I'm a Furuno guy but there's a Lowrance tutorial that is very helpful if you want to utilize all the bells and whistles of any fish finder. Here's the part on fish arches:
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Screen Resolution
The number of vertical pixels that the screen is capable of showing is called Screen Resolution. The more vertical pixels on a sonar's screen, the easier it will be for it to show fish arches. This plays an important role in a sonar unit's capability to show fish arches. The chart below lists the pixel sizes and area they represent down to 50 feet for two different screens.
PIXEL HEIGHT PIXEL HEIGHT
100 VERTICAL PIXEL SCREEN 240 VERTICAL PIXEL SCREEN
RANGE PIXEL HEIGHT RANGE PIXEL HEIGHT
0-10 feet 1.2 inches 0-10 feet 0.5 inches
0-20 feet 2.4 inches 0-20 feet 1.0 inches
0-30 feet 3.6 inches 0-30 feet 1.5 inches
0-40 feet 4.8 inches 0-40 feet 2.0 inches
0-50 feet 6.0 inches 0-50 feet 2.5 inches
As you can see, one pixel represents a larger volume of water with the unit in the 0 - 100 foot range than it does with the unit in the 0 - 10 foot range. For example, if a sonar has 100 pixels vertically, with a range of 0 - 100 feet, each pixel is equal to a depth of 12 inches. A fish would have to be pretty large to show up as an arch at this range. However, if you zoom the range to a 30-foot zoom (for example from 80 to 110 feet), each pixel is now equal to 3.6 inches. Now the same fish will probably be seen as an arch on the screen due to the zoom effect. The size of the arch depends on the size of the fish - a small fish will show as a small arch, a larger fish will make a larger arch, and so on. Using a sonar unit with a small number of vertical pixels in very shallow water, a fish directly off the bottom will appear as a straight line separate from the bottom. This is because of the limited number of dots at that depth. If you are in deep water (where the fish signal is displayed over a larger distance of boat travel), zooming the display into a 20 or 30 foot window around the bottom shows fish arches near the bottom or structure. This is because you have reduced the pixel size in a larger cone.
Chart Speed
The scrolling or chart speed can also affect the type of arch displayed on the screen. The faster the chart speed, the more pixels are turned on as the fish passes through the cone. This will help display a better fish arch. (However, the chart speed can be turned up too high. This stretches the arch out. Experiment with the chart speed until you find the setting that works best for you.)
Transducer Installation
If you still don't get good fish arches on the screen, it could be the transducer's mounting is incorrect. If the transducer is mounted on the transom, adjust it until its face is pointing straight down when the boat is in the water. If it is angled, the arch won't appear on the screen properly. If the arch slopes up but not down, then the front of the transducer is too high and needs to be lowered. If only the back half of the arch is printed, the nose of the transducer is angled too low and needs to be raised.
Fish Arch Review
Sensitivity
Automatic operation with ASP™ (Advanced Signal Processing) turned on should give you the proper sensitivity settings but, if necessary, the sensitivity may be increased.
Target Depth
The depth of the fish can determine if the fish will arch on the screen. If the fish is in shallow water, the fish is not in the cone angle very long, making it difficult to show an arch. Typically, the deeper the fish, the easier it is to show an arch.
Boat Speed
The boat's engine should be in gear at an idle or just above. Experiment with your boat to find the best throttle location for good arches. Usually, a slow trolling speed works best.
Chart Speed
Use at least 3/4 chart speed or higher.
Zoom Size
If you see markings that are possible fish, but they do not arch, zoom in on them. Using the zoom function lets you effectively increase the screen's resolution.
Final notes on fish arches
Very small fish probably will not arch at all. Because of water conditions such as heavy surface clutter or thermoclines, the sensitivity sometimes cannot be turned up enough to get fish arches. For the best results, turn the sensitivity up as high as possible without getting too much noise on the screen. In medium to deep water, this method should work to display fish arches.
A school of fish will appear as many different formations or shapes, depending on how much of the school is within the transducer's cone. In shallow water, several fish close together appear like blocks that have been stacked in no apparent order. In deep water, each fish will arch according to its size.
Why Fish Arch
The reason fish show as an arch is because of the relationship between the fish and the cone angle of the transducer as the boat passes over the fish. As the leading edge of the cone strikes the fish, a display pixel is turned on. As the boat passes over the fish, the distance to the fish decreases. This turns each pixel on at a shallower depth on the display. When the center of the cone is directly over the fish, the first half of the arch is formed. This is also the shortest distance to the fish. Since the fish is closer to the boat, the signal is stronger and the arch is thicker. As the boat moves away from the fish, the distance increases and the pixels appear at progressively deeper depths until the cone passes the fish.
If the fish doesn't pass directly through the center of the cone, the arch won't be as well defined. Since the fish isn't in the cone very long, there aren't as many echoes to display, and the ones that do show are weaker. This is one of the reasons it's difficult to show fish arches in shallow water. The cone angle is too narrow for the signal to arch.
Remember, there must be movement between the boat and the fish to develop an arch. Usually, this means trolling at a slow speed with the main engine. If you are anchored or stopped, fish signals won't arch. Instead, they'll show as horizontal lines as they swim in and out of the cone.
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The part I left out is what's referred to as "peaking" your unit. Forgive me if this is old news for some of you guys but for me, peaking was critical to learning how to get the most out of my Furuno unit and I thought I'd mention it on this forum
Let's say you're in 30 M of water. The way you peak the transducer is by setting the range of your unit to double, or even triple the actual depth of the water you're in. This must be done in manual mode. You'll know your transducer is "peaked" when you see two bottoms (or three if you set the range to 90 M) on your screen.
Once you're seeing the multiple bottoms, you can adjust the sensitivity to clean up noise--you'll start seeing way more things "peaked" then if the range matches your actual depth. The double (or triple) bottoms are caused by the pings hitting bottom, traveling back up to the water surface, then making the return trip two (or three times) according to the range you dialed in.
Once you're peaked and the noise is cleaned up, you can start zooming in on the portion of the water column that shows promising sign
One last (editorial) comment: those crystal clear arches you see in many of the adverts for Lowrance and Hummingbird are screen shots taken in zero to 10 M of fresh water, the best of all conditions. Once you get at depth in salt water with plankton and low-light and interference from jellies and fecal matter from those guys in the boat next to yours, those picture-perfect arches you saw in the adverts I don't think are necessarily in the cards.
Off La Perouse last August hovering over a major jag of springs, cohos and sardines, I fiddled with a 1000 Khz transducer wired to a high-end Furuno color scope. Even with lots of fish at 10 - 20 M under my boat, I never saw the arches we all want to see. I saw lots of salt and pepper in various colors depending on the size and concentration of what was under me but never any arches.
However, with both rods going off all morning long and with more springs to my boat then any god-fearing man deserves, I knew it wasn't condiments for my curly fries I was seeing there on the screen.
good luck
