Custom tank design and installation guide, custom made tanks

Drinking Water, Waste Water and Diesel

This guide has been designed to help you design your custom built tank, choose which type of fittings and accessories you need to use and install your tank.

All items mentioned are also listed in the
fittings section of the catalogue.

Index to this page

  • Where to start
  • Avoiding Holding tank problems
  • Installation of Holding tanks
  • Installation of Water and Diesel tanks
  • Hose Tail Fittings
  • Avoiding Baffles
  • Inspection Hatches
  • Senders
  • Holding tank installation items
  • Tank Drawing
  • Where to start
    Consider where the tank is to be situated and where pipework will run to and from the tank.  Perhaps one of the best ways is to make a cardboard model of the tank to fit into the space available, this will enable you to better consider the size and dimensions of the tank and also the fittings and connections required and their location.

    When designing your tank for drinking water, waste water or diesel, there are also some considerations to be made concerning installation of the tank.  regardless of which type of tank you are specifying, it is important to think about where and how you will install it and carry out maintenance checks of the system. 

    This guide contains a lot of information about holding tanks, the installation of the sanitory system and componant parts because there is a lot to consider when installing this type of tank. There is a natural reluctance to
    spend money on the black water tank and installation of the sanitation system as a whole, unfortunately, the regrets can occur much later such as out at sea or in the middle of the well earned holiday.

    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.

    • Make sure the dimensions of the tank overall mean the tank will fit into the boat, through any doors or apertures.
    • Make sure the tank is situated on a decent supporting platform
    • Fix, tie or hold down the tank in its final position
    • Make sure there is sufficient space around the tank so as not to abraze the tank, by itself, the hull or any sharp objects, wires, cables, tubes or pipes
    • Make sure that any items used to fix, tie or hold down the tank do not damage the tank
    • Do a test of the system so as to generally test the system and any senders, gauges etc
    • Check for leaks in joints, tubes or pipes
    • Do not situate the tank near sources of heat or direct heat
    • No not overtighten fittings or bolts on hatches
    • Ensure that wiring diagrams for senders and gauges are followed properly, i.e. for wema gauges the blue is negative and the black goes to the sender
    • Ensure there is a chamfer on the leading edge of fittings.  Soften hoses with hot water before pushing onto the fitting
    • Think about the orientation of pipes to make sure there are no excessive bends in pipework and try to minimise the number of bends.

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    Avoiding Holding tank problems

    • ISO 8099 specification calls for certain requirements in the tank and the system. This relates to items such as the size and availability of tank hatches, deck pump out fittings (even if you only want to discharge through the hull), tank capacity, minimum vent size, etc.  We can give full advice on ISO8099 and BoatWide can provide ISO8099 compliant tanks.
    • There will always be the temptation to install the smallest and cheapest holding tank solution just to comply with the law, however when the system is used, as it inevitably has to be, the shortcomings become real issues that adversely affect the pleasure of the boating activity

    Some of the shortcomings include the following;

    • Odours - caused by thin walled tanks, poor grade sanitation hose, insufficient vent size starving the tank of oxygen.
    • Insufficient capacity - too small a tank
    • Exploding or imploding tanks
    • Leaks - thin tanks, flexible tanks, poor fittings, poor hose, cheap hatches
    • Blocked vents (very nasty to clean out) - overfilled tank due to insufficient capacity or no tank level warning system fitted
    • Gravity empty system - leads to blocked outlets as paper forms into solid masses in the outlet connection, inability to empty tank

    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)

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    Installation of Holding tanks

    • Holding tank vent pipe should be at least 1" (25mm) inside diameter, preferably 1˝" (38mm). You must ensure that vent fitting diameter is maintained along the whole of the vent installation. This will avoid blockage and possible explosion with power flush type toilets or when overfilling the tank, and also implosion when using Marina pump-out. It will also allow air to circulate within the tank thus helping with the natural biodegrading.  An option would be to fit our Tanksaver safety valve. See catalogue
    • Think about the orientation of pipes to make sure there are no excessive bends in pipework which could cause resistance and blockages in the pipe or stress on the tank and its connections.  Also try to minimise the number of bends. Note, low points will retain contents which will lead to early permeation of odours through the pipe, therefore avoid pipe droops. Support with suitable clips or bulkheads.
    • When Inlet or Outlet Hoses are plumbed overboard, below-waterline and terminating inboard, below- waterline, they must be looped above the waterline to prevent siphoning. If any part of the system is below the waterline vented loops must be installed.
    • Be sure to use SANITATION GRADE HOSE and Stainless Steel hose clips.
    • Holding tank inlet, water tank inlet and diesel tank inlets are usually 38mm (1 1/2"), holding tank outlet is 38mm (1 1/2"), Note: macerator outlet to the hull valve is 25mm
    • When installing a system all connections should be double-clamped, only materials specified for marine sanitation should be used, and any below the waterline intake lines should include a seacock that is easily accessible by the boat owner.
    • There are one or two toilets on the market which require pressurised water and call for tapping into an on-board clean water supply. Note: Allowing the sewage system to have any contact with potable water presents an unacceptable health hazard. The manufacturers assure us that check valves prevent any contact, but check valves can and do fail. Therefore we recommend that these systems utilise a separate on-board water tank to supply the toilet via a separate pressure pump.

