plug with primer, and apply two coats of auto polish, buffing each coat out. The wax acts as a release agent.  One of the nice things about working with polyester is that as long as you do not use finishing resin (which contains wax), you get a chemical bond between layers even if you don't work "wet-on-wet" There is no amine blush to worry about and no sanding between layers.

The tank

     Cover the bottom with two layers of mat (1.5 oz.) and a layer of biply (24 oz. roving with 1.5 oz. mat attached) wet-on-wet, with roving to the inside and mat to the outside, and let it dry at least to where it can be handled. The fiberglass should be folded over the edges about 2 inches. Apply the same schedule to the top. You can either let this dry or proceed directly to the next step, depending on your comfort with working on a vertical surface. Wrap two layers of mat and a layer of biply around the sides in one continuous length, staggering the seams. If the top is still wet, this must be done vertically but it really is not that hard, just messy. (Be sure to wear long sleeves and good gloves in addition to your respirator.) If you elect to let the top dry first, the plug can be turned on its side and rotated while three of the four sides are applied, then turned back upright to apply the fourth. Of course, you will have to repeat the process three times to get all the layers on. I am not sure this is really easier - I did it the first way. Apply another layer of biply to the top and bottom (waiting for one to dry before turning it over), again overlapping 2 inches.
      A note on fiberglass schedules:  The double layer of mat on the inside is necessary to ensure that the tank will be waterproof. My finished tank has a schedule of mat-mat-roving-mat-roving-mat with double that on the edges. It is about 1/4 inch thick and nearly bomb-proof. You can certainly use alternative schedules; the important features are the double layer of mat on the inside and a layer of mat between layers of roving to ensure good bonding. Cloth or non-woven bi- or tri-directional fiberglass could be used, but cost significantly more and are not needed in this application.
 
 

The fun begins

      Measure down 3 inches from your 2-inch overlap, and draw a line all the way around the tank. Using a circular saw with a carbide blade or a jigsaw with a fiberglass blade, cut the tank in half along this line. Remove the plug. Unless you did a better job of waxing the plug than I did, this will involve smashing the drywall and peeling it off the fiberglass in pieces. (A trick that I learned after this project is to coat the plug at the last minute with no-stick cooking spray just before applying the laminate. I am not sure if it would allow the plug to pop out intact, but it is worth trying.)
      Now is the time to install a baffle if you want one. It can be fiberglass or coated plywood since the worst that will happen is that it will slowly decay, leaving a tank with no baffle. Paint the interior of the tank with primer and gloss enamel to make it easy to keep clean. Leave a 3-inch band unpainted on the lip of the top.
      Sand or grind (a 7-inch right-angle grinder does a great job here, but a belt sander works) a 3-inch scarf on the inside of the top and the outside of the bottom. Great accuracy is not needed; just draw a reference line at 3 inches to start and taper to a feather edge. Wet out the scarf with straight catalyzed resin. Make a glue by adding chopped fibers (easily made by cutting your scraps into 1/4-inch pieces) to the resin, apply this to the joint, and assemble the two halves of the tank, smoothing the squeezed-out glue with a putty knife. Be sure to tap the top into place until it is parallel with the bottom. Wrap another layer of biply around the sides of the tank. If you want a non-tacky finish, you can either use finishing resin for this step or wrap the tank in plastic wrap while it dries. The top and bottom can likewise be coated with finishing resin or with plain resin and covered with plastic, but this is entirely optional. 
       Test fit the tank again. (It had better fit!) I had to grind the lower corners a bit as the overlaps created a total thickness of more than 1/2 inch. With the tank in place, mark locations for the clean-out, vent lines, and sewer lines. I used a 5-inch clean-out, 3/4-inch thru-hulls for the vent lines, and 1 1/2-inch right-angle elbows glued into flanged fittings I got from an RV outlet for the sewer connections. (1 1/2-inch thru-hulls stood too high.) Remove the tank, cut the necessary holes, and install the clean-out and fittings using plenty of 5200 sealant. (Make sure you can reach the underside of each fitting through the clean-out before cutting the holes - you may have to relocate something to accomplish this.) You may want to place the tank in the boat before setting the final direction of the fittings to be sure any critical angles are correct. Be sure the pickup tube reaches nearly to the bottom of the tank; I cut a 45-degree angle on the end of the tube and let the tip hit the bottom to ensure placement. You probably want the vent lines and discharge into the tank to be near the centerline of the boat so that they are not submerged on either tack, but the pump-out can be at the most convenient corner.

