Custom tank design and installation guide, custom made tanks
Drinking Water, Waste Water and Diesel
Where to start
General points for specification and installation of all types of tanks
All tanks require, as a minimum, an entrance and exit of the contents to and from the tank and a vent, all with associated pipework to and from the tank (also a return for diesel tanks). Once the tank is in use, it is useful have a level indicator by use of a sender and a gauge.
Avoiding Holding tank problems
Some of the shortcomings include the following;
Note that Metals such as Aluminium and even marine grade stainless should be avoided as urine is a highly corrosive substance. Polyethylene is an ideal material for the holding tank as it is resistant to the chemical effects of urine.
Many boats fit flexible tanks. These are invariably stuffed down any opening large enough to take it, and only rarely are they secured in place. Since sailing boats are typically so much more 'active" than houseboats or cruisers, heeling side to side, bladders move and chafe till they leak. Because the tank is in an inaccessible place, it is almost impossible to install the fittings correctly, and the tank is never checked or maintained. Some aren't even vented, and it isn't at all uncommon for a flexible tank to blow out its fittings. Furthermore it is all but impossible to control odour in a flexible tank. The very qualities that make bladders attractive to install, make them undesirable for use.
Rigid tanks as made by BoatWide have sufficient wall thickness to prevent odour escaping through the wall. If the wall thickness doesn't continue to increase with the size of the tank, the tank walls will be too weak to support the weight of the sewage. There are polyethylene tanks being sold as holding tanks through most of the marine catalogues which have walls as thin as 1/8". These are just not suitable. (All our holding tanks have a wall thickness of 10mm, and even 12mm for tanks with very large capacities)
Installation of Holding tanks
of Water and Diesel tanks
Hose Tail Fittings
First decide where the fittings need to go.
Once you have established the position, the size has to be decided, taking into
account the overall height or length of
the fitting. Each fitting needs to be checked against the tank design and the boat structure
to avoid any problems later when the tank is fitted. Time spent now checking the
size and position of each fitting will pay off, as it is normally not possible
to modify the tank. At the end of this document there is a typical custom tank
drawing and how the fittings should be marked up. You may wish to provide the
information in a different format, which is no problem as long as all the
information is clear.
1. White nylon hull fittings, they are secured from the inside of the tank and the nut is tightened on the outside of the tank, available in a variety of sizes.
2. Black polyethylene tube which can be straight or angled, (Please specify the direction for angled fittings of this type).
3. Grey polypropylene hosetails which are screwed onto a BSP threaded adaptor which is permanently welded into the tank, all adaptors will stand 24mm off the tank face when the fitting is removed, dimension B on the diagram below includes the adaptor.with a collar. Straight or elbow hosetails are available (elbows are multi directional, so the direction they need to point does not need to be specified. Once the tank is installed they can be swivelled in the right direction and the locknut tightened).
Please specify which fittings will require a dip pipe. For water tanks, diesel tanks and holding tanks, a dip pipe is used for the extractracion of the contents from the tank.
Diagram and table shows the Grey Polypropylene hosetails.
When designing your tank, it is important
that the fittings avoid any baffles that we put in. A custom tank can have
fittings in almost any position, the only restriction being the baffles. Nearly
all custom built tanks will have baffles unless the tank is very small, then it
will have none.
Senders and gauges
Regardless of the type of tank, it is important to easily be able to monitor the amount in the tank. In the case of holding tanks, the tank full indicators need to be fitted in the toilet where the user can see them to prevent overfilling the holding tank and causing the contents to be pushed up the vent pipe.
Tankwatch 1. For drinking water, grey water and black water. Tank level full sender and indicator light (12V only). It is screwed to the top of the tank with 4 self-tapping screws and is easily removed if required.
Tankwatch 4 22" or 45" For drinking water, grey water and black water. Tank level sender, tank empty, half full and full plus indication lights. (Needs install kit). Tankwatch 4 screws into a flange, which is bolted to the tank and sealed with a gasket. The flange can be removed by unbolting the 5 bolts.
