When you start the motor, voltage drops of course due to the heavy current drawn. Many pieces of equipment require a minimum voltage to operate. Go below that and the unit will shut down.
If this always happened, it will be harder to trace since the initial wiring may be under sized. If it is something new, then you need to look for a bad connection or even a bad battery.
Note that most of the electronic loads on the accessory bus may not so sensitive to low voltage and turning them on won't help identify the problem.
Tracing can be tedious but if you replaced something (like that starter) and disconnected the battery cables when doing that job, then those connections should be looked at first. Make sure that the tie cables that connect the negative poles of all the batteries are connected. If all that works out, check the fuse for corrosion at the helm that the plotter is connected to. If none of that works and you have multiple batteries, swap the battery that the accessory bus is wired to with a different battery to see if the problem still occurs. If it doesn't the original battery may be weak.
Remember that ground connections are as important as the 12v feeds.
One test that can help identify excessive voltage drop is to measure the voltage drop under load. You don't need to test while cranking. To test you need a digital volt meter. Measure the voltage on the accessory bus at the helm with all devices turned off. Measure between the ground at the helm and the positive feed to the plotter. Let's say you get 12.5 volts. Now turn on all accessories , including spreader lights, radio plotter etc. Now measure the voltage. The plotter voltage should not be less than 3% lower than the first reading with no load, so something like 12.1 volts. If the difference is a lot larger than 3%, you have excessive voltage drop somewhere. That somewhere is not hard to find, you just need to do a similar voltage drop measurement starting at the battery and working forward.
Good luck.
