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The in-rush can easily exceed 250-500A plus. Continuous average will likely be in the 150-200A range.. Starter motors are very poorly rated and the kW rating is pretty meaningless. These motors are nearly creating a dead short in the system for a very short period of time called the in-rush. For your boat 1/0 is pretty standard for an engine of that size. Even Catalina ships 2/0 on engines of that size these days. Early on they shipped 4GA and many owners chewed through starters due to excessive voltage drop between the batteries and starter......

Here's a Mitsubishi with a 1.2kW starter motor (your Vetus is a Mitsu block). Note that this starter motor exceeded 640A and as seen below averaged 286A for the start duration (loaded starter to unloaded)


The AMPS number is averaged over the start duration:


The short answer is that unless you own the expensive equipment to test your starters in-rush and average, and that equipment is fast enough and designed to do this, go big with the wire.....;) The difference in cost between 1GA and 1/0 is not worth worrying about..
 
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