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The Bilge-Pump Alternative

The Bilge-Pump Alternative

By Richard Harker

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When you consider the differences between the natural reef and captive reef systems, it is surprising that reefkeepers are as successful as they are. Our success speaks to the adaptability of reef inhabitants and their ability to cope with less than ideal conditions. It also speaks to the ingenuity of reefkeepers who continue to develop better approaches to re-creating realistic reef conditions in their reef aquariums.

One area in which the hobby continues to produce innovations is in re-creating water motion. Adequate water motion is critically important for sessile (immobile) animals. Water must bring oxygen and nutrients to such an animal and carry off waste products. A coral can literally suffocate if water motion is inadequate. Scientists have found that as water motion increases, calcification and growth increases as well. (For a more complete discussion of water motion, read the article “Measuring turbulent flow in reef aquariums” in the August 1998 issue of Aquarium Frontiers.)

Adequate water motion is also important for the general health and appearance of a reef aquarium. Cyanobacteria blooms are often caused by nothing more than inadequate water motion.

The traditional approach to creating water motion in a reef aquarium is to use one or more powerhead arranged around the top of the aquarium. (For an explanation of water pump basics, check out the “Product Review” column in the October 1998 issue.) Powerheads have significant limitations, as they create very unnatural water motion. Water motion on the natural reef is turbulent and chaotic. Water flows around corals in all directions. In contrast, the water flow from a powerhead is unidirectional. It is a stream of water moving in a single direction.

The water energy generated by the most powerful pumps available do not come close to the water energy found on the natural reef. However, the unidirectional jet-like force of even a small powerhead is greater than most corals can handle. The stream of water can literally tear the tissue off a coral. The challenge for hobbyists has been to create realistic turbulent water energy in the reef aquarium using devices poorly suited to the task.

One solution has been to direct the jet of water toward a side of the aquarium. The water strikes the glass and is dispersed. Another solution is to use “wavemakers,” devices that periodically switch off some of the pumps. A third approach has been to use rotating powerheads that swivel to change the direction of the flow. Unfortunately, all of these approaches have limitations. Directing a strong stream of water toward the glass wall of the aquarium produces a wave that can push water over the side of the aquarium. Hobbyists are forced to use smaller powerheads than they might otherwise so as to keep the water in the aquarium.

By switching powerheads off and on, wavemakers can create more turbulence, but at a cost of water energy. Total water energy generated by powerheads is a function of the total flow of the pumps. If water motion is created by a powerhead that generates 200 gallons per hour and then the hobbyist adds a second pump also generating 200 gallons an hour, water energy has doubled. However, if the hobbyist then uses a wavemaker to alternate between the two pumps, more energy is not being generated. The only thing that has changed is its direction of flow.

An idle pump is not creating water movement, which means that if one uses a wavemaker, the number of pumps must be increased to make up for the pumps that are not running. There’s also the problem that few powerheads are designed to be frequently switched on and off.

Rotating powerheads are the latest innovation in water motion. They move the stream of water in an arc, so the jet of water sweeps across the aquarium. This has the potential to create somewhat more realistic turbulence, but it also has its limitations. While the stream of water is moving, the intensity of a strong jet of water can still do damage to corals even as it moves past. This means that rotating powerheads must be fairly small to avoid damage to corals. Because total water energy is a function of the total flow in the aquarium, creating high energy requires that the aquarium have quite a few rotating powerheads. This raises the total costs for a hobbyist. Reliability of the first generation of rotating powerheads was also a serious shortcoming, but the latest generation seems to be an improvement over the initial design.

At this year’s Marine Aquarium Conference of North America (MACNA), to be held in Ft. Lauderdale, Florida, the weekend of September 29 to October 1, I’ll be giving a talk on the design and construction of a 2000-gallon reef aquarium (Saturday, September 30). One of the areas I will discuss will be water motion — how to measure it and how to design a reef aquarium with adequate water movement, regardless of size. As a preview, this month I will look at a new alternative to the traditional approach to creating water motion.

The greatest limitation of powerheads that hobbyists have the work around is the narrow jet of water that powerheads generate. Powerheads were originally designed to aerate the water in a fresh or saltwater system. Nearly every powerhead has an optional "aerator" that draws air into the nozzle of the powerhead. To create adequate draw, the nozzle is narrow. This approach works well when one wants air bubbles to the jet of water. For reef purposes, however, the aerator is rarely used. A given size powerhead creates the same amount of water energy regardless of the size of the nozzle. (This is not true when the size is reduced to the point of creating significant back pressure, but for our purposes this is not an issue.) What changes is the velocity of the water. The narrower the nozzle, the higher the velocity. Velocity is the enemy of the reef hobbyist. It is the high velocity of powerful pumps that we must control to prevent damage to corals. If we can slow the velocity of the water while maintaining the same flow, then we have more control over water motion. Aquarium Products distributes the Gemini pump, an excellent air-cooled pump for creating water motion in the reef aquarium. It is rated at 960 gallons per hour (gph). The nozzle on the Gemini pump has a cross section of less than a square inch, producing a water velocity of over 120 inches per second as it exits the nozzle. While almost all aquariums can handle the water flow of the Gemini pump, only large aquariums can handle the velocity of the water exiting the Gemini. The solution is to increase the cross section of the pump nozzle, which maintains the flow of the pump while lowering the velocity of the water.

Nearly all powerheads have nozzles with a small cross section and therefore present the same challenge. Otto powerheads have a swivel in the nozzle that can be removed, thereby enlarging the nozzle cross section. I have used Otto powerheads without the swivel nozzle for many years. Unfortunately, Otto is the only manufacturer that has this option.

One solution with other powerhead brands is to simply remove the nozzle close to the body of the pump. A very short nozzle will allow the stream of water to expand more quickly, dissipating the energy across more of the aquarium. A second option is to use PVC fittings to create a flow manifold. This effectively increases the cross section, lowering the velocity of the water.

Recently, I’ve been pursuing another option — using bilge pumps designed for boats. Bilge pumps offer a number of advantages over traditional powerheads. First, they are very powerful. A small palm-size unit can pump 1100 gallons per hour and yet is considerably smaller than a comparably sized hobby pump. Bilge pumps come in a wide range of sizes from a few gallons per hour to larger models up to 5000 gph. They are heavy duty, waterproof, will operate partially or fully submerged, and use efficient direct drive stainless steel drive shaft impellers. Most importantly, bilge pumps are designed to be connected to large-diameter hose, so they have a nozzle with a large cross section.

The 1100 gph pump pictured above has a nozzle cross section of nearly an inch. This means that the velocity exiting the 1100 gph bilge pump is less than 30 inches per second, only one quarter of the Gemini pump velocity. The lower velocity means the pump can be closer to corals without damaging them, while at the same time generating a great deal of water energy.

Perhaps the greatest advantage to bilge pumps is also their disadvantage. They run on 12 volts D.C. Converters are readily available, but add to the cost of the setup. The 1100 gph pump consumes only 40 watts, but requires a converter with a 4 ampere capacity. In spite of the greater complexity of dealing with D.C. equipment, there’s good reason to have at least one 12-volt pump available. We’ve all faced power outages and the toll that power failures take on reef aquariums. A 12-volt pump could serve as an emergency pump powered by a car or smaller battery. In an emergency, I’ve powered my aquarium with my car battery for up to an hour. This is less expensive than purchasing an inverter with sufficient power capacity to drive a pump.

Because bilge pumps use a D.C. motor, one has more options to control water flow through voltage regulation. The speed of the pump varies in proportion to the voltage applied. Using a variable power supply to run a bilge pump enables a hobbyist to vary the water motion in the aquarium with the turn of a knob. D.C. wavemakers could vary the pump output in a way that would create realistic wave action. This has been available for Tunze users for many years, but at considerable cost. If entrepreneurs begin developing D.C. wavemakers, realistic wave action can become affordable for all hobbyists.

I’ll have much more to say about water motion at MACNA. In the meantime, check out bilge pumps at boating supply stores or at the West Marine web site, and explore these new options for creating water motion in your reef aquarium.

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