Holding tank installation for under £500

May 14, 2023

Keith Calton fits a compact, economical and user friendly heads system for less than £500

Keith Calton decided to fit a two-way waste holding tank to his boat. Credit: Keith Calton Credit: Keith Calton

Holding tank installation for under £500. Keith Calton shares his DIY tips

Spending more and more time in harbours, marinas or just pristine environments inevitably means ‘holding on’ or a long walk or boat ride to facilities.

There are many more of us on the water these days and what was thought of as ‘OK, it's biodegradable’ no longer can go into the water and while a quick pee can be acceptable anything else is not!

Keith Calton's 31ft Macwester Wight, Salter Boy. Credit: Keith Calton

If you want to fit a holding/waste tank then the ability to bypass it much of the time is, to me, an essential element so what I’ve done is to take items from other trades which are perfectly serviceable and use them to make a compact, economical and user-friendly installation.

I took the opportunity to strip down and refurbish my Jabsco toilet as I already had a kit, it was not a pleasant job as it was installed in 1978 and, to be honest, while it was not difficult as they supply full instructions, I could have bought a whole replacement pump for £60/£70.

I have a Macwester Wight, a 10m motor-sailer and the heads department is pretty small so inevitably I had to move the actual toilet a bit forward.

Main tank installation finished – the holding tank lives behind the bulkhead fascia. Credit: Keith Calton

That's no big deal but tight if you’re over 6ft 2in.

The first job was to cut out all old shelving and sort a position to fit the tank.

Solder free copper bend. Credit: Keith Calton

I chose a Nuova Rade ‘Zefiro’ model as it is an upright blackwater tank and was available in the right 40lt capacity for my needs.

Roto moulded with extra reinforcement built in, they’re among the cheapest on the market.

The main reason for fitting an upright tank is that it is well above the waterline and will discharge by gravity although my installation has a facility to pressurise the tank, not something I’m enthusiastic to do as blown joint does not bear thinking about.

When installing, ensure you have sufficient room above the tank for the angled filler and space on deck for the extraction pipe, plus space at the side and below for the Y-diverter valves.

Top of the tank showing connections. Credit: Keith Calton

I shortened the long extraction deck fitting and the tank extraction fitting so the tank could be mounted closer to the deck head.

The image above of my deck (which badly needs repainting!) shows the breather for the filter and pump out waste tank fitting.

It's best to leave plenty of room around the pump-out point for easy access for the Marine Sanitation Device, as it is called.

Water intake filter converted into an activated carbon filter. Credit: Keith Calton

The two major sizes for waste pipe are 38mm and 19mm neither of which likes to bend and if you wish for a compact installation, don't rely on putting these through a tight radius, I tried some heat but the lesson learned was that for a very small curve you should heat the inside of the bend and it will compress slightly, heat the outside and it will flatten off; best not heat at all though if you can help it.

My solution was 35mm plumbing solder-free copper bends, the outside of these is about 38mm which fits the standard tube perfectly and they are robust and internally smooth, perfect for the job.

Normal DIY stores won't have these but a good plumbers’ merchant will.

The copper fitting has a wall thickness of 1.25mm and the plastic fitting is 3.5mm which is a total of 2.5mm against 7mm loss of internal diameter with little lip to restrict the flow.

These bends are about the same curvature as some fittings supplied by the marine industry and quite a bit cheaper.

Full front with fascia removed. Credit: Keith Calton

Tek-Tanks do a fine range of longer curvature if needed. You will also need two diverter valves, I bought two which looked good for £16 each, but it was a mistake.

One I could hardly move and I thought it would free off OK and when I took the other out it immediately snapped.

The Jabsco Radial Y-Diverter valves, though over three times the price, are excellent and compact.

I found it best to get most of the major components ahead of fitting as nothing beats offering and mocking stuff up to see how it will actually work.

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I wedged the tank in place and checked all gaps to ensure I had the space needed.

Next I fabricated a support structure, bearing in mind that 40lt weighs 40kg and you don't want that flying about when you change tack or come off a big wave.

The support consists of two transverse 25mm x 20mm mahogany beams directly under the tank with cross supports plus a 25mm x 20mm at the front of the tank with a 4.5mm PVC decorative plate fascia which is also a structural retainer.

Once the tank was safely sitting on its beds I could start route the pipework and position exactly where the diverter valves were going to fit.

Deck fitting. Credit: Keith Calton

These had to have easy access for both pipework and hands.

The first diverter valve sits high on the left and takes waste from the loo and diverts it either to the tank or to the lower valve; this is easily accessed via the hole cut in the PVC fascia plate.

The lower exit diverter valve takes waste from either the tank or straight from the first diverter valve to exit overboard when the valve is closed off from the tank, it allows waste to accumulate in the tank.

Lower diverter valve. Credit: Keith Calton

The exit diverter valve, being on the bulkhead, needs to exit straight through and onto an angled joint to feed into the one way valve just before the stopcock which is in a separate cabin.

The reason for the inexpensive Jabsco non return flap valve was really a belt and braces idea.

Note that for all connections below the waterline and before the Jabsco non-return valve I have now fitted double Jubilee clips as extra security.

Hull exit seacock and Jabsco non return valve. Credit: Keith Calton

If I need to pressurise the tank to speed emptying, I can turn the upper valve to ‘tank’ and the lower valve to ‘tank exit’ and switch off the breather tap but being careful not to over pressurise – this shouldn't be necessary in practice.

The breather filter I used was a ‘Seaflow’ water intake with the inner ‘sieve’ removed and an activated carbon bag inserted which fits perfectly and is much cheaper than a purpose-built filter.

A tip with Jubilee clips is to use a small socket rather than the slotted screwdriver, for better control, but be careful not to overtighten them.

Breather tube and tap. Credit: Keith Calton

NOTE: no paper will go down the loo except possibly in ‘straight out’ mode, plus there is a pressurised water douche and lower isolating switch supply by the bowl (it's cold water, you have been warned!).

The insulated tube of the heater exhaust runs up the bulkhead which keeps the heads cosy in the winter.

The scruffy insulated tube running up between the douche and filter is the heater exhaust. Credit: Keith Calton

I chose to use two diverter valves but I could have used one with a switch at the bottom of the tank.

The Jabsco Y-diverter valves have adjustable 38mm exit and entry tubes and in retrospect I should really have used these but I’d already committed to the cheaper version.

Three terms are commonly used to distinguish the different types of waste water created on boats:

More guidance from the Royal Yachting Association (RYA) about the legislation relating to the discharge of black water from holding tanks at sea can be found at: rya.org.uk/knowledge/abroad/holding-tanks

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