A 'swan neck' is an inverted 'U' in a pipe line, and it is generally designed to prevent the influx of unwanted fluid into a specified area. The importance of swan necks was raised recently by reader Alan Waller who had suffered various problems when he bought a second-hand sea-going motor cruiser that had spent its early years on the Norfolk Broads.

After having the boat surveyed, Mr Waller made an offer, subject to several reported faults being corrected. One such problem was a badly routed fuel tank breather pipe that was noted to contain liquid within a downward loop. The pipe was rerouted in line with the surveyor's recommendation and the purchase duly completed.

On his first long trip to the Channel Islands the trouble started. The starboard engine failed, while the port engine began running very poorly. After limping into harbour, local engineers diagnosed salt water in the diesel fuel. Although they believed that the water came via the tank breather, Mr Waller felt sure that the problem lay with contaminated fuel bought in Guernsey.

After an expensive repair job, the cruise continued but within half an hour the glass fuel-separator bowls were showing large quantities of water that had to be repeatedly drained off over several days.

The fuel and tank had been tested for salt water following the repairs and had shown no water contamination so, as Mr Waller admits, he was wrong and the engineers were right about the source of the water. The culprit was the breather hose, which had no swan neck; it was able to collect water while the boat was under way and dispense it into the tank under suction from the engine's fuel lift pump.

In this way, lots of water entered the tank and seriously contaminated the fuel. To prove the point, the breather that ran directly into the tank was removed and allowed to breathe within the boat itself. This immediately stopped the problem. A clam-shell ventilator was then fitted over the breather skin fitting and a huge swan neck introduced into the pipe, which prevented any further problems.

When the engine manufacturer's installation instructions were checked, they showed that a swan neck was a mandatory feature of the tank breather but this had not been provided. This story clearly shows just how important seemingly insignificant details can be when preparing a boat for cruising at sea.