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When the pleasure boating season is almost over and you’re going to winter store your boat, no matter whether wet or dry storage, one of the most common and needed procedures is cleaning the water and fuel tanks.

Let’s analyse the crucial steps of accurate cleaning, first of water tanks and then of fuel tanks.

Cleaning of the fresh water tanks

Draining the tank is very simple since an aid can come from the fresh water pump: open the taps of all sinks on board as long as most of the water has been pumped out.

I recommend that you fill up some jerrycans or bottles for two reasons: first of all to save precious water and then to have some at hand during winter storage. Most probably the hose of the marina or of the port will be at your disposal but it will be safer to wash your hands and clean your boat with water from a bottle instead of from a water hose under pressure!

When the fresh water pump starts jamming and having a hard time pumping water (don’t worry, the pump doesn’t break for a few minutes of overcharge) and taps starts spitting out water and air, you’ll have to go on manually.

If yours is a mixed distribution system, go on with the foot-operated pumps until they are empty and drain as much water as possible in the plumbing system. Next step: you’ll need a small drain pump, one of those very simple fuel pumps. Open the taps of the tanks and, very patiently, finish draining. Then you’ll just have to dry and clean the tanks.

The game will be over once you’ve wiped the tank with a sponge cloth trying to reach its remotest corners by twisting your body into nearly inhuman positions – of course only when you want to be meticulous.

All said so far is especially true when the fresh water system has been frequently and regularly used during the boating season otherwise seaweeds, harmful biofilm and mucilage are likely to be found both in the tanks and in the plumbing system (of course this is possible also when the system is used regularly).

If so, before draining the tank as described above, a thorough cleaning will require that a chlorine-free product – to prevent damaging the rubber parts such as the O-rings – is dissolved into the water of the plumbing system. Let it react for at least 2/3 hours and then drain.

The optimum solution – even though demanding and tiring – actually is to clean first, then fill the system with clean water and finally add the sanitizing product.

If you have carried out a correct cleaning, you only have to leave the plugs of the tanks unscrewed in order to let some air in and prevent bad smells when back on board.

Specific note for the water heater

Almost every boat is fitted with a water heater, even if many people forget about it. Correct maintenance and draining is very important when the water heater is not used for a long period in order to prevent malfunction or dirty water in the system once you need it.

Since it’s nothing more than a traditional household water heater with an electrical coil inside, it suffers from the same problems, wear first. Moreover, most of the models feature a double heat exchanger system via the motor cooler therefore the anti-freeze solution will run through the water heater. For this reason I recommend that you drain it when cleaning the tanks and winter-store the boat.

The real problem – and the main reason why few people deal with it – is that draining the water heater is anything but simple. It is tightly clamped (because of its weight) in a low position on the boat, often under a bed or near the engine, and opening a plug or using a fresh water pump is not enough: its cylindrical shape and horizontal position prevent the complete draining of the two liquids inside the water heater, anti-freeze and water.

Fastening brackets are actually simple to open but easy access to the water heater depends on the situation and the boat. Once the fittings disconnected and the water heater removed from its position, you’ll have to lift it and bring it to a suitable off-the-boat place to drain it off completely by gravity.

In short, as you can easily guess, the operation is uncomfortable and tiresome but you’ve got few options unless you choose to leave things as they are, draining the water heater as much as you can without removing it from its position and hoping it lasts as long as possible. I have installed shut-off valves on the engine and the water heater to prevent using it when not needed. If you sail in summertime the consumption of hot water is very limited: you’d better use some solar camp shower bags for any specific requirement, heating water in a kitchen kettle instead of going through the whole cleaning operation when the boating season is over. Of course, I repeat, this is only possible if you think you can do without the water heater, leaving it totally empty as long as you can.

Cleaning of the fuel tanks

Cleaning of the fuel tanks is not always necessary, on the contrary boats are usually winter stored with full tanks in order to limit the presence of air that would consequently condensate and turn into water. Draining the fuel tanks completely is almost impossible because of the draft – usually 3 cm taller in addition to the hull thickness – therefore this option is not recommended.

As an option, I recommend full draining and thorough cleaning every 2 or 3 years because, even though I have adopted a good straining system and paid due attention, biofilm and seaweeds are inevitable, also because gasoil is partially made of vegetable oils and animal fats (that’s why it is called biodiesel).

Before draining the tanks and as the wintering date gets nearer, it’s crucial to plan and calculate the consumption of fuel in order to leave as little as possible, about 30-50% – also considering some safety margin – in order to prevent pouring too much fuel into jerrycans: depending on the size of some tanks, this operation might be inconvenient and implies large storage spaces.

How to drain the tanks

Let’s make an example: if we have to drain 2 tanks, each containing 100 litres, we’ll get jerrycans for a total of 100 litres capacity, that is 5 20-l capacity jerrycans.

Once ready, unscrew the plug of the first tank and pour the content into the jerrycans by means of a submersible drain pump. As with the fresh water tanks, finish draining by means of a manual pump (same type used for water). While pouring, use a funnel with strainer to trap sludge, etc., if any.

Afterwards you’ll pass to the real cleaning of the tanks helping yourselves with sponges, newspapers and any other means needed to remove residues and to absorb the little gasoil left inside.

Dirty gasoil (ineffective to clean it) can be poured into used water bottles – convenient are those containing 5 litres – because it will be useful as degreaser, for example during winch maintenance. Once general cleaning is over, use large quantities of acetone and many clean cloths to degrease.

Now you’re ready to clean another tanks: screw the plug of the first one and open the second.

Use a submersible pump to pour the fuel into the tank you’ve just cleaned (don’t forget the straining funnel).

Once the second tank is clean, fill it up with the fuel from the jerrycans and you’re done.

If you chose to drain the tanks completely, don’t forget to add some antibacterial liquid to the diesel in the jerrycans so the fuel will be clean and ready to be used at the start of the next boating season.

The same applies even to full tanks.

  • Before
  • After

Well, this is what I recommend for an accurate cleaning of the tanks. I can only wish you all the best in this task and should you have any doubts or need for details, don’t hesitate to contact me. 

Giampaolo Gentili
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