Equipment Selection (continued)

Tanks

The size of the holding tank selected should be based on the boat's intended usage (day trips, weekend or extended cruises, etc.) and the number of people using the toilet. Some experts have estimated that the average effluent (sewage and flushing water) per person per day may be as low as three gallons; in many cases it will be higher. Leakage typically occurs at fill, pumpout and vent line connections. The more flexible the tank, the more difficult it is to maintain leak-free connections. Connection sites on a flexible tank must be reinforced to minimize the deformation of mating surfaces.

Plastic (polyethylene) Tanks

Plastic tanks are rotationally molded and, therefore, have no seams. They typically have a wall thickness of 1/8, 1/4 or 3/8 inch. Wall thickness is a key to a tanks structural rigidity and resistance to odor permeation. Odor permeation is more likely to occur in tanks with a wall thickness under 1/4 inch. Thicker walls offer greater rigidity and permeation resistance, and are generally well worth the price differential.

Advantages:

• Readily available in a wide range of shapes and sizes.
• Generally the least expensive option.
• Provide visual indication of tank content level.

Disadvantages:

• None

Advantages:

• Can often be used when other types of tanks will not fit in the space available.

Disadvantages:

• Susceptible to permeation.
• Susceptible to leakage due to chafing and to flexing at piping connections.

Advantages:

• Highly resistant to permeation.
• Relatively inexpensive as a "do-it-yourself" project.
• Can be custom fit to the space available.

Disadvantages:

• Typically cost more than a plastic or flexible tank.

Advantages:

• Highly resistant to permeation.
• Easy to fabricate to space available.

Disadvantages:

• Susceptible to corrosion. Sewage, additives and cleaning agents may aggravate corrosion rates.

Tank Level Monitors and Warning Systems

Federal regulations (33 CFR 159.83) require that holding tanks have a means of indicating when the tank is more than 3/4 full by volume. A translucent plastic tank can meet this requirement if its location permits easy regular access for viewing contents level. Tank level monitoring systems are fairly simple to install. These typically include a sensor installed at the top of the tank and a small remote panel with a warning light that indicates when the tank is more than 3/4 full.

Seacocks and Through-Hulls

Toilet intake and overboard discharge lines should be equipped with seacocks (valves) to stop the inflow of seawater in the event of a hose failure. A seacock is operated by a lever-type handle that gives clear indication whether the valve is open or closed. These seacocks should be readily accessible for maintenance and oriented so that their handles are easy to operate.

Seacocks and through-hulls are made of bronze or glass reinforced plastic. Both can be used successfully for toilet intake or overboard discharge lines. Bronze seacocks and both bronze and glass reinforced plastic ball valves may be used for toilet in take or overboard discharge lines. Its generally a good idea, however, to fit plastic valves on reinforced plastic through-hulls. Either glass reinforced plastic or bronze valves may be fitted to a bronze through-hull.

The use of gate valves, which are made of brass, should be avoided because they corrode rapidly and their stems tend to break.

Deck Fittings

The federal standard for a deck fitting for pumpout is 1 1/2" inside diameter pipe thread. Most pumpout stations have adapters that will fit boats not so equipped.

The top surface of the deck fitting should be clearly and permanently labeled "Waste." Check to be certain that all of the boats other deck fittings are labeled "Water" and "Gas" or "Diesel" as appropriate. You can ruin your whole day if you accidentally fill the waste tank with fuel or siphon out the fuel tank at the pumpout station.

Piping/Hoses (intake, vent and discharge lines)

Rigid plastic (PVC) piping
comes in different thickness (strengths) known as schedules. Schedule 40, the standard for residential plumbing, works well for boats too. Structurally superior schedule 80 pipe can also be used.

Advantages:

• Highly impermeable; will not corrode.
• Resistant to chafe.
• Readily available in hardware stores with a wide range of fittings.
• Easily cut with a hacksaw; can be glued with PVC cement. Can be readily modified later if needed.
• Virtually maintenance free.

Disadvantages:

• Susceptible to damage from flexing. Provide additional support and use flexible (rubber hose) connectors to reduce potential for damage.
• Often takes longer to cut and fit rigid plastic PVC pipe in tight spaces than to run flexible hose.
• The "nominal" size stamped on PVC piping differs considerably from the pipes actual size inside or outside diameter.
• Subject to damage from freezing if not properly winterized.

Flexible hose
Choose a hose made with one of the special compounds designed to minimize permeation of waste system odors. These are typically made of flexible PVC or rubber and labeled sanitation hose. The heavier the wall thickness, the greater the resistance to odor permeation. All waste lines should be smooth on the inside to avoid trapping waste and reinforced to prevent collapse. Rubber hose is generally more expensive, tends to have a longer service life, and is easier to stretch over fittings than flexible PVC hose.

Advantages:

• May be easier to install in tight quarters.

Disadvantages:

• Susceptible to permeation.

A wide variety of fittings (adapters, couplings, elbows, t-fittings, etc.) are available for use with either rigid PVC pipe or sanitation hose. Each fitting joint creates a potential sewage catch point which may lead to a clog. When laying out the system prior to installation, use curved hose sections rather than fittings where possible. Where a fitting must be used, "sweep" fittings are preferable to "elbows".

Some lightweight spiral ribbed vacuum cleaner-type hoses will not make tight joints without a special screw-on cuff fitting. If you use this type of hose, be sure to use the required cuffs.

Plastic fittings are recommended for installation in plastic holding tanks because the coefficient of expansion is equal for the two materials. Each hose line run to a tank fitting should be restrained (e.g., with a plastic wire tie or clamp) so that it will not put a strain on the fitting and perhaps eventually cause the tank to crack.

Sealants

Sealants can be applied to the mating surface of the adapter before fitting the hose.

Caution: Using adhesive type sealants will make removal of hoses for maintenance or replacement very difficult.