Another article on the subject of winter:
“We must have been a bit careless when clearing out the boat in the autumn,” came the comment from the galley. A bottle of mineral water had been left in the cupboard – and now, with temperatures at minus 15 degrees in winter, it had burst. A striking example of just how important it is to winterise the boat properly and make it ‘frost-proof’.
When water freezes, it expands. That’s a well-known fact. To be more precise, water increases in volume by up to ten per cent as it freezes. If it is contained within a closed space, such as a container or a tank, it exerts considerable pressure on the outer walls. At low temperatures, therefore, water needs space to expand. If not out to the sides, then at least diagonally upwards. This is also the reason why guttering in Northern Europe is designed in a semi-circular shape. If they were rectangular, as they are in the Mediterranean region, plumbers in this country would have considerably more work to do. However, structural measures cannot always provide a solution. That is why frost, alongside corrosion and wear and tear, is the third major enemy of ship engines.
Advice on preparing for winter
“You’ve got to put antifreeze in all the pipes so they don’t burst,” is the sort of advice a novice might be given when winterising the boat for the first time, “It’s the alcohol in it that keeps them frost-free.” After all, the pungent, sweet smell of alcohol hits your nose when you switch on the windscreen wipers in the car.
But could I, for example, also use windscreen washer fluid to winterise my drinking water tank? That’s why many boat owners find themselves in the DIY store every autumn, wondering about the differences between the various coloured fluids. Does it really matter what I use for antifreeze, as long as it contains enough alcohol?
Not at all. Antifreeze is by no means intended solely to protect against frost, and the various types differ significantly in their properties. If used incorrectly, the vessel could suffer serious damage and, in the worst-case scenario, even sink.
Glycol instead of alcohol
The reason why a mixture of ethanol (commonly known as alcohol) and water only freezes at very low temperatures lies in the differences between the molecules of ethanol and water. They are so different that it is virtually impossible for the water to form a coherent ice crystal lattice, as this process is constantly disrupted by the ethanol molecules mixed in with it. This is why the mixture remains liquid even in winter – at least right down to very extreme temperatures. In the case of most antifreeze fluids, this is down to minus 50 degrees. The ethanol therefore lowers the freezing point of water.
In principle, ethanol, with its specific freezing point of minus 114 degrees Celsius, is very well suited to protecting against frost. That is why it is also used in de-icing products, such as door lock de-icers. However, an antifreeze has other functions as well. When used in an engine, it is primarily a coolant. However, ethanol, with a boiling point of 78 degrees, is completely unsuitable for this purpose, as the coolant would start to boil even sooner than water. Boat and car engines usually operate at 90 degrees.
This is why the dihydric alcohol ethylene glycol – colloquially known as ‘glycol’ – is used in coolant instead of a monohydric alcohol. Although the melting point of pure glycol is only minus 15 degrees, when combined with water the freezing point is even lower. A glycol-water mixture in a 1:1 ratio does not freeze until it reaches minus 40 degrees.
Although glycol is derived from ethanol, it is a completely different liquid. Its viscosity, for example, is much higher, and the liquid feels almost oily to the touch. This is an advantage, as it also lubricates moving parts such as the coolant pump and thermostat.
Heat capacity
Unlike in a car engine, where waste heat is channelled via the cooling circuit into the very large radiator grille and then dissipated by the airflow whilst the vehicle is moving or, when stationary, by the fan, it is all the more important in a marine engine that the heat capacity of the coolant is high enough. This is because, unlike car engines, ship engines are usually located deep in the hull of the vessel. Fresh oxygen often only reaches them via a fan, but heat frequently cannot escape. The ship’s engine dissipates its heat solely via the heat exchanger, which appears minuscule in comparison to a car radiator.
Use in the engine
The dual-circuit cooling system commonly found in boat engines today consists of an inner and an outer circuit. Both are connected via the heat exchanger, in which (as the name suggests) heat from the inner circuit is transferred to the outer circuit. The inner circuit is filled with antifreeze, which is pumped by a circulation pump through the cooling channels in the engine block, where it absorbs heat and transfers it to the outer circuit in the heat exchanger. The outer circuit is fed by fresh lake water, which is drawn in by the cooling water pump and injected through the heat exchanger into the exhaust system to cool it. The exhaust gases are then mixed with the lake cooling water in the water collector and discharged to the outside in surging waves via the exhaust.
