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Pumps and priorities

Mar 28, 2013

Voyagers need serious dewatering capacity to respond to rapid flooding

A high-capacity manual diaphragm pump like this can move a large volume of water fast.

A high-capacity manual diaphragm pump like this can move a large volume of water fast.

Keeping unwanted water out of the hull is the first principle of staying afloat. Voyaging boats need an effective dewatering system. Bilge pumps give a crew aboard a leaking or down flooding vessel a fighting chance to staunch the flow and remain afloat.

Bilge pumps come in a wide array of shapes, sizes and modes of operation. All carry some reference to their capacity to extract water from the bilge. More often than not, these gallon-per-minute (gpm) or gallon-per-hour (gph) ratings are a bit utopian and fail to reflect real world factors such as a four or five-foot pressure head, the circuitous route of a lengthy discharge hose or other restrictions to flow created by the installation. For example, an electric centrifugal pump with a 3,500 gph rating is likely to only discharge about 2,000 gph when the pressure head (height water must be lifted) is six feet.

Pumps deep in bilge areas have reduced capacity.

When it comes to the topic of leaks and pumps, flow volume becomes a big deal. The difference between a dribble and a gusher is relative to the extraction rate of the pumps in play. The rate of water ingress is a simple interaction between the surface area of the hole or void and the pressure based on how far below the waterline the opening occurs. A one-inch diameter hole a couple feet below the surface is about all that the best of manual bilge pumps and a fit, adrenalin-fired crew, can handle.
 

Manual pumps

The ubiquitous lever-activated, diaphragm pump built by Edson has for decades served as a bench mark of bilge pumps. The original version of this cast bronze, flap valve pump was designed to lift wet cement as well as water. Today, the modern iteration of this titan is a 30-gpm pump that’s worth its weight in silver if not gold. It’s available in bronze or aluminum, and although durable in the latter alloy, the $300 difference in price ($900/$1,200) between aluminum and bronze makes the latter a worthwhile investment. Picking the bronze option eliminates any issues with oxidation around the stainless steel fasteners holding the flap valves. There are also options when it comes to flap valve and diaphragm material that’s contingent on the type of liquid being pumped. The 30-gpm rating is based upon a 30-stroke-per-minute rate. A smaller well-built 18-gpm Edson pump is also available.

Ideally, these high-volume long lever pumps are permanently mounted, securely plumbed and remain at the ready when the need arises. Those with less room in the bilge often rig the big Edson on a plywood board cut to fit in the cabin sole over the deepest part of the bilge. Hull design is a key factor and vessels configured with an extreme sump-less canoe body are hard to pump. In a seaway, water will rush fore and aft as well as athwartship as the boat responds to each change in water plane. Keeping a bilge pump pickup in the right place can be challenging. In addition to the necessity of a movable pickup hose for boats with no dedicated bilge sump, there’s also the challenge of attaching a functional strum box to keep pump choking debris from clogging the check valves.

International Sailing Federation (ISAF) Offshore Special Regulations, which are quite specific in certain areas of safety, remain fairly sparse when it comes to bilge pump particulars. Under section 3.23 it states that a bilge pump discharge may not be plumbed into a cockpit drain or a hose simply led to the cockpit, unless water may run uninhibited to the sea. Even if the latter is the case, dumping an oily bilge discharge into the cockpit can create more hazard for the crew trying to handle the vessel. Also according to the ISAF regulations, pumps and strum boxes must be accessible for easy maintenance and there should be one manual pump that can be operated in the cockpit. Lanyards should be used to retain pump handles, and a couple of rugged 2.4-gallon buckets also need to be at the ready. It’s interesting to note that specific details regarding radar reflectors are far more explicit than those defining bilge pump capacity.

Advertised pump capacity may be a starting point for comparing one product with another, but when it really comes down to knowing what you have to count on there’s nothing better than a simple timed output test of each pump you have on board. This real world evaluation includes the effect of friction caused by a lengthy run of hose and the serpentine twists made as the discharge plumbing makes its way aft. There’s also the effect of lifting water as gravity pulls against the ascending volume. Little things like hose barbs, thru-bulkhead connections, and check valves add resistance to the flow and conspire to diminish output. So when you actually measure the discharge with a bucket and a timepiece, don’t be surprised to find that the water being delivered at discharge thru-hull is considerably less than the rating of the pump. This is also a good chance for the crew to get a feel for how a prolonged bout of manual pumping challenges shoulder and back muscles.

Nuisance leaks in timber boats were and are a fact of life and sailors became quite familiar with their diaphragm-type bilge pumps. Each time the ship’s bell coaxed the watch to man the pump, an entry of the strokes it took to clear the bilge was placed in the ship’s log. The ritual became a matter of discussion and any increase or decrease was considered in context with the sea state, amount of sail area set and how hard the vessel was being driven. Naturally, any sharp uptick in the stroke count caught everyone’s attention.
 

A centrifugal pump with a strainer assembly for keeping debris out of the impeller.