    Installation of Water and Diesel tanks
    Flush through drinking water tanks before use.  For diesel tanks make sure the feed and returns are connected correctly, the feed should connect to the dip pipe connection if such a connection is fitted.

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    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.

    There are 3 types of fittings available:

    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.


    Fitting Type

    Hose Bore (A) Height (B) BSP (D) Fitting Type Hose Bore (A) Height (B)
    - - - 3/4" STR10 10mm 62mm
    ELB12 12mm 60mm 3/4" STR13 13mm 64mm
    ELB16 16mm 62mm 3/4" STR16 16mm 64mm
    ELB20 20mm 70mm 3/4" STR20 20mm 68mm
    ELB25 25mm 74mm 1" STR25 25mm 74mm
    ELB38 38mm 108mm 11/2" STR38 38mm 92mm
    ELB50 50mm 110mm 2" STR50 50mm 100mm

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    Avoiding Baffles

    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.

    The baffle spacing is based upon the length of the tank. All custom built tanks have equal baffle spacing where possible. The spacing is normally between 250 and 350mm depending on how the length divides up. For example a tank 1200mm long will have 3 baffles with four compartments 300mm wide. A tank 675mm long will have 1 baffle with two compartments 337.5mm wide. If in doubt about the number of baffles a tank will need, then please phone for advice. The same will apply to inspection hatches. An inspection hatch cannot straddle two compartments; it has to be in one or the other.

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    Inspection Hatch

    The Wema hatch is the smallest and is ideally suited for tank access. It is fixed with a single central bolt that screws into an inner flange and seals with a large O ring between the top flange and the tank. Fitting and removal requires a spanner.

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    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. 

    Once the tank is full, the contents can only go one way, up the vent.  This blocks the vent and causes numerous problems.  A second tank full indicator is part of the solution, also having the biggest vent connection and pipe all the way through the system is another prevention.


    The Wema/Kus sender and gauge is used for drinking water and diesel.  We also have NMEA2000 senders, gauges and panels.

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    Holding tank installation items

    Sanitation Hose
    Sanitation hose is available in varying quality.  Be sure to use sanitation hose for all pipework, including inlet, outlet, vent and rinse connections and be sure that it is of the highest quality.

    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.

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    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.



    Y Valves

    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.


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    Odour Control

    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 toilet: 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 odours.

    In the holding tank, the key to odour control is the vent line; it must allow a free exchange of fresh air for the carbon dioxide generated by the sewage. Therefore, those bladder tanks which have no vent are all but guaranteed to stink; there's no source of air into them at all. Boat builders, boat owners and boat yard personnel who install holding tanks have always viewed the vent line only as a source of enough air to allow the tank to be pumped out without collapsing and an exhaust for methane (Many even believe methane to be the source of odour.) Some take the attitude that tanks are going to stink so the thing to do is run that vent line as far from people areas: cockpits, sun decks, etc. as possible, or make the line as small as possible. All of the above actually create the very problem you want to solve.

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    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). 

    If no tank level sender and gauge have been fitted to notify the user the tank is full, or the tank full indication light has been fitted in a place which is difficult to monitor, once the tank is full, the contents can only go one way, up the vent.  This blocks the vent, a number of things can happen from here:

    1.  The user continues to over pump the tank until it explodes due to over pressure
    2.  The tank gets a reprise as it is emptied, the tank gets air through the toilet joker valve (but the vent is still blocked and the tank explodes on the next filling, maybe when it is only half full). 
    3.  The tank implodes when emptied with marina pump out as the suction is very strong and the vent is blocked, the joker valve cannot provide sufficient air fast enough (Optional extra Tanksaver, see below)

    Item number 1 is helped by the fitting of a second tank full indicator and making sure the indication light is visible to the user.  Items 1, 2 and 3 are helped by having the biggest vent connection possible throughout the whole vent system.

    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
    This is a valve designed to prevent explosion of the tank when the tank is in use and the vent is blocked

    Available soon, currently in design.

    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.

    Tanksaver for when contents of the tank are being extracted by deck pump out, Tanksaver saves the tank from implosion when there is either an insufficent vent size, or the vent is blocked.  (Vent size should be a minimum of 1" (25mm), or even better 1.5" (38mm) to avoid problems in the system associated with insuficient vent size).





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    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.

    Tank Drawing

    It is important to provide us with as much information as possible when designing your tank. We will need the overall dimensions and a clear indication of where the fittings are going to go. If the position of the fittings is critical then they will have to be dimensioned from the edges of the tank. If they are not critical then we will put them as near as possible to the position indicated.  When positioning the hatch or hatches, just remember to bear in mind where the baffles will be, again give us a call if you are uncertain.

    On a very complicated tank with many angles, please indicate any square corners if any. This will help us greatly when we come to draw up the tank on our computer aided design system.  Another method of supplying us with the information we need is to supply us with a template or mock up in either plywood or cardboard. By doing this you can satisfy yourself that the tank can be easily installed and that you can actually get it in or out of the boat or vehicle. The template can then be marked up with all the relevant fittings and hatches and either sent to us flat packed or you can bring it along to the factory to discuss in detail.

    We can also accept all the information electronically, of course not everyone has access to design programs, a hand drawn sketch is fine as long as long as all the information is there..  The following drawing shows how we would ideally like the information presented.

    Tank design using computer with Google Sketchup

    Hand drawn tank design


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