CaptainDana - the West advisor on head
systems is excellent. However, I think it's important to note two simple modifications, if possible, that will improve the design. We used this exact design on two of our boats. But I found a few problems with the design and was able to make some changes.
1. The vent size and location is always an issue. I'd go to a slightly larger sized vent (IF POSSIBLE) and have it ported from the top of port and move steadily upward. I've had our side ported tank clog because the waste is capable of getting into the line and clogging it.
2. Again whenever possible (overhead space availability) have the input/output tank fittings top mounted. Not only does it eliminate the gravity factor of the output line waste, but it eliminates most any chance of liming. Liming was a major reason for our change in porting. Our first boat, a Shannon 28, simply didn't have the overhead capacity to do that.
3. If I had the option I'd use a two valve system instead of a y-valve. And all valves would be located above the tank.
Again, these are only valid options if the space is available.