How to Use a Simple Depth Finder From a Small Jon Boat

A basic depth finder can make a small jon boat much more useful on an unfamiliar lake. It won’t tell me exactly where every fish is, and it doesn’t need a screen full of advanced menus to earn its place on the boat. Its main job is simpler: show me how deep the water is, how the bottom changes, and where I should spend more time casting.

For beginner boat anglers, the biggest improvement usually comes from learning to read the water beneath the boat rather than chasing every mark on the screen. Once I can identify a drop-off, a point, a creek channel edge, or a firmer bottom patch, I have a practical starting point for finding bass, crappie, bluegill, and other freshwater fish.

Start with a clean, reliable setup

Most entry-level depth finders use a transducer to send sound waves downward and display the returning signal as depth and a scrolling picture of the bottom. A small black-and-white or color unit with basic traditional sonar is plenty for learning.

On a jon boat, the transducer is commonly mounted in one of two places:

On the transom: A transom-mounted transducer is usually the simplest option for a boat with an outboard or trolling motor that lets the boat move steadily forward.
On the trolling motor: This puts the sonar view where the bow angler is fishing, which can be useful when working shoreline cover, points, and brush slowly.

The important part is giving the transducer a clear view of the water. It should sit below the hull where it stays submerged, away from turbulence caused by rivets, strakes, or the propeller. If the screen loses depth whenever I accelerate, I first suspect turbulent water passing under the transducer rather than a bad unit.

Keep the power cable and transducer cable tidy but avoid sharply pinching or cutting either one. A dedicated small marine battery or a properly protected boat battery connection helps prevent frustrating shutdowns and interference.

Before wiring up at the ramp: Check your depth finder manual for the recommended fuse, battery connection, and transducer mounting position. Also confirm your state’s current requirements for required safety equipment and boat operation on the lake you plan to fish.

Learn what the screen is actually showing

Traditional sonar doesn't produce a live, side-view picture of fish and structure. The right edge of the display is the newest information. Everything to the left is history scrolling across the screen.

If the bottom line slopes from shallow to deep as the screen moves left, that means the boat traveled over a drop-off. If the bottom rises toward the top of the display, the boat moved into shallower water.

This is a key idea: the screen shows what passed under the transducer, not necessarily what is directly under the boat at this exact second. In a slowly drifting jon boat, the delay is short, but it still matters when I’m trying to line up a cast with a brush pile or ledge.

A basic screen usually gives me these useful pieces of information:

Current depth: Often displayed as a large number.
Bottom contour: The continuous line or band near the bottom of the screen.
Bottom signal strength: A thicker, darker, or brighter return can suggest a harder bottom.
Targets in the water column: Marks, arches, or blobs that may be fish, bait, vegetation, or debris.
Water temperature: Available on many transducers and useful for noticing broad patterns, though it isn't a fish-catching command by itself.

I recommend starting in the automatic depth range and automatic sensitivity mode. Those settings let the unit adjust as the boat moves from a shallow bank into deeper water. Manual adjustments are useful later, but they can make a simple unit harder to interpret when I’m still learning the basics.

Use depth to narrow down where to fish

Depth alone doesn't tell me where fish are, but depth changes give fish routes, feeding areas, and cover options. On a freshwater lake, I look for places where shallow and deep water sit close together.

Find the first useful break from shore

Rather than idling randomly across open water, I start near an area that looks promising from the bank: a point, a creek mouth, a riprap stretch, a dock line, or the outside edge of a flat.

Then I idle slowly from shallow water toward deeper water, watching for the first noticeable change. That might be a gradual slope from 4 to 8 feet, or a quick drop from 5 to 15 feet. Either can be worth fishing, especially if there is cover nearby.

When I find a break, I make a second pass across it from a different direction. One pass tells me it exists; the second helps me understand its shape. Is it a straight bank drop, the tip of a point, or a ditch leading toward a creek channel?

Read points and flats

Points are especially easy to understand with basic sonar. Idle from the shallow end of the point toward deeper water. The bottom should gradually fall away. Make passes along both sides, because one side may drop faster or hold more rock, brush, or bait.

Flats can look boring at first, but their edges matter. A wide 6-foot flat next to a 15-foot channel is more useful information than the flat by itself. Fish may feed on the flat at certain times and slide toward the deeper edge when conditions change.

