Originally Posted by Squeaks
I have been giving this specific conversion serious consideration for a while - any negatives?
Every sewage system has its negatives. There's nothing positive in my mind about having sewage on a boat. However, I've been far happier with this than any other system I've had. Another boat I've had sucked the sewage out from the top. The big problem with that was that there was always about 4 inches of sewage left on the bottom and that it would fail if the suction was anything less than perfect. I prefer to work with gravity than fight it.
Any hose can clog, any through-hull can freeze and drain fitting can leak. However, I feel a short straight gravity system offers less chance of this than other systems with fewer parts
to fail. Replacing the short section of hose (after pumping lots of non-sewage water through) would be easy and cheap
I personally don't see how sewage buildup would occur any faster with this system than one that pumps the tank out instead. Actually, I think the movement of seawater and ease of pumping lots of fresh in helps keep things cleaned out. (Since it just flows out it's little effort to toss all sorts of water in. I tell guests to pump a whole lot of raw water through the system) I also see putting the drain at the bottom of the tank as a plus as it will drain the tank more completely. No matter where you put the discharge fitting, you will have sewage to that level. I'd prefer that level be at the absolute bottom. While unpleasant, any failure here is far less dangerous than a through -hull failure and I think unlikely if the fitting is properly installed.
One can get sewage build up at the bottom of the tank but this is true regardless of whether the discharge hose is connected to a pump or gravity feed. However with gravity feed, there is no sewage pump to fail or clog. Ideally, I'd like to install an inspection
port on the top of the tank directly above discharge, so the discharge could be snaked out from above (As well a pressurized water to clean) Mine has never blocked, but if it did, it's a short straight cleaning
job from below with anything straight that fits. Maybe not the most pleasant, but I prefer that to taking a clogged hose off a full tank inside the boat...
I know all the Moorings boats use larger than typical discharge hose and through hulls, but several forum members have indicated they use standard size with this same set up. Consider that the downside of an anti-siphon discharge exits mostly by gravity as well. The pump pumps sewage to the top of the loop, but due to the anti-siphon, it's mostly gravity after that. There is no (or at least little) pressure due to the anti-siphon hole. The system I described uses a shorter, straighter run which is at least partially cleaned by sea water.
To address a previous post: The system on this boat is used almost exlusively where it can be emptied at sea often. However, one doesn't need to add y-valve or install a second outtake for pump out. A simple t-fitting above the through hull
will do the trick and preserve the straight inline discharge to the sea while using the same holding tank discharge fitting thus keeping it free. Since it's gravity release not pressure, having a hose snake far up to a deck
fitting won't compromise the system at all. Simply close the through hull for dockside pump out and you are emptying the tank to the very bottom. Alternatively you could use a second discharge fitting at the bottom of the tank for the pump out so you have a complete backup system for emptying, but the trade-off would be the issue Captain
For context, other systems I've had on cruising boats have included: RV type head (The other boat I own now) with integrated holding tank that can be pumped out, composting head, holding tanks
that get sucked out from above with hand pump and holding tanks
that get pumped out via lower fitting. For me the gravity feed system has proved to be the simplest and most odor free of all of them.
I think the big challenge with a pre-exisitng head is weather
or not it's feasible to place the holding tank just above the water line so that it flows easily and hopefully straight down to a through hull. Often existing through hulls, cabinetry, liners, etc make this a logistical challenge. On many of the Moorings boats, the tank is just behind the bulkhead and the through-hull is conveniently located under the bathroom sink. (Meaning it's not quite a straight discharge, but close.) Some have the through-hull in the cockpit
lazarette which is a really an inaccessible location.
I'll add: Every boat, cruising location and use is different. While I really like this system in general, I'm not arguing it's the best choice for every boat or boat owner.