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  Polarity Reversal With Twin Engines

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Author Topic:   Polarity Reversal With Twin Engines
Chuck Tribolet posted 10-04-2010 01:04 AM ET (US)   Profile for Chuck Tribolet   Send Email to Chuck Tribolet  
Adm. Linda and I launched the whaler at Pt. Lobos to go diving
this morning. This fellow diver that I've seen around and
chatted with a little pulls up next to us and asks me for some
help with a problem. He has about an 18' boat with twin 40s.
He'd blown the starboard engine and replaced it the previous
week. The day before he'd gone diving, and come up to find
the boat battery dead. No problem, the little 40s pull-start.
He gets back to the dock, pulls the boat, and
there's white stuff all over the port engine (the one that
wasn't replaced).

He was camping, so he swaps his truck battery into the boat,
jump starts the truck, and lets the truck recharge the dead
battery (kinda creative, actually).

But he's concerned about the white stuff. Of course, it's
galvanic corrosion. I grab my multimeter out of the tool box
on the boat. As soon as he turns the battery switch on,
there's 12V between the two motors. Starboard motor is at
+12. The bug: the battery cables had been hooked up backwards
on the new motor. The block and lower unit were at +12.

Ouch.

Obviously, the alternator diodes are blown.
What else?

And it's a miracle there wasn't some connection between the
two motors that wasn't fused (like steering cables). The
+12V path to the block of the starboard engine and the -12V
path to the block of the port engine were unfused.


Chuck

jimh posted 10-09-2010 10:18 AM ET (US)     Profile for jimh  Send Email to jimh     
Reversal of polarity of the starting battery to an outboard will often blow the diodes in the alternator, but may not, if there is a voltage regulator in the circuit. Some voltage regulators use a steering diode in their output which can prevent polarity reversal from damaging the rectifier. However, that said, there is a darn good chance the alternator is blown.

If the outboard is a simple 1970's technology, that is, no micro-processors running the works, there may not be much more damage. If the motor is a modern outboard with micro-processor controls, there is much more potential for damage (and pardon that pun).

This incident recalls another one I saw a few years ago. A fellow had his engine serviced. During the work a connection was accidently established between battery positive and the engine chassis, resulting in current leaking to the engine. The current did not blow any fuses and as long as the engine was running did not discharge the battery. A stainless steel propeller began to show a dull white finish after a few hours in the water with the engine running. Fortunately the short-circuit was found and repaired before more damage occurred.

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