I have a 1989 17' sea ray with a 3.0 mercruiser and a 21" pitch prop...It runs fantastic. At 4500 rpm's i'm hitting 42 mph with no problem......My problem was ... I was cruising along and my boat just litterally shut off....I mean like I turned the key off. Alternator was putting out 9 volts instead of the 12plus needed to charge battery.I was told by my mechanic that it WAS the alternator that caused my problem plus having to be towed in......Is this true? Can a bad alternator shut the boat right down??????