Use your map, if available, with the depth finder rather than treating the screen as a replacement for a map. A paper lake map, a phone mapping app, or a simple GPS display can help me identify likely points and channels. The sonar then confirms what is actually beneath the boat.

Recognize bottom hardness without overthinking it

Many basic units display harder bottoms as a stronger return: often a thicker, darker, or brighter bottom line. A soft, silty bottom may show as a thinner or less intense line.

This isn't a perfect test. Sensitivity, depth, boat speed, and unit settings all affect the appearance. Still, comparing nearby areas under similar conditions can be useful.

For example, if I idle across a point and see a stronger bottom return on one side, that could indicate rock, gravel, packed clay, or a firmer bottom. That is worth a closer look, particularly around spawning areas or places where crawfish and bait may be present.

Some displays show a second, fainter bottom line below the real bottom. This is commonly called a second echo. It can occur when the sonar signal bounces off a hard bottom and returns again. A visible second echo often supports the idea that the bottom is relatively hard, but I treat it as a clue, not a final answer.

The most reliable approach is to compare signals and then fish the area. If one bottom transition keeps producing bites, I remember what it looked like on the screen and look for similar water elsewhere.

Interpret fish marks carefully

The fish symbols on a basic depth finder are tempting, but I don’t rely on them too heavily. Fish ID features turn sonar returns into little fish icons. That may look simple, but the unit is making an educated guess based on the strength and shape of a return.

A return could be a fish, but it could also be a stick, a clump of weeds, a turtle, suspended debris, or a bubble trail. Turning Fish ID off, if the unit allows it, often gives a more honest view of the raw sonar returns.

On traditional sonar, individual fish may appear as arches when the boat passes over them. A full arch generally requires the fish to pass through the center of the sonar cone while the boat moves. Partial arches don't mean the fish is only half there; they often mean it passed through the edge of the cone.

For practical fishing, I pay more attention to patterns than individual marks:

Is bait or activity close to a drop-off?
Are marks holding near the bottom, suspended midway down, or near the surface?
Is there a brush pile, weed edge, or dock near those marks?
Are similar marks appearing on several nearby points or channel edges?

A screen full of marks in featureless water may be less useful than a modest cluster of marks near a clear depth change and cover.

Set a sensible boat speed and screen view

A jon boat is well suited to slow scouting. I generally idle at a steady pace that gives the screen time to draw a readable bottom contour. If I move too fast, the display can become harder to interpret, and turbulent water may interrupt the reading.

Use the chart speed setting that makes the image look natural rather than stretched or compressed. On many simple units, the default or auto chart speed is a good place to begin. If a stump, brush pile, or small hump flashes by too quickly to understand, slow down before reaching for complicated settings.

I also keep the depth range broad enough to show the water I expect to fish. In 5 to 20 feet of water, a range that includes that full zone makes contour changes easy to see. If the screen constantly resets because the range is too narrow, switch back to auto range or widen it manually.

Turn screen information into a fishing plan

The depth finder becomes most useful when I stop staring at it and make a decision. After idling over a likely spot, I mark the location with a waypoint if the unit has GPS, toss a marker buoy where appropriate, or simply line up the shoreline with a visible reference.

Then I fish the feature deliberately.

On a shallow-to-deep break, cast from shallow toward deep and work the lure down the slope.
On a point, make casts across the tip and along each side before leaving.
Near a brush pile or stump, approach quietly and make several casts from different angles.
Along a channel edge, follow the contour and check for spots where cover, bait, or a sharper bend adds something different.

A simple depth finder is best at helping me eliminate unproductive water. It tells me whether I am actually fishing the drop-off, flat edge, or cover I had in mind instead of just hoping it is somewhere nearby.

Keep the first trips simple

For the first few outings, I would give myself one goal: identify three features on the lake and fish each one. A point, a drop-off, and a flat edge are enough. Take note of their depths, bottom appearance, nearby cover, and whether they produced bites.

Don’t worry about mastering every menu option. Run the unit in auto mode, keep the transducer reading cleanly, and learn to recognize changes in depth and bottom shape. That foundation will do more for your fishing than a screen full of settings ever will.

Once those basics become familiar, a small jon boat and a simple depth finder can cover a surprising amount of useful water—without turning a quiet day on the lake into a troubleshooting project.