Posted: Tue Sep 07, 2010 11:49am

Need to fit a new alternator to sons narrow boat,it is a Lucas A127 mrine 55, to be relaced with the 70Amp version, boat has two leisure batteries (not sure of size) as he is living on boat I feel that he needs to fit another two batteries should we also look at uprating alternator and cabling any tips?

Posted: Tue Sep 07, 2010 9:25pm

First off if you are adding another 15 amps output the main charging positive and if fitted negative leads need uprating. No one can give good advice about the size of battery bank (feeling is simply not good enough - you need the best information available to you)until you have done a power or energy audit and then worked through the charging calculations to see how long he will need to run his engine for to recharge the batteries. See (http://www.tb-training.co.uk/Poweraudit.htm). Actually the answer to how long is probably longer than he can afford or is allowed to under BW regulations but the calculations should show how long it will take to get to about 80% of fully charged. If there is no shore power mains charging available I doubt his batteries will last much longer than two years and only one year is a distinct possibility so if his electrical use allows just two domestic batteries why add more? They will just cost more to replace. If you post the results of those calculations I may be able to give better advice. I suspect he would do better to take a very hard look at reducing his consumption, but that is a guess without a power audit. Once you get alternators much above 80 amp output you start running into drive belt problems so unless you can afford to re-pulley for twin or polly-V belts a significantly larger alternator may cause more problems than it solves. Many boats have sub-optimum charging wiring but without full details about the actual boat I can not be specific. Make sure all charging leads and all load supplies are connected to opposite corners of the bank, otherwise you will always have an/some undercharged battery(s). Tony Brooks