PS Advisor May 2011 Issue

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.

Comments (0)

Be the first to comment on this post using the section below.

New to Practical Sailor?
Register for Free!

Already Registered?
Log In