Features April 2015 Issue

A DIY Water Filter

We use the filter as a pre-filter for hose water (above) and for rainwater (below).


For those of you unfamiliar with Baja fuel filters, they are multi-layer strainers, purchased or handmade, used to filter diesel fuel of dubious pedigree before loading. Water, whether from a dockside hose or rainwater, presents a similar challenge. We’ve been exploring freshwater filtration possibilities for a series of articles to come, and while there are great pre-filtration products out there, none offer the versatility of our own home-grown solution, which we call the Baja water filter.

Cartridge filters are bulky, and while they are good for inline filtration at-the-dock systems, they also are susceptible to contamination since the greatest source of contamination is the junk growing in the hose itself. Hose-end filters are convenient, but the filtration is often coarse (10 to 40 microns), and they plug up easily, reducing flow to 2 to 3 gallons per minute after only a few fillups. With a few scraps of PVC pipe, simple tools, and common skills, a 4-inch by 14-inch, 1-micron bag filter becomes a multipurpose Baja filter serving all cruiser pre-filtration needs.

What You’ll Need

The total cost for all parts is approximately $25. Here’s what you’ll need:

1-micron polyester bag filter, seamless, trade size 4 (McMaster/Carr, 9316T211, specify 1-micron)
2 feet of 4-inch diameter PVC DWV pipe
2-inch by 3-inch reducer
2-inch by ¾-inch NPT bushing
¾-inch by ½-inch NPT nylon barb fitting
Silicone caulk
PVC cement

No. 4 polyester felt filter bag
A bottom view of the filter shows the bushings and reducers (far left). The No. 4 polyester felt filter bag (left) has worked best for us.


The 4-inch PVC pipe is small enough to support the top ring of the bag filter, yet large enough to give the bag an easy fit and good flow. To form an internal funnel and a bottom that can sit flat over a deck fill plate, slide the 2-by-3-inch concentric reducer about 5 inches inside the funnel (grind off small tabs that interfere). Seal the reducer in place with silicone caulk.

Next, glue a 2- by-3/4-inch NPT bushing into the reducer with PVC cement. The end of the bushing should be recessed about a half-inch from the end, so the funnel can sit on the deck plate without wobbling. Cut the 4-inch pipe long enough for the bag to hang free plus about 1-inch for stretch—about 15 inches inside the pipe. Insert a 3/4-inch PVC threaded nipple when used over the deck plate, or a 3/4-inch barb adapter for collecting rainwater. Smooth all edges, and add small holes near the mouth to hang from the gutter to collect rainwater.

In Use

Hose water: Drop it in the freshwater deck plate and watch the junk shed from the hose accumulate.

Rainwater: 1-micron filtration produces tank-quality water; rinse the salt and major bird bombs off your collector first.

Jerry cans: Upgrade water pulled from a grimy tap.

Clean: The bag can be laundered several times.

Sanitize: The bag can be dried and sanitized in the sun.

There are many bag filter materials and sizes available; we’ve tried them all and find polyester felt to offer the best economy and best removal efficiency of the particles of interest; it has something to do with surface charge and texture. While coarser bags promise more flow, the difference is small and the 1-micron filtration removes more silt and colloidal dust, ensuring a clean tank. There are also lighter versions using garden drain pipe instead of PVC for the main body; nipples and reducers can be adjusted for better flow rates or to tap into existing plumbing.

Comments (2)

We are about to build two Baja filters for our 47' Bill Dixon designed cutter which we live-a-board year round. We plan to use the deck for catchment and each filter will be installed in the boat directly under/after each of our deck fill ports. I'm wondering if you think there would be any benefit in installing two filters per fill port, the first being a 20-50 micron filter and the second being a 1 micron. Our thought is that by setting up the system in this way, we might see some cost savings as the 1 micron filters would last significantly longer. Do you think this assumption is correct or are we simply adding unnecessary complexity for too little gain?

Thank you for your genius,
Nathaniel Montague
S/V Poppycock

Posted by: Poppycock | December 20, 2017 10:20 AM    Report this comment

Practical Sailor developed this idea at the suggestion of a reader who keeps his boat on a mooring; he wanted to reduce hauling and use rainwater for cleaning, without it going all funky on him. After a season he reported complete success. After testing around our cruising grounds, we thought it would be a very versatile solution for the long distance cruiser facing a variety of water supply challenges.

For drinking water, we recommend some additional treatment--we do for any unsupervised source--which we will cover in upcoming issues.

Posted by: Drew Frye | March 27, 2015 9:42 AM    Report this comment

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