Gravity holding tank does not drain - SailNet Community
LinkBack Thread Tools
post #1 of 15 Old 08-16-2011 Thread Starter
Junior Member
Join Date: Aug 2011
Posts: 2
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
Gravity holding tank does not drain

I'm new to this but would certainly appreciate any thoughts and/or advice. I purchased a 2001 Beneteau 331 last year, and the Jabsco manual head was plumbed so that a Y valve allowed you to either pump directly overboard, or into the holding tank. The holding tank could only be emptied by pumping it out. I believe this was/is the standard arrangement in the US.
Since I live and cruise in Canada, where pump-outs are less common and discharge is allowed in open water, I replumbed the head so it pumped into the holding tank, and put the Y valve on the holding tank outlet, so the tank could either be pumped out, or drained overboard. I also installed a holding tank gauge, along with new hoses and clamps, etc. Outlet hose is standard 1 ˝ inch.
The system worked great in tests - put water in the holding tank, put the Y valve in the drain overboard position, open the through-hull, and whoosh - the tank drained completely. The bottom of the holding tank is a couple of feet above the waterline. However, while on our first longer trip I noticed that the tank was not draining when both valves were open (glad I installed the gauge). This led to some rather unpleasant work with a long stick to unclog the drain. The same thing happened a couple of days later, and this time I could not get it to drain at all. I was able to empty it at a pump-out station when we returned home.
The holding tank vent is clear and the tank discharge hose is relatively short, although it does have a right-angle bend as it leaves the tank. I believe that solids are collecting at the bottom of the tank, and gravity isn't enough to push everything through the outlet hose. It seems as if there are two possible solutions:

1) Install a manual or electric pump to empty the holding tank
2) Install a macerating head

The first option involves providng additional suction to empty the tank, instead of just gravity. However, the best location for the pump would be at the bottom of the tank, and there's virtually no room there at all. To allow the tank to be emptied at a pump out station, the pump would also have to be downstream of the Y valve, which would again put it further from the tank and where any blockage would likely be.

The other option would involve installing an electric macerating head, which would hopefully eliminate the problem by grinding everything up before it goes into the holding tank, allowing it to drain by gravity as intended. It would also simplify 'head handling' for guests etc. I understand Jabsco makes electric heads which have the same dimensions, connections etc. as their manual heads.

I'm leaning toward the latter idea, but welcome your thoughts and advice. I guess the main question on my mind is if the tank will drain without problems if it contains only macerated waste. Can anyone vouch for this arrangement? Thanks for your posts! (I long to go on another trip without carrying a ‘stir stick’…)
malaspina is offline  
Quote Quick Reply Share with Facebook
Sponsored Links
post #2 of 15 Old 08-16-2011
Senior Member
CapnBones's Avatar
Join Date: Sep 2010
Posts: 167
Thanks: 1
Thanked 2 Times in 2 Posts
Rep Power: 7
My thoughts are to just add a macerator pump, between the y and the seacock. I would think that the pressure pump would, or should be able to clear any solids from the tank. It would be a bit cheaper than a macerating head and probably easy enough to install, though I know you did say space was limited.
CapnBones is offline  
Quote Quick Reply Share with Facebook
post #3 of 15 Old 08-16-2011
Senior Member
PaulfromNWOnt's Avatar
Join Date: Aug 2010
Location: Fort Frances, ON, CA
Posts: 342
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 7
I'm not certain that these things will help, but here's my thoughts:

In my camper I use a half cup of laundry detergent, and a half cup of water softener (calgon) in my black tank. This combination contains no formaldehyde, and will not damage any of the components in the system. In combination with traveling down the road, it keeps everything broken up and in suspension, and as a side bonus it lubricates the valves as long as they are kept wet.

For the above reason, I would suggest emptying after you've been sloshing it around a bit.

When the tank gets to the halfway point, let it slosh, then top it with water and empty it immediately.

Don't empty the tank when there is very little in it, or when it has been stationary for a period of time. Flow is your friend when carrying out waste.

One other thing that's kind of related -- People talk about the bacteria in black and grey tanks, but these are supposed to be holding tanks only and don't require bacteria for operation.

I hope some of this helped.
PaulfromNWOnt is offline  
Quote Quick Reply Share with Facebook
post #4 of 15 Old 08-16-2011
Senior Member
Tim R.'s Avatar
Join Date: Mar 2003
Location: Portland, Maine
Posts: 1,551
Thanks: 1
Thanked 28 Times in 26 Posts
Rep Power: 14
My Caliber has a gravity emptying holding tank and it goes straight to the seacock at which point it turns 90° and through the seacock. My head is an electric jabsco macerating head and I have only ever had one clog when dumping overboard. Eventually the waves off shore freed the clog.

I believe that chopping into the tank will be more effective and less prone to problems. Trying to pull clogged material(paper and solids) though the hose to the macerator may be difficult and even make the clog worse.

