No hard and fast answer
'Cuz there are too many variables--how much flush water is used by the toilet, availability of pumpout facilities and/or easy immediate access to open ocean beyond 3 miles...space available, even how many people are usually aboard. My book includes an entire chapter on the subject, but my "rule of thumb" for sailboats under 30' is 12-20 gallons--mostly because it's almost impossible to find space to put one any bigger without giving up too much storage space, and anything any smaller isn't worth the effort and expense. Where is the head in your boat--at the aft end of the cabin, or right behind the v-berth? Unless it's aft, the best location for the tank is under the v-berth. Your best source for a tank is Ronco Plastics...they've been the OEM water and waste holding tank supplier to Catalina for decades (although 1976 predates laws requiring holding tanks, so there won't be an identical replacement, but they're likely to know what fits), plus they make TOP quality tanks for a very reasonable price. You'll find their catalog on their website at http://www.ronco-plastics.com You might want to check out the link below too.