Electrical pumps

Today, electrical and electronic bilge alarms have caused many crews to abandon the practice of counting strokes of a manual pump at least once during each watch. Instead there are many elegantly simple approaches to an automated bilge alarm. For example, a simple mechanical float switch placed at a point in the bilge that’s just above the omnipresent low volume automatic electric pump can do the job. If the smaller pump malfunctions, or is overwhelmed by increasing volume, the high water alarm float switch lifts and a car horn or other unmistakable sound signal catches everyone’s attention. This is the time for a high-capacity DC pump to be switched on or automatically toggle into action. With a 3,500-gph rated capacity (about 2,000 gph with a six-foot head), these centrifugal pumps have the capacity to move plenty of water. However, their appetite for amperes can be considerable, and a 15-amp rating will quickly spike to more than 20 amps if debris starts to clog the intake. Care in engineering the installation involves both plumbing and electrical implications.

Straight runs, heavy gauge wire and large diameter hose are the answers to electric bilge pump efficiency. The plumbing is meant to minimize restriction and keep the pressure low and volume high. Restrictions in the discharge loop do just the opposite, causing pressure to increase but volume to drop. Bilge pump discharges near or below the waterline need a centerline high loop and an anti-siphon break that prevents backflow that can flood the bilge when an electric or manual pump is inoperative. As a vessel moves through a seaway, its dynamic waterline changes. This can submerge bilge pump discharge thru-hulls that at rest sit well above the static waterline. Centerline goose neck like risers, anti-siphon valves and check valves have been used to lessen the chance of such back flooding, but each of these efforts adds more twists, turns and restrictions to the discharge plumbing.

Many sailors have set their high-capacity electric pump up with as straight as possible a discharge run and a large ball check valve at a discharge point well above the waterline. The valve is kept closed unless an emergency warrants the use of the pump, at which time the lever on the valve is thrown open prior to turning on the breaker for the pump. This approach is based on the question: “Why introduce unwanted complexity, inefficiency and potential down flooding problems to a system that will spend about 99.9 percent of its time in a standby mode?”

Properly wiring a high volume bilge pump is as important as providing large diameter, straight run plumbing. In an electrical context, the goal is to avoid voltage drop caused by excessive resistance. The more water to be moved through a given discharge system, the more energy in watts or lever thrusts will be consumed. And that’s why heavier gauge wire like larger diameter hoses, or a manual pump operator with the countenance of a fullback, make sense.

As the cross section of copper wire increases, its resistance to conducting an electrical current decreases. This results in maintaining a higher voltage, causing more watts to be available at the pump. In short, instead of wasting energy heating up small gauge wire, more current reaches the pump. Look carefully at the pump manufacturer’s wire gauge recommendations and realize that when a battery bank is 10 feet from the high-capacity bilge pump, the wire run is really 20 feet because the positive and negative legs must each be added to the circuit.

The more twists and turns a discharge hose makes, the more restricted the flow.

Don’t be misled by a pump’s wiring gauge. For example, a pump with 14 gauge leads does not signify that 14-gauge wiring is appropriate for the circuit. If it’s a lengthy run, you’re much better off with heavier 12 or 10 gauge wire. Make sure that all junctions are created with high-quality marine grade terminals and the crimping tool delivers a wide even compression of the fitting. Shrink tube sealing and other efforts to ensure watertight integrity are worth the effort. Corrosion at a wire to wire junction can be as inhibiting as a restriction in the plumbing. The corroded connection increases resistance and can cause a significant voltage drop despite the fact that the right wire gauge has been used.

An under-rated float switch wired in series with a heavy-duty pump can be another cause of poor performance. It’s the reason why many skippers use a high quality manually operated single pole/single throw switch to engage a high-capacity pump.
 

Engine as bilge pump

For decades, dockside wisdom has extolled the virtue of letting the auxiliary engine’s raw water pump save the day and rid the bilge of unwanted water. The idea is to swap the intake thru-hull for a hose to the bilge using either a dedicated Y-valve or a jury-rigged hose attachment. If all goes well, a steady stream of clean accumulating water will both cool the engine and be extricated from the bilge.

The problem is “clean” is seldom the adjective that depicts what’s sluicing around in the bilge when a sailboat or power cruiser has water rising above the cabin sole. The same debris that can clog strum boxes and the valves inside a diaphragm pump can also choke off water essential for the engine’s cooling system. The result is not just the loss of the newly-harnessed bilge pump, but now the engine is overheated and out of action. And so is the all-important alternator(s) that keeps batteries up and electrical pumps spinning. Incidentally, the total volume of water moved by the engine driven raw water pump on a mid-sized sailboat auxiliary isn’t as much as a big electrical pump can deliver. So you my want to think twice before turning the engine raw water pump into a tool for dewatering a boat in a jiffy.

Other options include adding a dedicated engine driven displacement pump with a mechanical or electric clutch as an emergency bilge pump. It can also be Y-valve plumbed to do double duty as a fire hose pump with an unlimited water supply.

Another choice is the portable gasoline-powered high volume centrifugal pump such as those made by Honda. These compact pumps hurl much more water than anything thus far mentioned, but they can be finicky to start and prime, and need to be carefully maintained. They are a smaller version of the emergency dewatering pumps used by the U.S. Coast Guard.

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Ralph Naranjo is a sailor, freelance writer and photographer based in Annapolis, Md. In 1975 he and his wife, Lenore, set sail on a five-year circumnavigation on their 41-foot sloop Wind Shadow, as told in his book Wind Shadow West. He is also the author of Boatyards and Marinas.

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Nov 20, 2013 10:02 am
 Posted by  xanaduran

Newbie creating initial account. Greetings to all!

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