Shurflo 181-201 Fresh Water Accumulator Tank, 21 oz

The Shurflo by Pentair 181-201 is a fresh water accumulator tank, and it does one job really well. It smooths out the pressure in your boat or RV plumbing so the pump stops hammering on and off every time someone cracks a faucet. If you've ever heard your water pump chatter through a quick hand rinse, or felt the water surge and sputter at the galley tap, this is the part that fixes it.

Here's how it works. The tank holds 21 ounces of water behind a butyl bladder, with a pre-charged air cushion on the other side. When the pump builds pressure, it packs a little extra water into the tank against that air charge. Then, when you open a tap for a short draw, that stored water feeds the line first. The pump doesn't even wake up for small demands. For longer draws it still runs, but it cycles far less, and that's easier on the motor, the check valve, and your ears.

We see these go into two kinds of systems all the time. On boats, it mounts down near the fresh water pump, often behind a headliner or tucked into a locker on a 28 to 32 foot sailboat. Give yourself about 3 inches of clearance at the port end so you can get a wrench on the fittings later. On RVs and campers it lives in the water bay or under the galley sink. Either way the footprint is compact, 8-3/4 inches long and 3-3/4 inches around, so it fits where a bigger tank won't.

Plumbing it in is straightforward. Both ends are 1/2 inch-14NPSM male ports, so it drops inline on standard 1/2 inch fittings, pump side in, fixture side out. Use PTFE tape on the threads and snug them by hand plus a light turn with a wrench. Don't crank on them. The body is nylon, and if you overtighten a fitting into a nylon port you can crack it. On a boat, a little dielectric grease on stainless fittings keeps the threads from seizing down the road.

Pre-charge matters. Set the air side a couple psi below your pump's cut-in pressure, which for most 12 volt diaphragm pumps lands around 20 psi. That's the sweet spot where the tank absorbs the small draws before the pump ever starts. Pair it with a fixed-speed diaphragm pump like the Shurflo 2088 and you'll really notice the difference in short-cycling. Running a variable-speed pump that already ramps smoothly? You'll see less benefit, since that pump is doing some of the same work electronically.

The bladder is butyl and the body is rated to 125 PSI, well above any normal fresh water system pressure, so it's got plenty of headroom. It's CSA and NSF listed, which means it's approved for potable water. The bladder keeps the air charge separated from the water you drink, so nothing tastes like a bike tire. Mounting is two 7/32 inch D-holes spaced 4-1/16 inches on center. Bolt it flat to a bulkhead. On a planing hull or an aluminum boat where engine vibration is real, use locknuts or a dab of thread locker so the mounts don't shake loose over a season.

What it's not: a substitute for a properly sized pump or a fresh water tank. The 21 ounce reserve is a buffer, not a supply. If you've got two faucets wide open on a liveaboard, the pump will still run steadily, and that's normal. This tank is about killing the annoying rapid cycling on the small, frequent draws that make up most of a day aboard. For that, it earns its spot. Set the pre-charge right, don't overtighten the ports, and it'll quietly do its job for years.