You are experiencing voltage loss, likely due to some corrosion of the wiring over the years. You are connected to the boat’s harness, and, as suggested above, it is best to bypass the boat’s wiring and have your electronics on their own separate circuit. You can do it as above using a block for both batteries, or, simply wire each graph directly to the battery, fusing each positive wire right at the battery. Block is a better idea for two graphs. BCB offers the harness with block in their online store.
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