Thursday, 28 July 2011

Freshwater Pumps for Sailboats

Part of your sailing 'adventure of a lifetime' planning process will be selecting the correct freshwater pumps for your sailboat. Like everything else these days it is not as simple a process as it has been in the past when you could go into a chandlery, ask for a freshwater pump for you boat and probably be handed the only pump available.
Nowadays there is a whole raft of models to choose from so you need to take into consideration all of your and your vessels' requirements. The following article from Kurt Kupper of  outlines some of the features that you need to check for before making your selection: 

'On any sailboats other than day sailers or runabouts, fixtures that allow the convenience of water on tap for showering, hand washing and dish washing are no longer considered a luxury. Driving these systems are freshwater pumps.

The most commonly used fresh water pumps used on boats are electrically driven diaphragm pumps. These are ideal for this application, because they are self priming, operate fairly quietly and can be run dry without immediate damage to the pump.

These pumps have built in pressure switches which turn them on and off on demand. The pump thus pressurises the plumbing system that it is connected to until the cut out pressure is reached. The pressure switch then opens to switch the pump off. As soon as a tap is opened and water runs from the system, the pressure in the system falls and the switch closes for the pump to run again. The pressure switches typically maintains a system pressure of about 3bar.

The immediate problem with this system is that when a tap is opened, there is a delay until the switch has been activated and the pump has sped up to restore pressure to the system. This results in uneven water flow. To counteract this, accumulators are often fitted into the system.

Accumulators are usually cylindrical tanks with spherical ends with a capacity of one up to twenty litres. At one end a connection is provided to allow water into the tank. At the other end is an air valve which is connected to a bladder inside the tank. This bladder is pressurised with air. When there is no water pressure, the air pressure in the bladder inflates the bladder like a balloon. When the water pump is run, water enters the cylinder and compresses the air in the bladder. Whenever the water pressure in the system starts to drop, the bladder can expand again, thus keeping the water circuit pressurised. This has a dampening effect and results in more even water flow.

Recent pump developments have brought us pumps fitted with sophisticated electronic control units that regulate the speed of the pump instead of just having a simple pressure switch. These are very responsive to fluctuations in water pressure and can be used without an accumulator in the circuit, yet produce flow smoother than in systems fitted with an accumulator. As can be expected, these pumps are somewhat dearer.

Pumps are available with different flow ratings. Although there are some baby pumps suited just for one washbasin that pump about 4L/min, most boats are fitted with pumps capable of supplying between 10L/min and 25L/min. At the lower end of this range these are suitable for boats where it is unlikely that more than one outlet will be used at any time. Boats with multiple showers and basins require larger pumps to ensure that water pressure and flow are maintained if several taps are run at the same time.

When selecting a pump, carefully consider what your pressure, flow and voltage requirements are. The more pump chambers the product has, the smoother the flow is likely to be.

Diaphragm pumps are very susceptible to dirt and therefore require protection by fitment of a fine strainer before the inlet to trap any contaminants. These are often supplied as standard with the pumps and should be cleaned regularly to ensure optimal performance.

The pipe connections on pumps are sized relative to the flow they are capable of producing. It is important to note that larger pipes should be fitted throughout the system when the larger pumps are used, as the benefit of their capacity will otherwise be lost through restrictive resistance. Great care should be taken to ensure that all joints in the pipes are carefully made and don’t leak. Leaks in the system will cause the pressure to fall, resulting in the pump being automatically activated periodically to boost the pressure again. This can be most annoying, especially at night.

As with all electrical equipment, use only tinned cable of adequate cross sectional area to wire up pumps, make sure that the joints are protected from moisture and fit a fuse or circuit breaker with a rating corresponding to the manufacturer’s recommendation'.

Article and images courtesy Kurt Kopper

You can read more about boat pumps and their repair whilst on passage in my ebook 'Voyage of the Little Ship Tere Moana' downloadable from my website

1 comment:

Anonymous said...

interesting blog. It would be great if you can provide more details about it

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