All About Pumps: Go with the Flow
Yeah, we know – articles that focus on technical topics are about as exciting to read as wiring schematics. But right now, you’ll just have to tough it out – peruse and then percolate these pumping particulars, and you’ll be better prepared to pursue your pastime. (Boating, we mean, which we would have said in the sentence prior if it began with a “p,” but it doesn’t and we are trying to keep things interesting here, folks!)
Why should you give a darn about pumps? Because they’re a very important piece of equipment that let you down quite often. The average raw water washdown pump (at a cost of several hundred dollars) usually lasts no more than a few seasons. Bilge pumps seem to fail less often but usually, the results are a little more serious, and when it comes to a fishbox pump-out you’re lucky if it survives two seasons. Why? Stated simply, because most boats are built with the wrong (read: cheap) pumps for the job. If you’re sick and tired of constant pump hassles, take the newfound knowledge that’s about to come (assuming you’re not yawning, already) and replace all the pumps on your boat immediately.
Centrifugal Pumps
These are the type you’ll most commonly be depending upon to drain the bilge. This type of pump has a rotating impeller which spins the water outward, imparting centrifugal acceleration. As the water flows away from an impeller vane, a low-pressure area is created, drawing more water into the pump. There are actually three types of centrifugal pumps: radial (all pressure is developed by centrifugal force), axial (the pressure is developed by the pushing action of the vanes on the water itself) and mixed flow (a combination of radial and axial).
Why You Should Care. Centrifugal pumps lose efficiency in two ways which often affect boaters: lift and flow restriction. For every foot in height your pump has to lift the water before expelling it out through a through-hull fitting, your pump loses about five percent of its efficiency. In other words, if there’s a two-foot difference between the bilge pump and the outlet and your pump is a common 800-gph model, it’s actually evacuating water at 720-gph. Wait a sec—it gets worse. Bends and restrictions in the hose will also slow the flow even more. Just as a point of reference, note that a 1” hole in your hull that is 5” below the waterline will let in about 2,600-gph. Now, walk out to your boat, peek in the bilge and ask yourself: should that pump be replaced with a more potent one before you leave the dock again?
How do you address reliability in a bilge pump? Truth be told, most failures are due to float switch or electrical problems, as opposed to a problem with the pump itself. If your float switch falls and gets stuck in the “on” position, that pump is going to burn itself up no matter how high quality it is. There is, however, a simple solution: get rid of the float. There are many float-free auto switches on the market these days, many of which work on solid-state electronics (when water touches both terminals, the pump kicks on).
Diaphragm Pumps
These aren’t found on boats as often as the other types of pumps… and that’s a shame. They work by lowering and raising a diaphragm to suck water in through one valve and push it out through another. They get a bum rap because the valves on some models clog easily.
Why You Should Care. Most fishboxes are fitted with macerator pumps which are designed to grind waste – human waste, that is. The macerators were used in marine heads long before integrated below deck fishboxes came into vogue and these pumps were never really meant to chew on crab shells, scales, and rib bones. So naturally, they fail quite often.
One boatbuilder, tired of complaints from customers about how often these pumps failed, tested every type of pump on the market until they found one so tough it could pump wet sand. Guess which one it was: a diaphragm pump called a Gulper (made by Whale). Extra diaphragm bonus: it can run dry without burning up as centrifugal pumps do. If you’re sick and tired of dealing with dead macerator fishbox pumps, swap them for high-quality diaphragm pumps and be done with it.
Multi-Chamber Diaphragm Pumps
These pumps are good for building pressure, which is exactly what you need in a raw water washdown or a livewell pump. They also deliver a much smoother flow than the single diaphragm pumps.
Why You Should Care. Unfortunately, many of the factory-installed washdown pumps that come on new boats produce a stream of water that has less pressure than your own internal piping produces after you chug a six-pack of Budweiser. Washdowns commonly come in all types and sizes, from flexible impeller to centrifugal to diaphragm designs. Why? Because they’re less expensive than multi-chamber diaphragms. In some cases, you’ll even see bilge pumps used for washdowns and livewell pumps. The worst offenders have a single shared pump to do both jobs, rigged on a Y-valve. Try to use both accessories at the same time and you’ll be sorely disappointed. The simple solution is to replace that chintzy pump with one that produces some serious pressure and has a sealed and coated body that won’t corrode or leak. Look for a three-chamber diaphragm model that provides at least 40-psi and a five-gallon per minute flow.
Rigid Vane Pumps
When there’s a load to be transferred between tanks, such as fuel or ballast, rigid vane pumps are the common answer. These pumps create a vacuum for self-priming, are very reliable and can be reversed – ideal when you want to go from tank A to tank B, then back again.
Why You Should Care. Unless you own a large boat with multiple fuel tanks or a dedicated wakeboarding boat with a ballast tank, you probably don’t care one iota. Still, you should know what these pumps are for. If someone tries selling you on an expensive rigid vane pump when its intended use is in the bilge, for a livewell or a washdown, you’ll know something’s fishy.
Flexible Impeller Pumps
You’ve seen these if you’ve ever changed the water pump on your outboard. Flexible impellers, usually made of rubber or neoprene, spin in a housing to create suction and shove fluid around. Their number one best feature is reliability since they have only one moving part – the impeller itself.
Why You Should Care. These pumps are extremely vulnerable to failure if they run dry. With an outboard, this isn’t a common problem (unless you fire your engine up without a water source – in which case you should give up boating and take up a sport you’re better suited for, like knitting.) While you will occasionally see flexible impeller pumps that are used for applications other than engine water intakes, it’s rare. And if you do see one used in a way that leaves a distinct possibility of running the pump dry, get rid of it.
Whew! We made it through the boring stuff. Now, wipe that glaze off your eyes – this next part is important. Pump failures occur for a multitude of reasons, but the most common is one that people rarely think about – lack of use. With virtually every pump on the market, regular use is necessary to keep the seals in good shape, the parts lubricated, and the prime strong. You want every pump on your boat to run for the next 10 years? Then make sure each and every one gets a workout at least once a week. Now, if you’ll excuse us, we have a wiring diagram that we need to read.