Power Failure – battery nuts

Report Text:

Vessel built 2008 is an auxiliary sailing yacht. Incident: On leaving the marina berth for a day’s duty as a Regatta Committee Boat in a borrowed boat, all the electrically-powered domestic accessories suddenly failed – no bow thruster, no anchor windlass, and no instruments. Problem: Subsequent investigation showed that the domestic battery pack (3 x 12V, 130 AH leisure batteries) had been linked in parallel, using wing nuts on the terminals. One wing nut had vibrated loose, causing arcing/sparking, hence localised heat increase, increased resistance and even higher temperatures. Final result was partial meltdown of the battery connecting terminal lug. The batteries are stowed under a screw-down panel under the galley floor.
Lessons Learned: Check the tightness of battery terminals at regular intervals. Suggestion: NEVER use wing nuts only to connect battery terminals. (But the evidence is, that was the way in which the builder had supplied the boat!). Far better and safer to use hexagon head nuts with lock washers, tightened with a spanner.

CHIRP Comment

Contact with the boat manufacturer revealed this is a modification; the current design does not use wing nuts on batteries and uses locking nuts.
MCA and NWA (National Workboat Association) Reference 8.5.4 States: Adequate provision should be made for securing electrical connections, e.g. use locking washers.