Lagos marina has no 'yacht club' or similar venue where sailing people can congregate to meet up with their fellow sailors and exchange the odd yarn or experience over a pint. Mac on Moonstone and Bob of Trilogy, neighbours on the exalted F pontoon, decided they would try to do something about it. Mac has a flat near the Jardin das Dumas club, and knowing the proprietor, negotiated use of their facilities on a Wednesday evening.
Again a good turnout for the second meeting. This time Mac had volunteered to share his knowledge of batteries, and John Holloway had volunteered to talk about the problems of galvanic corrosion on boats.
Mac began his talk about batteries by explaining that the effective capacity of a battery was usually much less than we imagined. A normal lead acid battery, of say a nominal 100AH capacity, could only effectively deliver about 30AH. These batteries are intolerant of a heavy discharge, and should not be discharged beyond about 50% of their capacity. However, most charging systems were incapable of bringing them up to the full 100% charge, often stopping short at only 80%. The charge could be improved by using a charge controller to adjust the alternator output to the batteries' actual requirements. Having a larger bank of batteries also provided an advantage, as the effective capacity of a battery depends on how fast it is being discharged: slow discharge provides more effective capacity compared with a rapid discharge. Mac wondered how many of the audience had carried out an audit of their power requirements. Each device should be listed, and the consumption measured and multiplied by the number of hours to give an estimate of the 'ampere hours' required in a particular day. Mac reckoned that many boaters needed 150AH or more per day. Particular attention should be paid to unnecessary lights: he showed examples of LED lights that provide a much more economic light source than conventional bulbs. Mac went on to describe the advantages and disadvantages of other battery types. Gel batteries he felt were a good compromise: they are more tolerant of heavy cycling and offer a longer life in terms of discharge cycles, but cannot be charged up rapidly. His favourite batteries were AGM (absorbed glass mat) batteries. The electrolyte is held in suspension in glass mat between the plates. They are capable of heavy discharge and can be rapidly recharged, are fully sealed and leak proof, and last far longer than other battery types. They therefore justify their increased cost over the longer term. Mac recommended that an ammeter should be fitted using a shunt in the battery leads, so that usage can be monitored.
John sketched for us the type of boat that Maurice Griffiths would have been sailing on the east coast some 40 years ago. It would lack the modern conveniences that we now take for granted, and might not even have had an inboard engine. It almost certainly would have not had a battery other than for engine starting and a little lighting. The problems of galvanic corrosion, he contended, arose from the modern pursuit of comfort afloat. We now have many devices for navigation, and pilotage, together with lights, pumps and other contrivances, that use DC power drawn from our batteries. The complexity of the wiring involved required high quality wiring and proper attention to earthing. A boat floating in water, having a propeller and stern tube, etc. was in effect a battery, and currents would flow where possible between these elements depending upon the position of the metals used in the 'Noble table': these currents can cause erosion of the parts, and aluminium for example, would not last long in water. A sacrificial zinc anode was essential, properly bonded to the boat's earth, in order that it would erode in preference to expensive propellors. Not only is our boat to be considered, but adjacent boats as well. A boat with poor wirng moored nearby might cause a problem too.
The position was further complicated by the pursuit of comfort, when alternating current devices such as washing machines, kettles, hair dryers, not to mention the ubiquitous battery charger, find their way on board. A small potential difference between the earth circuit on a boat compared with that of the marina will potentially result in massive erosion of the sacrificial anode, opening up the way to serious damage. John quoted one example of £5000 worth of damage on a new boat within a few months of launching. A fault in the marina earth, barely noticeable, or in an adjacent boat could cause severe problems. John described a simple test that could be carried out to measure the earth leakage: currents of significantly more than 20ma would cause damage and he had seen currents as high as over 400ma on some boats in Lagos over the years. The answer he suggested was to fit a galvanic isolator in the boats earth circuit, that would continue to provide the protection of a safety earth but at the same time prevent unwanted earth leakage current from causing havoc.
A queue of anxious owners formed after he had finished speaking, anxious to have their boats checked out.