Tim R.
Our Carina is for sale
1997 Caliber 40LRC

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Tim R. is offline  
Quote Quick Reply Share with Facebook
post #5 of 15 Old 08-16-2011
Sailor Dance
Join Date: Oct 2001
Posts: 13
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
We have a t-connector on the holding tank on our Caliber 33. One side of the T goes to the deck pumpout fitting, the other side of the T is plumbed to manual Whale hand pump then to another T thats plumbed to the overboard thru-hull.

We can flush toilet to holding tank or directly overboard with Y-valve. We can get pumped out at a pumpout station or pumpout manually with Whale hand pump overboard when offshore.

S/V Sailor Dance
jlmarra is offline  
Quote Quick Reply Share with Facebook
post #6 of 15 Old 08-16-2011
Senior Member
jackdale's Avatar
Join Date: Dec 2008
Location: Calgary, AB, Canada
Posts: 9,186
Thanks: 30
Thanked 73 Times in 68 Posts
Rep Power: 9
Scale can accumulate at the bottom if the holding tank and block the discharge hole.

I had to run a snake up the hose after removing it from the discharge thru hull and break the dam. We flushed the tank several times with fresh water while at a pump out before we used the snake.

ISPA Yachtmaster Ocean Instructor Evaluator
Sail Canada Advanced Cruising Instructor
IYT Yachtmaster Coastal Instructor
ASA 201, 203, 204, 205, 206, 214
As I sail, I praise God, and care not. (Luke Foxe)
jackdale is offline  
Quote Quick Reply Share with Facebook
post #7 of 15 Old 08-16-2011
Senior Member
mitiempo's Avatar
Join Date: Sep 2008
Location: Victoria B.C. Canada
Posts: 7,761
Thanks: 0
Thanked 109 Times in 100 Posts
Rep Power: 10
The ideal way to install a gravity drain holding tank is with the deck pumpout fitting directly above the seacock so you can clear a clog easily with a rod from on deck.
chef2sail and jackdale like this.

Living aboard in Victoria Harbour
mitiempo is offline  
Quote Quick Reply Share with Facebook
post #8 of 15 Old 08-16-2011
Senior Member
fallard's Avatar
Join Date: Nov 2009
Location: Mystic
Posts: 1,392
Thanks: 3
Thanked 33 Times in 32 Posts
Rep Power: 8
I added a macerator pump to drain my holding tank beyond the 3 mile limit. I added a Y-valve in the overboard discharge hose from the head. In one position the Y-valve allows the head to directly discharge overboard. In the other position, the macerator output goes directly into the overboard discharge hose.

BTW, you need to be careful not to run a typical (e.g., Jabsco) macerator pump dry.
fallard is online now  
Quote Quick Reply Share with Facebook
post #9 of 15 Old 08-17-2011
Senior Member
Join Date: Feb 2010
Location: Narragansett Bay
Posts: 13,866
Thanks: 64
Thanked 335 Times in 315 Posts
Rep Power: 8
Originally Posted by PaulfromNWOnt View Post
........these are supposed to be holding tanks only and don't require bacteria for operation.
If you empty it and rinse it out almost daily, that may be true. However, we would have to motor over to the fuel dock for a pump out before we left each Monday, and that just isn't practical. It may not even be open when we leave. With three to four days away baking at 85 degs in the cabin, the aerobic bacteria in the holding tank better be healthy. By strictly avoiding any chemicals or soaps in the tank and using sufficient flush water, we have no problems.

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Jeanneau 54DS

In the harsh marine environment, something is always in need of repair. Margaritas fix everything.
Minnewaska is offline  
Quote Quick Reply Share with Facebook
post #10 of 15 Old 08-17-2011
Senior Member
JimsCAL's Avatar
Join Date: May 2007
Location: Glen Cove, NY
Posts: 3,563
Thanks: 3
Thanked 129 Times in 121 Posts
Rep Power: 10
My previous and current boats have a manual diaphragm pump in the holding tank discharge line for pumping it overboard when permitted. Works great.
JimsCAL is online now  
Quote Quick Reply Share with Facebook

beneteau 331 , holding tank , macerating head

Quick Reply

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page

Similar Threads
Thread Thread Starter Forum Replies Last Post
Convert existing steel freshwater tank to blackwater holding tank? patrickstickler General Discussion (sailing related) 7 09-24-2010 08:55 PM
need ideas for gravity feed diesel tank piav Gear & Maintenance 11 11-13-2009 01:40 AM
Dye in holding tank davidpm Gear & Maintenance 5 09-27-2009 01:05 PM
Holding Tank cvsheppard Pacific Seacraft 1 06-01-2009 11:27 PM
Gravity Running Tank for Engine capecodphyllis Gear & Maintenance 11 01-18-2007 11:29 AM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome