PS Advisor: Plumbing a Multi-pump Bilge


Your September 2010 issue had a great article on electric bilge pumps. My Pearson 424 ketch has a bilge sump that has to be 4 feet deep, which would be a lot of water to move if it ever started to fill. In the article, you recommended a mix of a small pump for “everyday duty” with a couple of large-capacity backup units, you didn’t mention how to plumb these. Would each pump have to have a dedicated through-hull, or is there a way to utilize only one through-hull? More holes in the hull doesn’t sound reassuring.

Dan Kalinowski
Lady Leanne II, Pearson 424 ketch
Honolulu, Hawaii

We’re glad you liked the September 2010 review of high-capacity bilge pumps. Be sure to check out the October 2010 review of low-capacity electric pumps, which included a detailed rundown of our rules of thumb for bilge-pump installations.

In an ideal setup, each bilge pump would have its own discharge hose, anti-siphon valve, and through-hull fitting with a seacock. The point of having multiple bilge pumps is to have system redundancy. That way, if any part of one pump’s plumbing fails (which could include a clogged discharge hose or damaged through-hull), you have a backup that operates entirely independent of the failed pump. We always try to heed Murphy’s Law, and when you increase the number of bilge pumps onboard, you decrease the odds that if one fails, your boat will sink.

While multiple, independently plumbed pumps are what we recommend, we realize that’s not always feasible. According to the safety standards set by the American Boat and Yacht Council (ABYC), “If the discharges of several pumps are manifolded to discharge through a single through-hull fitting, the system shall be designed so that the operation of one pump will not back feed another pump, and the simultaneous operation of each pump will not diminish the pumping capacity of the system.”

What this means is that if you have to plumb more than one bilge pump to the same through-hull, then their discharge hoses should have their own anti-siphon valves and be manifolded just before the seacock/through-hull. Also, the larger, combined discharge hose (after the manifold) and the discharge through-hull should be large enough to effectively handle the output flow of both pumps. So if you’re manifolding multiple pumps to a single through-hull to avoid putting more holes in your hull, know that you’ll likely still need to make the existing through-hull larger. A standard rule of thumb is that it should be the diameter of the pump discharge-hose diameters combined.

Although you can apply the geometric equation used to find the area of a circle (Area=∏r²) to determine a functional outlet diameter for two combined hoses, this yields a smaller diameter than the one we would recommend. Every boat has different challenges. To accomodate these and other variables, we take a conservative stance on hose sizes. To ensure that your second pump encounters no added resistance in the line, we recommend that you simply add the circumferences of the two hoses to determine the final discharge diameter. For example, if one pump has a ¾-inch discharge and a second pump has a 1½-inch discharge, you need at least a 2½-inch through-hull discharge (3/4 + 1½ = 2¼, rounded up to the next largest size) to ensure optimal flow if both pumps are running.

To prevent backflow, we’d plumb the pumps to vented anti-siphon loops. Both pump makers and the ABYC discourage using check valves. When possible, plumb the discharge(s) out of the transom near the centerline or at the trailing edge of the overhang. We also recommend plumbing manifolded pump lines to a through-hull that is above the heeled waterline and fitting all through-hulls with a seacock.

Keep in mind that if both pumps are running, the problem could be serious, so you want the highest efficiency from your pumps. You can easily test actual flow rates of various installation options with a bucket and a timer. When testing a proposed system, make sure the fittings, hose, hose run and discharge height closely matches the actual installation.

Bottom line: We recommend using separate plumbing and discharge outlets. If this is not practical or the pumps are not essential to emergency pumping, a combined system is an option, but be sure size the outlet conservatively and follow industry guidelines to prevent backflow.

Lightning and LEDs

Are there ways of protecting LED lights during a lightning storm (like disconnecting a power source, etc.)?

Jim Liggett
Via e-mail

When it comes to lightning, LEDs are no different than any electronic device or incandescent lights. We’re not aware of any product that will protect LEDs from lightning. Opening the breakers or shutting off non-essential equipment, so that those items are no longer connected to the system, is good practice to minimize lightning damages, but it is not a guarantee. According to Mike Moriarty, an engineer with LED-maker Imtra Corp., if lightning finds its way through the boat’s DC system to Earth, it will destroy anything on that system, LED bulbs included.

Darrell Nicholson, editor of Practical Sailor, grew up boating on Miami’s Biscayne Bay on everything from prams to Morgan ketches. Two years out of Emory University, after a brief stint as a sportswriter, he set out from Miami aboard a 60-year-old wooden William Atkin ketch named Tosca. For 10 years, he and writer-photographer Theresa Gibbons explored the Caribbean, crossed the Pacific, and cruised Southeast Asia aboard Tosca, working along the way as journalists and documenting their adventures for various travel and sailing publications, including Cruising World, Sail, Sailing, Cruising Helmsman, and Sailing World. Upon his return to land life, Darrell became the associate editor, then senior editor at Cruising World magazine, where he worked for five years. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


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