The most important thing to know with a holding tank is when
it is near full. If one Tankwatch 1 or 4 is fitted and the tank full
moving piece gets clogged with paper, (especially if the toilets are being used
whilst the boat is in the marina and the tank contents are not moving with the
motion of the boat), there is a danger the tank will be overfilled and the
contents will go up the vent and may cause a blockeage. The answer is to
fit more than one tank full level senders, if one sender gets clogged, there is
a good chance the other will sound the alarm. For example a Tankwatch 4 or
Tankwatch 1 as the primary indication, AND another Tankwatch 1 as a backup.
This gives two tank full senders. The chances of getting a tank full
indication are doubled or the chances of a failure to receive an indication of
tank full are halved, whichever way you want to look at it.
The Wema/Kus sender and gauge is used for drinking water and diesel. We also have NMEA2000 senders, gauges and panels.
There have been problems with hose in the past, and there is hose on the market today that is totally unsuitable for sewage, and it is very difficult to determine, just by looking at it, whether a particular white flexible PVC hose is suitable for use in sanitation systems or not. Some inferior hose can be identified by bending the hose and looking at the thickness of the material between the strengthening spiral, if the material is very thin and nearly splits in your hand then this will not stand up to the pressure of the securing clips and it will not effectively retain the odours. Good quality hose is quite thick when viewed end on and there is little to be seen in-between the strengthening spiral when the hose is bent.
If possible, run your hose without any low spots where sewage can stand, and always be sure to flush the toilet sufficiently to push all the sewage out of the hose and rinse behind it, alternatively, if limited holding capacity makes extra flushing each time the toilet is used impractical, hen pour about 2 pints of fresh water into the toilet and flush that through the system completely. Following this will solve most odour problems.
As part of the routine of closing up the boat, close the intake seacock, flush the toilet dry, then pour about 2 pints of fresh water into the toilet and flush that through the system completely.
The standard for sanitation hose is flexible smooth-walled PVC, 1˝" (except for the discharge from a macerator to an overboard through-hull installed below the waterline, for which the standard is 1"). Since the standard fittings on holding tanks are 1˝", when coming off a macerator to a holding tank, it will be necessary to break the hose, using a 1" to 1˝" adapter. The standard for the hose from a holding tank to the deck fitting is also 1 ˝" inside diameter.
A range of pipe adaptors are available to suit sanitation hose.
Holding tank pumps
Manual Diaphragm Pumps
Manual diaphragm pumps offer light lever operation to diaphragm mechanism. Excellent soft solids handling capability. These pumps will pass small hard objects. Generally they offer simple maintenance and inexpensive spares. Both behind and through bulkhead mounting is available.
Electric Macerator Pumps
These pumps are very compact and lend themselves to installations short on space. Matter to be discharged passes through a bronze cutter which reduces the waste particle size, and is then pumped by the flexible impeller pump, discharging through a 1" (25mm) hose. The macerator cutter will not handle rags or hard objects.
A "Y" divert valve or "Y" connection valve is useful when making connections from the toilet to a macerator or manual pump, or to a deck pump out or hull valves etc.
You may have read or heard, that the key to odour control is the hose, that hose permeates with sewage and causes the system to smell which leads to the kind of advice about hose we mentioned above. That's a half-truth. The key to odour control is in the installation of the entire system.
First of all, a marine holding tank must be exactly the opposite of a septic tank. Holding tanks smell when they become "septic tanks." Why does a septic tank stink? Because it is in an airless environment. So a sufficient supply of fresh air, in and out of the tank is what keeps a holding tank from becoming a 'septic tank’.
Sewage contains two type of bacteria: aerobic bacteria (which need oxygen), and anaerobic bacteria which thrive in an airless environment. Anaerobic bacteria can't survive in an aerobic environment. It's the anaerobic bacteria in sewage which produce the foul-smelling gasses; the aerobic bacteria break sewage down, but do not generate odour. So as long as there is a sufficient supply of air to the tank, the aerobic bacteria thrive and overpower the anaerobic bacteria, and the system remains odour free.
Enzymes do little if anything. Chemical products only mask one odour with another odour, they kill anaerobic and beneficial aerobic bacteria, and therefore create more problems than they solve. Chemicals, are also unwelcome in landside sewage treatment facilities.
