How thin can my gravity drain holding tank be??

[Annie in WA]

We are relocating our holding tank to a gravity drain system with a wye for also pumping out but we mainly plan to cruise internationally so pumping out will be rare. I am planning on placing the holding tank on a bulkhead 30" wide 40" high and 6" deep. Is there a problem with 6 or 5 inch deep tanks? There will only be one 1.5" 45 degree fitting between the tank and through hull

[David M]

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.