More tank capacity, more self-sufficiency: Planning and installing additional tanks for diesel and water

Until not so long ago, the maxim in automotive engineering was that there was no substitute for displacement - except for more displacement. Well, that's over now. In yacht building, however, the maxim still applies in a slightly modified form, at least with regard to tank capacity. If you want to sail long distances or avoid crowded harbours on holiday, you can hardly have enough diesel and fresh water on board.

Less diesel consumption, more fresh water required

Typical cruising yachts between 32 and 36 feet usually carry 150 to 200 litres of water and 70 to 150 litres of diesel - enough for a week without refuelling, assuming reasonably conscious consumption. On a fully occupied charter boat, however, the water tank can run low after just three days. And for an Atlantic passage lasting three weeks or longer, the reserves are definitely not enough.

While diesel consumption is generally on the decline because solar cells and modern lithium batteries mean that the engine hardly needs to be used for power supply, the demand for fresh water has risen and is still rising. The old rule for long journeys - ten litres per person per day - is hardly feasible today when sailing close to the coast and mooring in port every few days.

Low tank capacities on many yachts

There are several reasons for this, first and foremost the increased demand for comfort. But there are also technical trends that are increasing demand. For one thing, the foot pumps for washing up in the galley sink that used to be commonplace are now an absolute rarity, meaning that the supply of fresh water can hardly be stretched. They are practically no longer found in series production and are only available as an optional extra at shipyards that specialise in blue water boats. On the other hand, more and more owners are also using fresh water instead of seawater for flushing toilets, as this minimises the build-up of odours in the hoses.

In addition, there are still a large number of yachts with limited capacities. These include modern lightweight performance cruisers such as the Pogo 30 (40 litres of diesel/140 litres of water) and, in particular, older used boats, as illustrated by three examples: Dehler 33 (60/90 litres), Luffe 37 (30/100 litres), X-362 (80/135 litres).

Especially in areas where the infrastructure is still full of holes, this doesn't get you very far. This applies to small communal harbours in Greece as well as to large parts of the Caribbean, where you sometimes even have to wait for a tanker to bunker. Therefore, more volume not only means more independence, but also simply more relaxed cruise planning.

Block-shaped series tanks or customised individual production?

For these reasons, more and more owners are considering installing additional tanks. Or they have already expanded. Sometimes inexpensive standard tanks are sufficient for this - especially if only an additional 60, 80 or 100 litres are required because the standard tank already has a reasonably sufficient capacity. Such tanks are available from equipment suppliers or specialised suppliers such as tanksdirekt.de. They are industrially manufactured, usually have a square or rectangular shape and can be easily installed in the back box or technical room. For those who do not have enough space or want to accommodate more volume, there is no way around customised tanks.

That's when specialised companies like Steffi Schüler's, who is the second generation to run Schäfer Kunststoffverarbeitung, a company founded by her father, are in demand. Although located in Rhineland-Palatinate far from the coast, the company specialised in yachting early on; occasionally, workshop manager Sascha Schmidt and his colleagues also build tanks for motorhomes and industry. Schäfer is one of a small, select group of experts who, thanks to their expertise and a vertical range of manufacture of almost 100 per cent, can find a solution for every special requirement (see below). They mainly supply private customers and shipyards at home and abroad with special tanks.

HDPE is the ideal material for tanks

HDPE, a high-density and durable polyethylene, has established itself as a construction material. When correctly dimensioned, it is very robust, relatively light, food-safe, odour-impermeable and resistant to acids, alkalis and diesel, which is why it can be used universally. Black sheets are usually used for tank construction because they are more UV-resistant and reduce the formation of germs if water or fuel is stored in them for a long time.

Compared to aluminium and stainless steel, HDPE is also significantly cheaper - in terms of material costs, the plastic is almost half to two thirds lower than aluminium or V4A sheets for a comparable strength. Almost as importantly, all pipes, flanges and sleeves can also be made from polyethylene, resulting in a homogeneous component that does not require a sealant.

Additional tank or larger main tank?

The processing is basically very similar to that of metal tanks. After cutting to size, the panels and connections are first pre-stapled and then permanently welded. There are practically no limits to the shape. Depending on the volume, even curves can be realised, but only with wall thicknesses of up to eight millimetres. Tanks with a capacity of around 200 litres, as in the example documented here, on the other hand, require a panel thickness of ten millimetres; in this case, the adaptation to the frame shape of the boat is carried out using the buckling frame construction method, which is more complex but also provides greater strength.

Fixation at the installation location can be provided by bulkheads or frames and stringers; individual tanks should also be lashed down using lashing straps. When planning, it is also important to consider hose routes, ventilation and possibly fill level sensors. Depending on the position in the ship, a larger main tank may ultimately make more sense than extending the existing installation with an additional, smaller tank that requires its own supply and discharge lines.

A complex matter? Tank builder Sascha Schmidt reassures: "Not possible, doesn't really exist!"

Costs & contacts

It is difficult to make concrete price comparisons in tank construction because it is not just about volume, but also about shape and cost. Here are examples of a 200-litre model:

• Simple prefabricated tankswhich are manufactured using the rotomoulding process, are available from 300 euros without connections, complete from 400 euros. However, they can hardly be used on yachts, except in large forecastle boxes.
• Customised PE tankslike the one shown here, cost between 750 and 900 euros complete; simpler forms from 350 to 400 euros including inlets and outlets as well as inspection covers. Lead time and construction time: four to six weeks, often more at the start of the season.

We accompanied and documented a production at Schäfer Kunststoffverarbeitung in Oberwambach. Other suppliers specialising in yachts:

• DL Plastics Technology, Unna
• Leto Plastics Technology, Wilhelmshaven
• Tanknology, Wentorf near Hamburg