Need help on totally conflicting advice

[Rick Evans]

I just attended a marine electronics class. The instructor stated that shutting off the key does not hurt the engine. It only disables the tach and the alarms but, contrary to popular belief, does not blow out the alternator because it is still able to discharge into the batteries. Yet, the boat manual for my Beneteau 321 with a 27 HP Yanmar 3GM30F warns in bold print never to shut off the key without first shutting off the engine. Second, the instructor also said that at dockside, you could start the engine and run it even with shoreside power running and the battery charger working. Yet, the boat manual warns in bold print never to start the Yanmar with the battery charger working. I presume they think it will overload the batteries or do something evil to the alternator. Yet, my instructor said the batteries don't care where they are getting a charge from. If the alternator senses the charger has topped off the battery, the regulator will simply shut off and the alternator will be happy. Any thoughts on these two issues and the different pieces of advice on got on each one? Thanks.

 

[Marc]

Instructor is correct

Let's look at the second part of your instructor's lesson first. The alternator's voltage regulator cannot differentiate voltage. If the battery voltage is high enough the regulator will taper the output of the alternator. If the shore power is on the battery charger will raise the battery voltage and thereby "fool" the alternator's regulator. When the shore power is disconnected and the battery voltage begins to fall the alternator will once again supply charging voltage to the batteries. Turning off of the "battery switches" while the engine is running will definately damage the alternator's diodes. Turning off the key will not.

 

[BILL TRIPP]

ADVISE

I follow the safe road, pull out the kill throttle control, listen for the horn to sound and extract the key. My boat has a Yanmar enginewith specific instructions (why were they written in such detail on shutting off the diesel engine?). I also shut down all electronics before the throttle control is pulled.

 

[Rick Evans]

The final answer

I posted the same question on the general forum to ask all sailors. I got a ton of replies with lots of interesting insight. You will note in one of my responses that I called Beneteau directly for input. The bottom line is that my instructor appears to have been correct although nobody is exactly comfortable ignoring the warnings in the manual. Beneteau says they have heard of a few cases where the alternator was blown under these circumstances but really didn't have any hard & fast proof that it was anything but coincidence.