Marty, when in waters less than 50' I run my fishfinder in dual frequency mode. My machine runs a low frequency (LF) 50kHz on the left and high frequency (HF) 200kHz on the right. The transducer I run is an Airmar p66 600w transducer (very common for inshore people). The "cone" of the signal with this transducer for 50kHz is very wide. It is 45 degrees. The cone for the 200kHz is narrow at 10 degrees.
http://www.airmartechnology.com/uplo...chures/p66.pdf
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Most people typically have been told to use 200kHz for inshore because it is more sensitive and 50kHz for offshore since a low frequency will travel deeper into the water, though "less sensitive" due to typically wider cone angles. Before digital processing fishfinders like today, you often got a lot of noise or clutter with low frequencies like the 50kHz in shallower waters. That is NOT the case today with digital fishfinders since the processing filters out a lot of the noise/clutter.
Because fishfinders are so good today, I recommend knowing the strength and weakness of each frequency and running both side by side even in the bay. I personally think this is better than going with a new CHIRP unit unless you are bottom fishing in deep water. (I wont get into the CHIRP differences here).
Now given my transducer, like most, offers a wider cone at 50kHz, I am able to gain these advantage by reading this frequency.
1. I cover more water under the boat with the wider cone.
2. I get more of a defined "arch" for bigger fish than smaller fish and this allows me to better differentiate between game and bait. This makes it great for suspended fish.
"Arches" are formed as a fish's swim bladder passes through the cone of the transducer frequency like this:
This is why you dont see as big of arch on the 200kHz side because the cone is either missing some fish and/or the fish is in the cone for less time.
The 200kHz narrow cone gives me these advantages:
1. narrower, more targeted view of the bottom since the signal is concentrated on a smaller area of the bottom. Because of this it more likely to pick up fish right on the bottom for bottom fishing
2. the narrower more sensitive cone shows bottom density changes (harder to softer bottom - hard bottom is a thinner line, soft bottom is a wider line)
3. the narrow beam is more likely to pick up water breaks like thermoclines. Sometimes i need to turn up the gain on that side to find them in the spring and fall. (increasing gain likely fills in targets for less discrimination though)
All the way to the right on my screen is a flasher. That just gives me the instant returns vs seeing what we have already passed over as the marks scroll to the left.
Here is a labeled screen shot from the screen from last May to kind of put it all together for you:
When I troll deeper water in the spring and fall, I usually use the 50kHz and split the screen to read the full top to bottom on one side and just the top 40' of water on the other. I dont care as much about bottom density changes when trolling so the narrow cone of the 200kHz doesnt have much benefit other than to know about where the thermocline is to run some baits along it. In the summer when I am looking for spot or areas to chunk on bottom changes, I use the 200kHz a lot more and will even do bottom zoom with white line features.
The squiggly lines above were just us drifting with the fish staying under the boat for an extended period of time.
Clear as mud. We need to fish together we can talk more about it
