1999 Mercruiser 30 that wonbt start unless Jumped

"I have a 1999 Mercruiser 3.0

"I have a 1999 Mercruiser 3.0 that won't start. It cranks and cranks but doesn't start. So, I hooked on a Battery Jumper and it started.

So, I thought the battery was bad. I bought a brand new Optima (Blue Top) Battery and installed it. The engine still would not start. I hooked the battery jumper again and it started.

WHY?

The battery is 12 volts while resting. Once started and idle, the battery voltage is about 13-14 volts. With the Jumper, its up to about 17 volts.

Why does the engine start with the jumper, but not without?

Please Help.

-Eddie"

what is the voltage while cran

what is the voltage while cranking? is the engine EFI or carb.? EFI engines will not start if voltage drops below 10.6 volts i am not sure about engines with points but when mine had a bad battery I could jump it and get the engine to crank but the points would not fire. If your cranking voltage is low try an amperage test on the starter

The 2 batteries double the cur

The 2 batteries double the current for the recipient boat's electrical system; no voltage drop as Michael mentioned. You may also have internally corroded cables.