Re: How thin can my gravity drain holding tank be??
Is this a transverse or longitudinal bulkhead?
It should be fine as long as the boat does not list so much that the drain becomes uncovered. A 1 1/2 inch line is the minimum diameter you want for a gravity drain. The higher the tank is above the waterline, the better. This will create more of a suction effect when draining it.
I would not use a Marelon valve. The last thing you need is for your discharge valve to seize up or for the handle to break off. Make the handle so that it can be locked closed in no discharge zones. The authorities may one day want to see that you have done this. The Coasties sometimes check for this when they board.
Life begins where land ends.