Not rare. Search for "Blown Rectifier" and you'll find many with low voltage issues. If the voltage at the battery is that low and does not ever move, then you have a charging system problem.
Check all of the connections and fuses first.
Check the AC volts going to the rectifier from the stator coil at the yellow and yellow gray wires. Should be well above 12 volts.
The round non-regulated rectifier can be tested by following this:
http://forums.iboats.com/showthread.php?t=190638
if no AC volts then probably the stator coil is bad. Ohms test across the disconnected yellows should be a very low reading.