Whether you are looking to add additional water or fuel capacity to your boat or hoping to upgrade the system you have, flexible tanks can be a solid option. They are reasonably priced, relatively easy to install and they fit just about anywhere.
Flexible tanks, also known as bladder tanks, come in a variety of sizes, shapes and materials that allow you to store water, diesel fuel or sewage. A prime advantage of these tanks is that they conform to the space they’re allotted and can be located almost anywhere, from an oddly-shaped locker to the area under the cabin sole.
Another great thing about flexible tanks is that they can be removed easily for cleaning, inspection and winter storage, so long as you have proper access. 
Whatever your need, flexible tanks have flexible uses – from providing more ballast in a sailboat (ballast that can double as an emergency supply of drinking water) to replacing a rigid holding tank. It is even possible to cut the top off a leaking polyethylene tank and install a flexible tank inside it.
Flexible tanks come in rectangular and triangular shapes with capacities from 5 to 58 gallons, and prices from about $95 to more than $300. If you’re willing to pay considerably more, there are manufacturers who will build custom tanks that can hold up to 1,000 gallons.
Installing a flexible storage tank is a project that can be completed by most boaters with basic skills, and it can often be done in a single day or weekend. As usual, it begins with a little research.
TANK CONSTRUCTION
Bladder tanks are made by companies such as Nauta, Plastimo and Vetus, all of which have web sites that provide details about their tanks, as well as installation guides and tips.
Be sure to buy a tank rated for the material you intend to contain. You don’t want to use a water tank to hold diesel fuel. Likewise, make sure that the tank you want to use to hold potable water is approved by the U.S. Food and Drug Administration for that purpose.
Water tanks are constructed in several ways. One variation is a tank manufactured with three separate layers. The outer layer is a tough, abrasion-resistant polymer or rubber, designed to protect the tank from wear. The middle layer is a high-strength fabric designed to handle the weight of the liquid. The third layer is designed to keep the water pure and free of any odor or taste.
Vetus water tanks are built with this type of construction. One additional design feature is a strip of material that bridges the seam where the top and bottom layers meet, to remove stress from the joint. The company’s holding tanks are constructed similarly, except that the materials are selected for their resistance to odor permeation.