The anaerobic bacteria in sewage produce a variety of sulphur monoxides and dioxides (which are the malodorous gasses), methane, which has no odour but is flammable, and carbon dioxide which also has no odour but creates the environment in which the aerobic bacteria cannot live, but the anaerobic bacteria thrive. Carbon dioxide does not rise or fall; it is ambient, like the atmosphere. Without a sufficient flow of fresh air through the tank to allow it to dissipate, it simply lies like a blanket on top of any pool of sewage (whether inside a hose or a holding tank), suffocating the aerobic bacteria and creating the perfect environment for the anaerobic bacteria to take over. The system becomes "septic," and the result is a smelly boat.
To prevent this, let's start with the
If at all possible, the discharge hose, no matter whether it goes overboard, or
to a holding tank, should be installed, if at all possible, with no sags or low
places where sewage can stand. When a marine toilet is not flushed sufficiently to
clear the hose of sewage, that sewage sits in low spots in the hose. Bits of it
cling to the walls of the hose. With no fresh air present the anaerobic bacteria
thrive and create their stinking gasses. If sewage stands in a low spot, which
gets no air, the smell will eventually permeate the hose. This is what has given
rise to the myth that the "wrong" hose causes odour. Therefore, as we have
already said, flush your toilet thoroughly enough to clear the entire hose of
sewage and rinse behind it. And when you leave your boat to go home, flush the
toilet thoroughly one last time, this time with fresh water. Until holding tanks
came along, the hose was the source - but not the real cause of most
The Vent system and optional Pressure Relief Valve
Vent the tank with as short, straight, and
level a line as is possible, with no sags, no arches, and no bends. The minimum
internal diameter of the hose should be 1"(25mm). Ideally, it should be no more than
2 metres long. If it
has to be substantially longer, or if running the vent line uphill can't be
avoided, or if it's impossible to run a vent line that does not go around a
corner, increase the size of the vent line to 1˝"(38mm). (In fact, when possible we
put 1˝" (38mm) vent lines on all installations, this is becuase a 38mm hose
is quite difficult to block even when overfilling. The British Standard MAS 101 actually
specifies a 1˝" (38mm) breather or a multiple of breathers meeting the same cross
sectional area as a 1˝" (38mm). Note that the European standard ISO 8099,
only specifies 20mm which we consider to be very unacceptable. At BoatWide
we will not build tanks with vents less than 1" (25mm).
The vent fitting should not be the same type as a fuel vent fitting, a cap with a slit in it, but should be a completely open type.
Pressure relief valve
Anti Odour Vent Filter
An anti odour vent filter may also be installed between the deck skin-fitting and the tank to further reduce smells. Connections for hose sizes 19mm and 25mm.
Pump-Out Deck Fitting and optional Tanksaver
It is a good idea when specifying your tank to put in a connection for deck pump out, we can put a blank plug on this connection which can remain in place, available to use at a later date if the need arises. The deck fitting is for 38mm hose. The deck fitting should not have a chain linking the cap, (you can't connect a pump-out with a chain in the way), and should be clearly marked 'Waste." Although the best quality deck plates are cast stainless steel with stainless steel caps. Colour coded plastic caps, blue for water, red for fuel and black for sewage are quite suitable and are far less likely to mistake one deck plate for another.
Routine Maintenance of holding tanks
Never put anything into a marine sanitation system that is not specifically marketed for that purpose. Use the cheapest "no-name" single-ply paper which is the same thing as "marine" toilet paper, (and it's a whole lot cheaper!) Just don't put anything else down the toilet, above all no female sanitary products. Do not use detergent, washing up liquid or other cleaners, especially cleaning products which contain pine oil, petroleum, or alcohol. Those substances will break down the seals, gaskets, and valves in the system, and will also break down the walls of the hose, causing it to be more susceptible to permeation. Above all, do not pour vegetable oil down the toilet to "lubricate" the parts. A layer of oil on the sewage will only seal the contents of the tank, keeping air out of it. Follow manufacturers’ recommendations for periodic maintenance and lubrication.
At least once every two years, put a complete rebuild kit in your toilet as part of your regular spring re-commissioning; if you lay your boat up every winter, we suggest doing it every year. When seals, gaskets, valves and impellers dry out they become brittle and prone to crack. By replacing them regularly you all but eliminate the possibility that you will have to make emergency repairs to the toilet. Although it's impossible to predict someone putting something in the toilet that's too large to pass through, a well-sealed pump that's working to factory specifications can often push a borderline object through that a worn system can't.
Hand drawn tank design