As well as glycol, certain additives are blended into the engine antifreeze to perform various functions. The agents and active ingredients vary depending on the antifreeze specification. However, the functions of the additives are the same in every
Antifreeze also provides corrosion protection for the engine, water pump and cooling channels, as the water content naturally causes corrosion on metal. It prevents mineral deposits (e.g. limescale) and protects against cavitation.
Cavitation
Many people are only familiar with cavitation in relation to ship propellers. The term describes the formation of vapour bubbles in a moving liquid. These bubbles form whenever the static pressure of a liquid suddenly drops below its specific vapour pressure. Unlike water, which has a vapour pressure of around 0.7 bar at operating temperature, the antifreeze mixture reduces this value to 0.5 bar. The likelihood of cavitation is therefore reduced by the antifreeze. The coolant circuit of a running engine is maintained at a pressure of approximately 1 to 1.5 bar by the coolant pump. Whilst the vapour pressure of a water-antifreeze mixture is still around 0.1 bar when the engine is started in cold coolant, at operating temperature (90 degrees) the vapour pressure is already 0.5 bar – still a long way from the engine’s operating pressure. However, the coolant system has a number of narrow points and obstructions. For example, when the liquid molecules slow down just before entering the inlet to the water pump in order to line up in the impellers, vapour bubbles may form there. Bubbles can also form during engine warm-up and cooling, as the vapour pressure of a liquid depends on its temperature.
If these bubbles are then carried by the flow into an area of higher pressure, they implode suddenly, which can cause damage to the coolant system and the engine. In older coolant pumps, distinct indentations in the metal are often visible at the outlet of the coolant passage, caused by imploding vapour bubbles.
Technologies
In the past, all antifreeze fluids contained silicates. Their advantage is that they protect the material by forming a protective aluminium silicate layer on the aluminium parts. The disadvantage is that this protection only lasts a few years and the antifreeze fluid has to be replaced frequently. To extend the service life, it was decided to use organic compounds for corrosion protection instead of silicates. However, there were now two different types on the market; the colouring of the fluids was not standardised, and problems constantly arose due to mix-ups.
This is because: if the two systems are mixed, clumping can occur, causing the channels to become blocked. Furthermore, mixing them produces corrosive acids. That is why the hybrid system was developed, which combines the advantages of both systems (compatibility with aluminium and a longer service life) and can also be mixed with the earlier systems. Furthermore, it offers better heat dissipation, better corrosion protection and, at 135 degrees, a significantly higher boiling point. However, even hybrid systems are not the ultimate solution and are not equally suitable for all engines. The choice of coolant for your own engine should always be based on the manufacturer’s specifications. The colour alone is not a reliable indicator. Furthermore, only equivalent coolants should ever be used, as otherwise the additives they contain may interfere with one another.
Ready-to-use mixture or antifreeze concentrate?
There are many antifreeze concentrates and ready-to-use mixtures available on the market. Anyone who thinks ‘more is better’ and, to be on the safe side, pours the concentrate into the engine undiluted is making a mistake. This is because, even though using excessive amounts of glycol lowers the freezing point and raises the boiling point, the disadvantage of pure glycol is that it has a much lower heat capacity (2.4 kJ/(kg·K)). In a 50:50 mixture with water, the heat capacity increases to 3.5 kJ/(kg·K). Consequently, when using pure glycol, a greater flow rate is required to transport the same amount of heat. The hoses would need to have a much larger cross-section. Another disadvantage of glycol is that, when air-permeable plastic pipes are used, oxygen can come into contact with the liquid and oxidise the glycol into carboxylic acids, which can corrode the system’s metal components.
The safest option is therefore still a ready-made mix. If mixing your own, you should use at least 30 per cent glycol to ensure adequate protection. However, never use more than 60 per cent, as this carries a risk of overheating.
Changing the coolant
Coolant does not last indefinitely. Particularly when subjected to prolonged and heavy use, it loses its properties and must be changed from time to time. Over the years of use, heavy metal particles, traces of engine oil and other contaminants also dissolve into it.
After draining the coolant – and especially if you do not know which coolant was last used – it is a good idea to flush the engine several times with water. Then top up with the correct coolant. Fill the expansion tank to the maximum level. Run the engine. When the engine is cold, top up to the mid-point between the minimum and maximum marks.
