CONTINUOUSWAVE

On a Boston Whaler 305 CONQUEST boat with twin engines, everything works well when [both battery switches involved in the distribution of power from two battery banks] are set to the BOTH position.

When the Port battery switch is set to the 1 position and the Starboard battery switch is set to the 2 position, then the Port engine almost immediately dies.

Give me some explanations of the cause.

GENERAL CASE: In any electrical circuit where a battery is the source of electrical power, if there is an electrical problem related to lack of or absence of electrical power, the first circuit element to test is the battery.

SPECIFIC CASE: Check Battery 1.

GENERAL CASE: If there is a switch mechanism involved in the distribution of electrical power, and there is an unexpected absence of electrical power in a circuit downstream of the switch, check the switch.

SPECIFIC CASE: Check the Port battery switch.