• You can shape the tank to maximize the usage of the compartment space.
  • You can move the tank to a different location.
  • You can increase the volume of the holding tank.
  • You can get rid of the odour from a leaky tank.

How-To Menu

Unless you are sure that the new tank size will fit into the location, it is a good idea to make a full-scale cardboard mock up of the tank and put it in place.  There is nothing more frustrating than to get a new tank and not be able to fit in place because it is a bit too large. The mock up can be pieces of cardboard duct taped together at the seams. Make sure to mark the locations of the inlets, outlets, vent and possible the location of the access port.

It is important that you are able to get the full size mock up out of the boat. Sometimes you underestimate the cabin opening and the new tank cannot fit in the boat.

If you are shipping the mock up to us, please mark the adjoining pieces at the seams – “A,A” – “B,B” etc. and then take a picture of the tank.  You can now cut the tank at the seams and lay it flat and package it for shipping to us at any of our locations.


Most marinas now use a vacuum to pump out holding tanks and it is important to keep vent pipes free of obstructions. Size the vent to a minimum of .75” diameter and use a non-collapsing vent tube. Some newer holding tanks use a spring actuated vacuum relief valve to prevent collapse.


Our holding tanks are made from .25” to .5” thick Marine Grade Polyethylene. We use a blow mold grade PE similar to many plastic jerry cans.  This material is a high molecular weight P.E., which means it has a lot more stretch than a conventional high-density polyethylene's.  Even at extremely cold temperatures this makes a superior material for outdoor holding container applications.


Unlike freshwater and even grey water tanks, black water tanks are usually pumped out rather than drained out. This means the tank can be subjected to negative pressure and there is a lot of flexing of the material and depending on the strength of the vacuum, there is a lot of negative pressure in the tank, especially once the tank is empty. This is because once empty, the vacuum CFM is the highest and if the outlet is a 1.5” pipe size the vent is .75” pipe size at the largest. This means the air speed in the .75” diameter end is about 3 times the speed at the 1.5” diameter outlet. What are we talking about? The small air vent causes the tank to come under a high degree of vacuum once the tanks have been emptied.

Most of the stock molded tanks are made from medium to high density P.E. and have a wall thickness from .187” to .375”. However, some P.E. resins are not as flexible and can crack under fatigue quicker than the high molecular weight resin of the custom tanks that we fabricate.


  • It is ideal to have most of the fittings at the top of the tank above the liquid line. This prevents the potential for leaks at fitting connections.
  • The inlet for the tank should be on the top or on the side close to the top of the tank designed to accept a 1.5” NPT fitting.
  • The outlet should be on top of the tank or on the side close to the top and a 1.5” diameter suction tube will be mounted in the tank angled at the bottom to pick up most of the tank contents.
  • The vent should be a minimum of a .75 NPT fitting mounted on the top of the tank.
  • The inspection port size we recommend is a 4” access hole large enough to remove any of the purge fittings in the tank.
  • Only the fittings mounted on the top of the tank will be replaceable bulkhead style fittings.  All other fittings mounted on the sides or bottom of the tank will be thick-walled, threaded, welded tank bosses.


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