Using 50-200 KHZ Transducers
Dual frequency sonar 50-200 Khz transducers are pretty popular on Lake Ontario especially when used in conjunction with the Furuno FCV 585 fishfinders and the like. But what do these numbers mean and how do we use them to our fishing advantage? This article will explain how we use them on our Lake Ontario fishing charters out of Oswego,NY.
According to the dictionary, the definition of sonar is,” A system for the detection of objects under water and for measuring the water’s depth by emitting sound pulses and detecting or measuring their return after being reflected.” Sounds simple enough! But what about these frequencies of 50/200 Khz? What are they?
200 khz means that the transducer is putting out 200,000 pulses per second. A 50 khz frequency is putting out 50,000 pulses per second. The higher the khz number, the higher the frequency, but the lower the cone angle that is required to produce the higher frequency. On typical 50-200 Khz transducers, the cone angles are 45 and 12 degrees respectively. It is counter-intuitive.
The lower the frequency (50khz), the deeper the penetration, and the wider the cone angle (45 degrees). This also produces less discerning of objects and more clutter on the screen. Conversely, the higher frequency, the more discerning of objects, but less penetration into the water column with a narrower view of whats beneath. So again, what does all this mean? How do we use this to maximize our fishfinder capabilities?
When to Use What Frequencies
When we are running out to fish and up on plain, we set the frequency to 200 khz. We also have the scroll speed of the Furuno set at 4/1 ratio. This is the fastest scrolling speed available for the Furuno Fcv 585 . Set this way, we can mark bait, fish, and never lose bottom out to depths of well over 600 ft. We deploy the 50 khz when trolling. On our charter boat the “Ace“, the lower frequency allows us to see our downriggers on the graph. But be advised, the downriggers are not at the actual depth that they are displaying on the graph. They are running shallower than where they appear on the fishfinder. See our blog post: Using Your Fishfinder Basics for further explanation!
We found that using the dual frequency 50-200 khz tranducers setting in shallow water works quite well when brown trout fishing. When you see marks on both screens simultaneously, you know that these fish are directly below the boat. If you see them on the lower 50 khz side of the display,the fish are more to the side of the vessel. This can be handy in determining the mood of the fish and if they are being boat shy.
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