De-Icing is accurately referred to as an art form more than an exact science. Each application and location is different and there is no set rule in determining how many De-Icers are needed for a given area or which mounting option is the best. The effectiveness of de-Icers largely depends on the depth of the surrounding water, obstructions in the water, and the geographic location of the installation. De-Icers work by moving warm bottom water that is heated by the earth to the surface. The shallower the water in the area surrounding the De-Icer, the less warm water there is available. It is recommended that the De-Icer is 2.5" to 5" below the water surface. Obstructions in the water such as floats on docks, boat lifts, large dock pilings, boats, etc. may restrict the flow of water causing a smaller area of influence around the De-Icer. Your geographic location will also play a major role in the effectiveness of your de-Icer. The longer and colder the winter is, the more the water will cool over time and open water may be limited accordingly. Other factors that may effect open water areas are bottom contour, salt content, mounting, etc.