Gravity Holding Tank Install... need advice

[Steven UK]

I am planning to update the sanitation / holding tank setup on a (pending survey) new to me boat that has been mostly used for racing. As such the current setup is just for decoration it is totally useless as a holding tank, in fact the ‘holding tank’ is a five gallon diesel jerry can with a length of vacuum cleaner hose clamped over the spout and no vent. So totally unusable… I have been looking at gravity systems online (and I will buy Peggy’s Book) and they all show a vertically oriented tank behind the head. The only place I can put a tank is high as possible under the ‘V’ birth just forward of the head. But as long as this is above the water line when not healed and slightly below when healed (starboard tack) then I foresee no issues… I have seen vents in the top of vertical tanks acting as vacuum breaks but my hoses will be almost horizontal. Comments highly appreciated.

something like this... https://www.sailmagazine.com/diy/boa...ng-tank-set-up

Also i would be using a very small tank in the 5 gallon range.

Best Regards Steven

[coopec43]

Steven

I have heard of "Peggy's book" but have not read it. I don't understand why you need a "gravity tank". Isn't there a space in the bilges for a 20-30 gallon tank? Mine will fit beneath the floors.

My holding tank is in the foreground.

[Steven UK]

i just want to make it as uncomplicated as possible, i like the elegance of the gravity system. minimal components, minimal chance of anything going wrong, minimal cost, seems like a win-win.

[J.Kitchens]

You should put it over the V berth and give it a glass bottom. It could be a new style of "aquarium". I bet there is something worth watching going on in there...if you have a strong stomach.

[Steven UK]

Funny but I’m eating my dinner...! I find this ‘stuff’ deeply disturbing... I definitely don’t want it above me when I sleep.

[atoll]

you really want to have your deck fitting (for suction out) directly above your outlet filling leading to the seacock, with as little bend in the outlet pipe.
this is so, when the tank gets blocked you can push a bit of rigging wire down it to unblock it.
also you want to have 2 inch suction and outlet fittings on the tank leading to a 2inch seacock,smaller gets blocked. the other fittings are standard inch and a half and 3/4 for the vent pipe

[denverd0n]

The section of the discharge hose that is between the thru-hull fitting and the tee will get filled with waste that will just sit there. That leads to a couple of implications...


First, if you do not use the direct overboard discharge for some period of time, it is likely that a plug of solid, unyielding waste will build up in there. That means the next time you open the thru-hull it is very likely that nothing will happen, and the tank will not drain. You will have to find a way to clear that clog. Probably as atoll mentioned, some way of fishing something down there to push through the clog.


For me, this would mean that I would not want to try this sort of an arrangement unless the great majority of my cruising was in an area where discharge regulations are pretty lax. In an area where I would have to keep the thru-hull closed most of the time, I don't think this is a good way to go.



Second, because waste may be sitting in that section of hose for quite some time, you want to be sure that it is the absolute best possible waste hose that you can get. This particular section, at the very least, will be a place that you do NOT want to try to get by with bargain-priced hose! And if you start to notice odors, that section of hose would be the first place to check.


Good luck.

[atoll]

Originally Posted by coopec43

Steven

I have heard of "Peggy's book" but have not read it. I don't understand why you need a "gravity tank". Isn't there a space in the bilges for a 20-30 gallon tank? Mine will fit beneath the floors.

My holding tank is in the foreground.

a gravity tank needs no pump when emptying into the sea and is less likely to get blocked as the toilet flushes through it when not being used as a holding tank,closing the seacock,turns the tank back into a holding tank.
no nasty 3way valves,macerator pumps or extra seacocks needed.

[gonesail]

as long as the tank is holding something .. chances are good that gravity will eventually fail to clear it. a hand sanitation pump seems a good candidate. for the more elegant solution one that runs on 12v.

[Training Wheels]

We’ve had 3 boats with gravity drain tanks, each with 1 1/2” thru hulls, and have never had a problem.

[atoll]

Originally Posted by Training Wheels

We’ve had 3 boats with gravity drain tanks, each with 1 1/2” thru hulls, and have never had a problem.

barnacles can be a problem in some areas,a larger diam is less likely to get blocked.
though i suspect the larger diam on my tank is to accept the sleeved down 11/2' suction pipe insert fitting on the top of the tank (that goes to the bottom of the tank)which is in line with the 2'outlet fitting at the bottom of the tank
.so no T fitting needed to suck out the contents from the discharge side.

[Rough Magic]

Lots of newer boat manufacturers use this system. I believe that Catalina does on almost all models these days. I would put a valve on the bottom of the tank, as well as the thru-hull. Nothing standing in the drain pipe. I would also arrange the vent in the top to align with the drain hose so you could clear any plug from on top. Good luck

[NorthCoastJoe]

If you want simple. porta potty is gravity, and they come with 5 gallon capacity holding tanks. No plumbing, ours has been perfect for the little we use it, never plugged up and simple to use. I got rid of all the stink of hoses and tank, and have more storage space.

If your only going to have a 5 gallon tank I don't see why it's worth it at all messing with hose and fittings.

[Hobie_ind]

Originally Posted by atoll

you really want to have your deck fitting (for suction out) directly above your outlet filling leading to the seacock, with as little bend in the outlet pipe.
this is so, when the tank gets blocked you can push a bit of rigging wire down it to unblock it.
also you want to have 2 inch suction and outlet fittings on the tank leading to a 2inch seacock,smaller gets blocked. the other fittings are standard inch and a half and 3/4 for the vent pipe

This is exactly how the holding tanks on our Amel were set up (with the exception of 1-1/2" outlet and thru-hull). Great system and the one time I had the outlet plug I was able to snake it from the top easily.

The one additional thing that Amel did was that the bottom of the tank had a 'funnel' shaped area on the bottom that the hose to the thru-hull connected to. I think this served two purposes in that it minimized the amount of stagnant material on the bottom of the tank and if you had to un-block the funnel area automatically directs a rod or snake into the outlet pipe.

[CapnBazza]

Originally Posted by NorthCoastJoe

If you want simple. porta potty is gravity, and they come with 5 gallon capacity holding tanks. No plumbing, ours has been perfect for the little we use it, never plugged up and simple to use. I got rid of all the stink of hoses and tank, and have more storage space.

If your only going to have a 5 gallon tank I don't see why it's worth it at all messing with hose and fittings.

It’s like this Joe, once properly established an electric dunny with a small gravity emptying holding tank is far simpler than carting the Porta Potti through the boat to the cockpit so that you can slosh the contents overboard during breakfast because the Admiral has a sudden need. If you had one you’d see it.