How To Unblock A Toilet On A Boat | Page 4 Of 4 | Become A Cruiser
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How To Unblock a Toilet On a Boat

How To Unblock a Blocked Holding Tank

Disgusting Scale: 5-10
Difficulty Scale: 2-24 Beer Reward

DSCF0789 scaled e1604440031614 How To Unblock a Toilet On a Boat
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Total Time Needed :





Total Cost:



Required Tools:

– Baños Gloves (see unblocking a jammed toilet bowl for more details)
– Disposable protective coverall suit
– Cut off Broom Stick slightly shorter than the depth of your holding tank

Things Needed?

Depending on the severity of the situation:

– Washdown hose/ dock hose
– Pressure washer (caution and one specific use)
– Counselling

Before You Start


Don’t be tempted to try a chemical solution. That includes Muriatic acid. You will only make your life more difficult down the line.

DO NOT try to blow the blockage out with pressure. You will only create a pressurized poo bomb! Holding tanks are not very strong and I have witnessed more than one explode. You do not want to experience that level of misery!

Step 1

Try to establish where the blockage is:

– Pump out Pipe
– Ventilation hose
– Holding Tank Inlet
– Holding Tank Outlet/ bottom of the tank (most common) this may also block the pump out pipe

Holding Tank Blockages in the order of difficulty

Scenario 1 – Bottom of Holding Tank Blockage

Inability to dump or pump holding tank

If there is an inspection hatch on the tank. Suit up and open it. Using the broomstick try to move/ break up the blockage. Depending on how bad it is, this may require you to submerge yourself in the tank’s contents up to your armpits. Try to siphon or pump out the tank to avoid this level of nastiness.

If there is a lot of ‘terracotta’ from urine crystals you may have to scoop out the crystals before the blockage will clear. The bottom of the tank may have a few inches or feet of what looks and feel like shale or pebbles.

Try carefully putting the hose up the thru-hull (seacock open). Making certain not to force the discharge hose off of the seacock. Try to work out with water pressure. DO NOT pressurize the holding tank. Be ready to get out of the way in a hurry when the blockage clears!

Alternate with putting the hose up the discharge hose and down into the pump out pipe. You may need to repeat working the broomstick. You can try to put a pressure washer wand or sewer jet into the holding tank through the open inspection hatch to try to blast the blockage free.

Once you clear the blockage (you are disgustingly filthy now anyway) go ahead and clean as much scale off of the sides of the tank as you can.

Scenario 2 – Holding Tank Inlet Blockage

– See ‘Blocked Sanitary Hose’ above

Scenario 3 – Holding Vent Hose Blockage

– See ‘Blocked Sanitary Hose’ above

A Blocked holding tank vent hose is generally a result of overfilling the tank to the point of overflowing. There is normally some form of mesh screen in the hose to keep insects out. And as the holding tank overflows pieces of solids block the mesh screen.

Scenario 3 – Holding Tank Pump Out Blockage

– See ‘Blocked Sanitary Hose’ above
If you are able to open the holding tank inspection tank you may be able to save yourself the hassle of removing the hose by pushing a running garden hose/ deck wash hose into the pipe.
Be very careful, if you somehow pressurize the line any contents in the pipe will come back out like a geyser.

Scenario 5 – Holding Tank Discharge Blockage

If you are able to open the holding tank inspection hatch (to prevent over-pressurization) you may be able to save yourself the hassle of removing the hose by pushing a section of stiffish garden hose up the discharge pipe via the thru-hull. Or even better a running deck wash hose.

Again make certain you do not force the discharge hose off of the seacock as this will result in flooding the boat. If this does not work it is most likely that the bottom of your tank is blocked with waste. See Scenario 1, Blocked Bottom of Holding Tank.

If you are 100% certain the blockage is in the discharge pipe. Empty the holding tank before beginning to removing the pipe. Then follow the steps above for clearing a blocked hose.

Please click on this link if you are looking for information on How to Unblock a Drain on a Boat.

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Hi, I’m Nic! Our Family of four have been out cruising since 2016. We have sailed about 15,000nm, almost halfway around the world. We sold everything, took the leap of faith, and bought a 10-year-old Lagoon 380 ex-charter catamaran. We’ve fixed every system on the boat, often more than once. Cruising has been such a wonderful, positive experience for our family that I want to share my tips to help you Become a Cruiser.

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