The holding tank should be sized by how you expect to use it. If you are going to spend the better part of your cruising at marinas, and you most often will be getting pumped out from your deck fitting, install the largest practical tank. If you are going to spend most of your time in waters where you will be mostly pumping straight through, a smaller tank will suffice. Small is about ten to fifteen gallons, large is 25 gallons and above. Rubber tanks will also work, but they are much harder to clean and flush and much harder to properly vent. If you figure a minimum required capacity of one half-gallon per day per person, you will be close to your size requirements.
     If you have a deck wash pump and hose, flushing through the vent is possible and practical. Just open the seacock, stick the hose in the vent pipe and fire away. The KISS MSD cuts the cost of installation considerably by eliminating many expensive parts. It makes operation virtually foolproof. No fancy instructions, no puzzling over which way the "Y" valve is turned, no more short-circuited rusty old macerator pumps. No more pumping of manual pumps. Elimination of the vented loop in the outlet, a major source of odor and malfunction, is axiomatic. The free flow of air from the vent to the tank and the open hose in the tank at a height considerably above the waterline subsume the vented loop function. Trouble-shooting guide In the event of a stoppage, except for the pipe from the MSD to the tank, the entire system is accessible from outside without any disassembly. Snakes, a water hose, or air pump can be used to back flush either from the seacock, the deck fitting or the vent pipe.
     If you expect many uninitiated users who might clog the system, I suggest creating a quick disassembly unit between the MSD and the tank. If you are using PVC pipe, I suggest a 1/4-turn valve with a standard coupling just below it, as close as possible to the MSD. Most stoppages from female sanitary products and diapers take place either in the joker valve of the MSD or at the entrance to the pipe. You need to be able to prevent the column of waste from back flushing when you disconnect the pipe from the MSD. If you are using a straight hose connection, you have a different problem and I suggest adding a valve between the MSD and the pipe. PVC 1/4-turn valves are available in all hardware stores. They have no metal parts to corrode and have no history of leakage. Certified locking device While it has been written that a wire tie is a satisfactory method of securing the seacock, I recommend a little overkill for the uninitiated, possibly nitpicking bureaucratic officer. Since the Coast Guard approves automatically all installations that meet their requirements ( CFR 159.11 Subpart B), there is need for this added precaution.
     In Key West we had an unprecedented case where a Florida Marine Patrol officer cited a boat operator who had a legal Type III device but had no certifying label. Since the USCG does not issue certification labels for type III installations, you must be prepared and cite the regulations yourself.
     A possible sign might look like this: USCG TYPE III NO DISCHARGE DEVICE Certified under Sub part B CFR 159.11 33 CFR 1 (7-1-94 Edition)
     An ordinary marine quality hasp can make an excellent locking device for the outlet seacock. Epoxy a block to the hull to receive the screws from the hinge side of the hasp. Locate it in such a way that when the seacock is closed, the hasp swings up, and the handle of the seacock in the closed position goes through the slot. Drill a hole through the handle of the seacock large enough to accommodate a small marine quality lock. Make an attractive sign to paste in the area designating "open" and "closed."
     Then relax and enjoy the simplicity and low maintenance of the KISS MSD holding tank installation.