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Kristin, upon reviewing your photos again, it is very hard to see how deep the rigging tunnel is...or whether it has a sump to catch and allow water to drain via a thru hull drain plug or installed bilge pump?? If the designed drainage path is actually into the port side well, then a new drain hole could be drilled on that side even with the existing tank top.

Or GChuba's suggestion about removing the tank, inspecting/renewing it, and raising it to the proper level would restore the limited drainage originally designed in.

Al