Installation

     Finally, install the tank permanently in place, being sure it is blocked securely so it cannot shift or rub underway. I used some of the urethane foam-in-a-can that is sold for caulking to create an exact fit along the edges. Attach all hoses securely, double clamping below the waterline and including a vented loop if necessary. Now go cruising in your good old boat and enjoy the independence of a truly custom marine sanitation system!
 
 
 
 
 
 

     Mark Parker, M.D., is director of the Emergency Care Center at The Cheshire Medical Center in Keene, N.H. He's been sailing since college - Sunfishes, Lasers, Hobie Cats. His work on a 16-foot trimaran, a "work in progress," was temporarily sidelined when the Pearson Triton, Always, received a higher priority rating. Mark sails with his wife and family in Narragansett Bay. Kim, the photographer is Mark¹s daughter.

Play it Safe

     Being an ER doc in real life, I have perhaps a greater than average concern for the toxicology of the chemicals we use in working on our good old boats. It turns out that both epoxy and polyester resins are potentially very dangerous - but in entirely different ways. The dangers of epoxy resin are well addressed in the Gougeon Brothers' On Boat Construction. Put in the simplest terms, epoxy in liquid form is dangerous if you get it on your skin. It is not dangerous to breathe, as is gives off no volatiles. Therefore, when working with epoxy resin, you must wear gloves and long sleeves at all times, but (contrary to popular belief) you do not need a respirator. Epoxy dust, however, is toxic if inhaled, so you should wear a particulate respirator whenever sanding or cutting epoxy.
     Polyester resin, on the other hand, is very toxic if the volatile gases released during cure are inhaled. (I use the generic term polyester to refer to both isothalic and orthothalic polyester as well as the slightly different vinylester.) Breathing even relatively small amounts can cause permanent brain, kidney, and/or liver damage. It is, therefore, mandatory that people working with polyester resin wear respirators rated for organic vapors. These are the canister types that usually have charcoal filters which must be changed periodically. A good rule of thumb is: if you can smell it ‹ don¹t. Change your respirator, get better ventilation, or do something so you cannot smell the polyester, and you should be safe.
     Given an understanding and respect for the differing toxicities, both epoxy and polyester can be used safely, and each has its advantages and disadvantages. Epoxy is a much better glue; it sticks (bonds mechanically) to things better than polyester. Once it is cured, however, subsequent coats must rely on secondary (again, mechanical) bonds. In contrast, polyester can bond chemically to itself ‹ regardless of the time lapse. This obviously results in a stronger bond. Epoxy is more flexible than polyester. This can be an advantage or disadvantage, depending on your application. The rate of cure of polyester can be adjusted by the amount of catalyst added; epoxy resin and hardener must be mixed in a fixed ratio with the rate of hardening determined by the particular hardener chosen. You can use fiberglass cloth or roving with either resin, though the more exotic fibers (kevlar, carbon, etc.) are usually coupled with epoxy because their properties are more complimentary. You must not, however, use mat with epoxy, even though I have seen other authors recommend it. The binder in mat is dissolved by the styrene in polyester, but will be unaffected by the epoxy. Therefore use with epoxy will result in incomplete saturation and very weak laminate.
     An excellent source of information on the pros and cons of epoxy and polyester is LBI, Inc. of Groton, Conn., (800-231-6537). They sell epoxy and polyester and have years of experience with both. Their catalog is informative, and the owners will answer any questions and make recommendations regarding choosing between epoxy and polyester for a given project. Of course, the Gougeon Brothers¹ technical department is very knowledgeable and anxious to answer any of your questions about West System epoxy. They may be somewhat biased, however, as they neither make nor sell polyesters. The same can be said for System Three which publishes a very entertaining and informative booklet on using epoxy but does not deal with polyester.