Hey Polo,
I saw your post and some of the responses to it. I don't want to get into sonar theory, etc and bore you to death, but I've been through more sonar classes than most people (part of my job) and know quite a bit about sonar. However, most of the posts to your question are right on the money. Vertical pixel (usually, but not always, the second number.... example 240 x 128 equals 240 horizontal by 128 vertical) are probably the most important. Power is usually very over-rated in most fresh water cases (<200 feet depths). I found some interesting info on the internet at the Bottom Line sonar website that pretty much sums everything up very nicely. I'll paste it below. Basically, determine your budget and then go from there. All the name brands have good units. I'm partial to Eagle (made by Lowarance without the Lowrance high price), however everyone will have an opinion. Good luck and PM me if you want to talk about it some more. Here's the info:
1. What is sonar and how does it work?
Greatly simplified, a sonar unit is just a combination of a speaker, microphone and stopwatch. Every fish finder "knows" that the speed of sound through water is about 4,800 feet per second. It transmits a sound pulse and then measures the time it takes for echoes to return from the pulse. Then it converts the elapsed time for each echo into distance. A built-in computer organizes all of this information and shows it on a display screen.
2. Pixels. Why are they so important?
The word "pixel" is short for "picture element". Pixels are the elements that the picture on a fish finder's screen is made from. Liquid crystal displays are really checkerboard-like grids of tiny dots (pixels) that darken individually when electricity is applied to them. A fish finder's computer forms the picture on its screen by darkening selected pixels and leaving others "blank".
The number of pixels on a unit's screen determines how much detail it can show. Remember that pixels are arranged in columns and rows. The more pixels a screen has in each vertical column, the less depth each pixel represents and therefore the higher the resolution. If a screen has 100-pixels in each column and you search for fish on the 0- to 50-foot depth range, each pixel represents 6 inches of depth (50 feet divided by 100 pixels equals ½ foot per pixel). A picture made with 6-inch building blocks isn't going to have a lot of detail. Take a Bottom Line® model with 240 pixels in each vertical column, and each pixel represents about 2½ inches of depth. Smaller changes in the bottom contour and subtle details in the structure features spring into view, giving you, the angler, better information.
The number of pixels in each horizontal row determines how long information stay on the screen before it scrolls off. This is especially important for units to show side-by-side displays of different kinds of information. Wide-screens, such as the Bottom Line Tournament NCC 6300 and the Tournament 5100, allow information to stay on the display a normal length of time even when the screen is split into separate features.
3. Fish arches. Now you see them, now you don't. Why?
The importance of seeing fish as perfect boomerang-shaped arches on the screen has been greatly exaggerated over the years. It all has to do with how fish arches are created.
Imagine sitting in an anchored boat with your fish finder turned on. Picture in your mind the transducer's cone-shaped scanning area under your boat. In order to print a perfect arch, a fish will have to enter the edge of the cone, swim directly across the middle, and pass out of the cone.
Let's say the fish holds a constant depth of 15 feet as he swims straight across the cone. The unit measures the distance to an object and starts to print out on the display; it's 15 feet below the surface of the water but probably 16 feet from the transducer. As the fish swims through the center of the cone, it passes 15 feet below the transducer. When it reaches the edge of the cone again, it's 16 feet away just before it stops printing on the screen. This causes an arch to start at 16 feet, curve up to 15 feet, then curve back down to 16 feet. The wider the cone angle, the more exaggerated the arch.
If the fish changes depth, passes through only one edge of the cone, or wonders around under the boat before swimming off, it won't print as a perfect arch.
4. Power. Why is it so important?
The output power of a fish finder's transmitter is stated in watts RMS or in watts peak-to-peak (P-P). The two terms represent different ways to advertise output power, and all you need to know to compare units that advertise different ratings is that P-P equals watts RMS times 8.
Power's importance is based on the perception that more power always produces a better echo. In reality, it is the combination of the unit's power and receiver sophistication that determines the unit's ability to find a fish and display an accurate image.
5. Which is better, wide or narrow cone angle?
Bigger is not necessarily better. A transducer with a wide cone angle scans more water as your boat moves along and can fish and structure features faster, but this advantage can also work against you. The wide cone may cover two or three important stumps on the bottom, for instance, and the lump their reading together, makes it impossible to see just the one with fish next to it.
