port engine voltage drops after starboard engine starts

[tunakiller]

I hope someone has some advise for this one. I purchased a "new " used boat with twin 2006 F250TXR / LF250TXR. Have had the boat about one month and everything was fine. Today when I started the starboard engine the voltage on the port engine started dropping from approx. 13.8 and continued dropping to below 12V. If I run just the port engine everything is fine, but the minute I start the starboard engine the port voltage drops. I have confirmed the voltage readings at the batteries and at the motor connections with a separate voltage tester. The starboard engine charges at about 14.4V. If I run both engines off of one battery the voltage on both engines read the same. I have already swapped batteries thinking I may have a bad cell but nothing changed and the batteries are only one year old. Thanks.

[fairdeal]

I tried to grasp what the problem is, and failed.

that suggests that the answer may be Rule #1 of Boat Electricity:

if the symptoms are so bizarre as to challenge either description or understanding,

the cause is a bad ground connection. Somewhere.

[99yam40]

I like that rule #1