Sunday, August 30, 2009

Finishing Projects - Holding Tank & Macerator

Here's the thing - as humans we're mostly interested in what goes into us and less so the other end of the process. This is certainly the case with me, so when I started the holding tank project a couple of years ago I stopped when the holding tank was installed and functional. But the design of the project was far more ambitious - it had to perform several functions and allow flexibility.

Here are the specifications:

  • The head discharge has to be able to be switched overboard or to the holding tank.
  • The holding tank needs to be well vented
  • The holding tank has to be able to be emptied through a pumpout, through a macerator pump and finally, as a last resort, a manual pump.
  • The pumps (manual and macerator) have to be able to take a suction from either the holding tank or the bilge satisfying the need for a manual bilge pump operable entirely from inside the closed up boat (SOLAS requirement).
So, with that said, the first thing I needed to do was put in a holding tank. Pelican didn't come with a holding tank - just an Electrasan. The Electrasan isn't very useful because it isn't legal in the U.S. and off shore you don't need it. In addition, it is a power hog using about 6 amps per flush. It really draws much more, but only operates for three minutes. True, you don't have to use it by just pumping through it added a lot more hose and wiring and took up valuable storage space.

Many people think that if you have a boat that's pre-MSD laws you don't have to upgrade to a legal system ever. That could not be more untrue. If you have a pre-MSD law boat without a holding tank and you change even one hose, you are required to upgrade to a legal system. So, adding an Electrasan constitutes an upgrade triggering the need for a fully legal system.

I purchased the new 30 gallon HDPE tank online from Plastic-mart.com - they had a standard one that fit in the lower bilge area. Tall and narrow and I asked them to spin-weld all the fittings I needed to one end - those being the inlet (1-1/2"), the suction (1-1/2" with a tube to the bottom), and two vents (1"). There is an excellent discussion by Peggie Hall on head and holding tank installation here: http://boatbuilding.com/article.php/MarineSanitationFactvsFolklore. Using her ideas and discussions and my nuclear reactor operations background I came up with system that would meet my needs.

Installing a holding tank is actually really simple. I added an 8" inspection port to the tank so that I could clean it if it really becomes necessary. I'm hoping never to do that, but hey, you never know. As you can see at the left, I've cut out the hole for the inspection plate. You can also see the tube that goes to the bottom for pump outs. When you order the tank, make sure they know its final orientation and that you need a bottom suction. I used the inspection plate as a pattern for the hole I cut.

You'll notice there are two 1" vent fittings on opposite corners. The more ventilation a holding tank has, the better. It is the anerobic bacteria that smell badly - the aerobic ones are the ones that actually break down sewage. That's why a sewage treatment plant aerates the sewage as part of the clean-up process. More often, holding tanks have insufficient ventilation in the form of one 5/8" vent.

The bilge behind the keel of Pelican is 5 1/2 feet deep from the cabin sole, about three feet below the top of the keel. I don't really need that much bilge space so I figured it was a great place for the tank. It's hard to show that here, but you can see there's a lot of volume down there. These two pictures show the bilge before the tank goes in and after it's mounted.

As an aside, the inspection plate from Seabuilt, Inc.
has been discussed before in another post. It is a sturdy, easy to install, and leak-proof system for installing inspection ports in almost anything -from fuel to water to holding tanks. They aren't for highly corrosive materials, but there are few of those aboard Pelican. The design is so elegant that I used it for putting an access plate in the base of the mizzen mast for wiring. It works like a champ.

Next, I plumbed in the tank as normal - vent hoses go to fittings on either side of the hull in as short a run as possible. Also, the hoses have no loops or sags. This is very important so that a water seal doesn't occur in them. The fittings are as high on the hull as possible below the bulwarks. There's nothing special about them except they're stainless steel. The deck pump-out fitting is installed on the starboard side because there's a closet I could run the hose through easily and many of the overboards are already there. You can see the 'Y' valve on top of the tank.

Once you get used to drilling or cutting holes in the hull, doing it in the deck is easy. I ordered a stainless steel waste fitting for the deck from Marine Parts Depot who I've used for all sorts of stainless fittings. The waste fitting doesn't need a key to open it. There's a lever built in so that you can leave the boat and the pump out person can do the job without searching for a key. When you cut a hole in the deck, though, where it's cored, make sure you seal the core with epoxy before installing the fitting.

I've installed a macerator pump and manual bilge pump in the same closet as the vent and pump out fitting. This looks really complicated but basically it allows you to use one pump at a time to either pump the bilge or pump the holding tank. For those of you thinking ahead, there is a check valve to prevent pumping the holding tank to the bilge. After seeing this all together I think I should have done it all with PVC pipe and PVC valves - it would have been a little less expensive, but far less leak prone. Someday, if it all goes wrong, that's the way I'll rebuild it. But for now, it doesn't leak, it doesn't smell, and it seems to work as planned. And what could be better than working as planned? I submit nothing!

Next up, water maker installation and shaft powered alternator.

I'm leaving to go south soon! I hope to meet you on the water!

2 comments:

plastic-mart.com said...

Captain Bob, thank you for the kind mention, we hope your tank fulfilled your needs. Happy sailing, Plastic-mart.com

Bob and Cathy said...

Hello Bob...
Glad to see you are on your way to tropical climates. And we look very forward to seeing you and catching up!
I love that Spot idea. Wow that is really neat!