If you’re a member of INA, you can look into holding tank replacement under menu item “store”

Background Notes:

As a service to its members, INA has developed sources to manufacture custom-made holding and water tanks for our boats. These replacement tanks are made of rotationally molded, seamless polyethylene with a 3/8" wall thickness. Rotationally molded seamless polyethylene is the product of choice for holding and water tanks. Color is translucent white. The molds for these tanks are custom built to Nonsuch specifications and manufactured by Companies that supply many of the top boat builders. The tanks come with threaded female outlets, ready for you to install the marine grade fittings of your choice. The tanks are warranted for one year against defects in materials, workmanship, and cracked fittings.

The tank designs were created and tested with the combined input of INA members and drawn by member Ed Charest. The designs provide several options for venting and discharge. For example, two 1" vents are provided in most holding tanks to allow as much air as possible to vent in and out of the tank. This will stimulate the growth of aerobic bacteria, which in turn will eat the anaerobic bacteria, the ones that produce the unpleasant odors. If you choose not to use the available secondary ports, you can close them off with threaded plugs or order your tank without them. Most tanks include 4" inspection ports for easier access.

Drawings and additional information for the tanks are linked below. Since each Nonsuch is somewhat unique, please be sure to check the drawing against your particular boat to make sure the tank will meet your requirements. For members with a NS 26U, the replacement-holding tank has been designed to fit through the lazarette and into the hold. The factory installed aluminum tank will not do that. Also please note that when ordering the 40-gallon water tanks for the NS 30C, NS 33 and NS 36, you must specify port or starboard sides.

NOTE: the drawing files are in PDF format and depending on the internet browser being used you will have a direct download option or use File > Export as PDF