thru hull in side, how thick?

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changed baitwell system around recently for better ride. got a portable 30 gallon tank now riding in front of the console. i want to drill a thru hull for drainage through the side of the hull. boat is a 1985 18 ft cc. i am wondering how thick the sides are. hole will be above waterline towards the front of the console. i am not a seacraft purist. i bought the boat for the ride and have been making modifications ever since. just want to "call before i dig".