A narrow cone zeroes in on fish and can detect small details on structure features that fish may relate to. Focusing the transducer's power into a narrow beam also concentrates the sound output, enabling it to reach greater depths. A di
vantage to a narrow cone angel is that it scans smaller amount of water as the boat moves along.
Bottom Line strives for the best of both worlds by using a transducer with a medium cone angle and automatically manipulating receiver sensitivity and echo filtering to provide wider coverage in shallow water and narrower, more detailed coverage in deeper water. Add the advantage of multiple transducer beams that look to the side as well as straight down, each with its own separate display, and it's easy to tell where fish are in relation to your boat.
6. What is sidefinding and how does it work?
Sidefinding is more than a sonar transducer pointed horizontally to look sideways. Directing sound pulses sideways results in echos from the underside of waves and from the bottom (especially in shallow water). Bottom Line pioneered and patented the technology needed to separate the echoes returned from fish and those returning from the water's surface and lake bottom. A considerable amount of sophisticated filtering is required.
7. How do I interpret what I am seeing on my fish finder display?
Some anglers don't stop and fish areas unless they see fish on the screen; this limits their success. Areas loaded with weeds, brush and submerge trees are great spots even if no fish are obvious. Developing the skill to see fish-attracting cover is as important as being able to identify individual fish on the screen.
Any object that is different in density from water can return a sonar echo. It's the density difference that determines the strength of an object's echo. Clumps of weeds and branches of submerged trees poking into the cone angle have different densities, and return different echoes.
It's often been said that 90% of the fish are in 10% of the water. Vast areas of any body of water are barren of fish, and fishing them is a waste of time. One great benefit of fish finders is their ability to help you bypass water that contains no cover, no baitfish and no lumps on the bottom that could be game fish. Ironically, one of their great benefits is showing you where NOT to fish.
8. Why do I need a sidefinding unit?
Everyone who has searched for fish with sonar has wondered, "How many fish am I missing that are just outside my transducer's cone of coverage?" When trolling a Sidefinder® can spot a fish to your left or right and help you the most productive track to take. When casting, a Sidefinder can search water not accessible with your boat. A Sidefinder with a trolling-motor-mounted transducer can sweep the area 360 degrees around your boat for fish. When sweeping a circle around your boat, rotate the transducer slowly to give the unit more time to work. The more slowly you move the transducer, the more time the Sidefinder has to pick out the fish echos.
9. How do I tell what kind of bottom I'm marking?
Generally speaking, a hard bottom will be indicated with a thicker (top-to-bottom) contour line because it reflects most of a transducer's sound pulse. A soft, mucky bottom will absorb sound and return a weaker echo, resulting in a thinner contour line. However, a feature Bottom Line® calls GrayScale makes it easier to see the difference between hard and soft bottoms. All Bottom Line units display GrayScale to indicate very strong reflections from the bottom. The thicker (top-to-bottom) the checkering, the harder the bottom.
10. LCD vs. flashers. What's the difference and what's "real time"?
Some anglers equate using a flasher to reading a meter, and using an LCD graph unit to looking at a picture. New sonar buyers apparently prefer looking at a picture, because only a few tiny percentage of sonar units sold to sport fishermen are flashers. Many professionals agree that it takes much less time to become proficient with a graph unit than it does with a flasher. The units are really more similar than most people imagine.
The vertical columns of information that cross the screen on LCD graphs can be compared to the separate revolutions of a flasher. Instead of flashing a bulb for an instant to mark a target's depth, LCD's darken pixels in a column that remains visible until the column scrolls off the screen.
Flashers show changes in bottom depth and the depth of suspended objects as quickly as electricity can pass through the unit. In contrast, LCD units look at the last few soundings and do some averaging and filtering before displaying information from them. Years ago, this meant a perceptible and sometimes annoying time lag between when you passed over something and when the LCD put it on the screen. Today's units have much more power and the lag is all but gone. You can test a unit for lag by driving your boat over a boathouse cable in a marina. Note when your transducer passes over it and when it appears on the screen.
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