The Front Line in Filtration

Materials matter. So does design. Plastic strainers are inexpensive, but they, or their bowls, are vulnerable to chemicals in some anti-freeze concoctions (see PS December 2017 ”Winterizing Wisdom from the Chemistry Lab”).

Some designs are clearly easier to inspect and clean. Given the propensity for weeds and algae to get sucked into our boat’s cooling system, ease of maintenance is high on our list of desirable features.

1. Testers found the square openings on the Vetus filter are more difficult to clean than other kinds of perforations.
2. This close up shows a perforated strainer basket typical of popular intake filters.
3. This inexpensive Sureflow filter has a wire basket. It’s nylon bowl is vulnerable to some antifreeze.
4. Groco’s poly propylene filter features square holes like the Vetus.
5. The Perko silicon bronze housing with glass bowl and perforated metal strainer has been around for decades.
6. The water-activated activated bilge switches we tested in September 2005 had small strainers to protect against debris.
7. Two levels of grating protect the automatic float-switches from clogging. Although these are not commonly vulnerable to clogging, a plastic bag or large debris could interfere with it. A DIY strum box would add another layer of protection.

The nemesis of inline strainers and pump inlet screens is limited area. High flow and the resulting pressure drop forces the debris deep into the holes, causing stapling and making cleaning difficult. Limited area reduces flow and increases cleaning frequency.

What if a strainer is built around the sump pump, allowing much greater area, so large that the pressure drop is reduced to no more than gravity? Instead of driving the trash into the mesh, debris often just falls off between cycles. If not, it is easily brushed off, because it is not driven into the holes.

 The ultimate in sump pump protection. The box frame can be wood, metal angle, or fiberglass. The sides are ¼-inch perforated metal. The top can be left open. About 8 inches deep and 1-2 feet on a side.

Minimum size is about 5 square inches per gallons per minute (GPM) flow, or about 150 times the pipe flow area, although twice this is better. This is more than 5-10 times the minimum recommend pipe strainer area.

Sure, keeping the bilge clean is the best practice, but even well maintained boats accumulate some hair and splinters, and in the event of serious flooding or a knock-down, all matter of paper scraps can wash down from nooks and crannies you didn’t know you had.

1. A variation on the strum box, this DIY strainer is simple to build, clog-proof (it will overflow down the drain when it clogs) and can provide a lot of surface area.
2. Do NOT use expanded metal as shown here. Use perforated metal for the sides.
3. This hardware cloth strum box keeps sticks out of tech editor Drew Frye’s backyard sump pump.

Cockpit drains that don’t drain can cause trailers to tilt, invite frogs to breed, and even sink boats if the vessel is small. Bits of cloth, leaves, and bird droppings can congeal into a concrete-like mix that can literally seal a small-diameter cockpit drain.

A variation on the strum box, this DIY strainer is simple to build, clog-proof (it will overflow down the drain when it clogs) and can be cleaned of the most stubborn hair and fibers with a single wipe of the hand. The mounting stub should have a loose but not overly sloppy fit in the drain, typically 1 to 1.25 PVC. The riser is a 1.5-inch PVC drilled with closely spaced, staggered 3/16-inch holes. The top is open. After drilling the holes, sand or scrape to get rid of all of the burrs, but do NOT chamfer the holes; that will actually make it harder to clean. A drill press helps, since there are a lot of holes

For winter boat storage, remove the grill over the cockpit drains and just drop these in. Neither leaves nor ice can block it, since it is tall enough to reach above the accumulation. The riser can be as tall as you like; 8 inches works for tubs and most cockpits, but taller could make sense in snow country.

The lazy variation: Roll up some hardware cloth (coarse wire mesh) into a tube and stick it into the cockpit drains with at least six inches sticking out. Not as durable, not as easy to live with, but suitable for winter storage.

This sounds dead simple, but having cleaned thousands of strainers, I’ve learn a few shortcuts. The following assumes we are cleaning a basket strainer with the debris on the inside. If the debris is on the outside reverse the process.

1. Scrape the outside with the back of a knife to shear stapled fibers. Alternatively, scrubbing with a wire brush may be easier, although the shearing action is not as strong.
2. Wipe the inside of the basket with a small brush in a smooth rotary manner, not back and forth. You want to pull the fibers out, not scrub them in.
3. Rinse from the outside with a strong stream. This should help push fibers through to the inside.
4. Repeat as needed.

Additional notes

A few small details can make a big difference when selecting the right strainer for the job

• Install valves near both sides of any inline strainer. If you can’t easily clean it, you won’t.
• Threaded covers are notoriously prone to jamming when over tightened. Clean the threads carefully , use silicone grease, and tighten only the bare minimum. Replace O-rings sooner than needed to avoid the need to over tighten. Get the right wrench, and only use it for removal (hand tighten only). Remove at least twice each year. 
• Plan ahead. How will you clean up the inevitable mess? There should be space for a pan.