Waste disposal
Glycol can be disposed of at most landfill sites or hazardous waste collection points. It can also be handed in at some garages that have their own collection bins. There are also a number of service providers who accept used glycol for a small fee or even free of charge, and will even collect it in larger quantities. Some larger marinas and shipyards even have collection bins where berth holders can dispose of their glycol.
Winterising the machine
It is important that, when winterising the engine, any remaining seawater is either drained or replaced with antifreeze. For single-circuit-cooled engines, this is best done whilst the engine is still in the water, as they must first be warmed up before winterisation so that the thermostat opens. Otherwise, only the cooling water circuit from the seawater valve to the water pump – and then the exhaust system – would be flushed with antifreeze. If the boat is already ashore, it is necessary to run the engine on fresh water first to allow it to warm up. This can be done by disconnecting the intake hose from the seacock and connecting it to a water barrel. (Or into a bucket and then topping up with a hose.) Alternatively, the cover of the seawater filter can be unscrewed and fresh water poured in there. To ensure that the fresh water is drawn into the engine and does not escape through the open seawater valve, it is important that the seawater valve is closed. Once the engine is warm, the antifreeze is then added in place of the fresh water until coloured coolant emerges from the exhaust.
In the case of dual-circuit-cooled engines, the seawater-cooled circuit is flushed and filled with antifreeze as described. The internal cooling circuit between the engine block, circulation pump and heat exchanger is already filled with antifreeze. However, a refractometer should be used to check whether the antifreeze is sufficient whilst the engine is cold. Ideally, the measured value should be between minus 20 and 30 degrees. The antifreeze should also be replaced every three years, as it loses its corrosion-inhibiting properties over time.
With Z-drives, the antifreeze mixture can be fed directly from a water bucket to the Z-drive’s cooling water inlet via a flushing nozzle, using an electric pump. A second builder’s bucket should be positioned beneath the drive at the same time to collect the recirculated coolant.
Engine antifreeze
Winterising the on-board toilet
The process for winterising the on-board toilet and waste tank is similar to that for the engine. The tank should be completely emptied before the motorhome is lifted off the trailer. The recommended antifreeze is then poured into the toilet and pumped out for a while to protect the macerator pump from frost. It is even safer to disconnect the hose from the ball valve and draw the antifreeze in directly from there, so that the intake line is also filled with antifreeze. If the on-board toilet is flushed using water from the fresh-water tank, it is also necessary to fill the hose running from the tank to the toilet with antifreeze to protect it from frost.
It is usually not possible to empty the waste tank completely, but the antifreeze pumped from the toilet into the tank will mix with the remaining water. However, it is also important to ensure that the tank’s bilge pump on the seaward side draws in antifreeze.
A common mistake is to winterise the on-board toilet using the simplest and cheapest antifreeze available from a DIY store – ‘it’ll be fine for the loo’. However, this is often antifreeze containing silicates, which attacks the rubber parts and seals. Even though leaks caused by antifreeze are rare, the product will damage the rubber hoses to the extent that odours will permeate through them more quickly. It is advisable to use specialised antifreeze approved for use in sanitary systems.
Antifreeze for toilets
Preparing the water tank for winter
You should also drain the water tank in autumn before winterising the system. It used to be common practice to winterise the drinking water pump with one or two bottles of schnapps. Schnapps is cheap, won’t freeze and isn’t harmful if any residue ends up in the toothbrush cup. To do this, you simply need to disconnect the hose from the tank, dip it into the bottle, pump three times, and you’re done. Unfortunately, however, this method of winterising isn’t really frost-proof, as the alcohol evaporates over the winter. Furthermore, it is likely that the alcohol will corrode seals and rubber parts. This ‘cheap solution’ is therefore strongly discouraged.
It is better, after emptying the fresh water tank and the boiler, to fill them from the top with an antifreeze mixture suitable for the intended purpose and to circulate it through the pressure water pumps to all tap points until coloured water comes out of the tap. Otherwise, in particularly cold winters, any water trapped in the washbasin taps may expand and cause damage. For this reason, the tap should also be left open over the winter. The outdoor shower must not be forgotten either, nor the shower sump with the float pump, in which a little water often remains.
Antifreeze for drinking water
