Features December 2002 Issue

Fishfinders (a.k.a. Scanning Depthsounders)

Why stick to a measly numerical readout if you can get a picture of the bottom? The Furuno LS6100 and Garmin 240 Blue finish one-two in our look at six units from $300 to $400.

Back in the mid-'90s, we compared cheap stand-alone depthsounders with cheap fishfinders. One of the conclusions we came to was that the discrete depthsounders were overpriced, and we could see no reason for it. The only explanation offered (in that case by the sales manager of Raytheon) was that instruments for powerboaters and sailors existed in "separate realms." 

A 25-foot center-console powerboat
served as the test platform. Trans-
ducers were all set to a frequency
of 200 kHz.

We concluded that depthsounders still had their place, especially when part of a larger instrument network, and they generally draw less power than fishfinders. However, we said, "a distinct information gain is obtained from having a fishfinder on board, if feasible."

The intervening years have done little to bring those separate realms together: Stand-alone sounders still cost too much, compared to fishfinders that use the same basic sonar technology and put the answers on a screen in great detail. Yet we regularly get letters from readers asking if they can install fishfinders when they have to replace their sounders: "Are fishfinders OK to use on sailboats? Are they only for powerboaters?"

Look how the separate realms confound us! Let's leap the boundary...

How They Work
Modern sonar equipment—a.k.a. fishfinder, bottom machine, echo sounder, depthsounder, or just plain sounders—uses sound waves to look through the water, then paints us a picture of what's below.

First, it sends a pulse of sound down, via the transducer. The sound bounces off objects in the water, like rocks, wrecks, and fish. Second, it measures the time that sound pulse takes to return. Finally, it uses that information to show us a screen display.

Sound waves reflect best off objects with densities different than the water's. So rocks, mud, metal, and the air-filled swim bladders of fish all show up well.

There are also side-scanning and forward-scanning sounders, normally found on commercial fishing, military, and research vessels. These are expensive units, but less expensive versions of this technology have trickled down to the recreational user. For example, one of the units we tested (Raymarine L470) does have the capability to side-scan with the addition of a special transducer.

We didn't test forward- or side-scanning sounders for this article, but we will when prices come down to a more manageable level. For this article, remember that everything on the screen is under the boat; if you're moving forward, it's essentially history.

Even with a restricted set of prices and features, sonar selection can be mighty confusing. Whether you use your machine to navigate the depths, determine bottom composition, or, for that matter, to find fish in some deep hole, you'll need to know the answers to some of the basic questions: Which transducer frequency should I use? How much resolution do I need? How much power do I need? Where should I set the gain? What the heck is that upside-down crescent shape on the screen? How different do these shapes appear on different machines? Other questions and variables include power output, transducer type selection, screen type, and screen resolution.

Let's start with screen type. All the units we tested have monochrome LCD screens. In the under-$400 range, these screens dominate the marketplace. The LCD screens are lighter, require less power, and are easier to waterproof than comparable CRT screens. The latest improvements in LCD technology such as glare-reducing coatings and internal backlighting make these screens easy to see even in direct sunlight.

Screen resolution is normally expressed in a horizontal and vertical pixel count, such as 234 x 320. This means 234 dots make up each horizontal line of the screen, and 320 dots comprise every vertical line. The horizontal pixel count tells you how much history your screen will show and how much screen space you will have for split-screen functions. We'll talk about some of them later.

The higher the vertical pixel count, the better the machine will be able to show a distinction between the sea bed and anything above it, especially in deep water. Here's why: Let's say you're scanning in 20' of water on a unit with 320 pixels of vertical resolution. Each pixel represents less than an inch of water depth, which means the machine should easily show even a small rock or object above the bottom profile. Now try the same thing in 300'. Now each pixel represents about a foot of depth, which necessitates the use of resolution-enhancing split screen functions to distinguish between the bottom and objects above it.

Four major functions increase a fishfinder's usefulness: bottom lock, zoom, A-scope, and whiteline. Bottom lock and zoom increase the screen resolution by showing only a portion of the water column on the screen. All the machines tested have zoom, but only the Furuno and Raymarine products have true bottom lock. The Humminbird Legend 2000 has a mode called bottom lock, but it could better be described as bottom zoom.

A-scope, called RTS (Real-Time Sonar) by Humminbird, shows in real time what is directly under the transducer. All except the Garmin and Lowrance have this function. As the machine receives information and writes a new vertical line of pixels to the right side of the screen, it also shows this information expanded horizontally as a split-screen. This horizontal expansion allows your eye to easily view this information.

Whiteline, which Lowrance refers to as Greyline, changes the way a machine displays the bottom echoes. With Whiteline activated, the bottom will show up as a thin black line under which varying depths of white or gray indicate strong or weak echoes. This makes it easier to determine bottom hardness and differentiate between bottom and objects close to the seabed. Aside from giving a view of rocks and obstructions, this can make a big difference in your anchoring decisions and techniques.

The maximum power output of most recreational depthsounders is between 100 and 1000 watts of RMS (root mean square) power. The definition of RMS power isn't important here. In this case we can take it to mean an average continuous power output. More power generally helps a machine work better in deeper water.

Transducers may vary from a pre-packaged unit selected for a specific machine to many other options. There are transom-mount, through-hull, and shoot-through units, usually in plastic or bronze options. (We used transom-mount transducers for this evaluation, for obvious reasons, and all the test units had optional transducers available, usually at an added cost.)

Frequencies from 28 kHz to 455 kHz are common, and some provide boatspeed and water temperature data. They vary from nearly the size of a football to smaller than a hockey puck. Lower frequencies penetrate deep water better, while higher frequencies show better detail. Some units are available with more than one frequency installed; three of the units we tested fall into this category, being dual frequency.

Virtually all displacement sailboats will need through-hull or shoot-through transducer mountings. The former will generally be more accurate; the latter is normally epoxied to the inside of the hull, and doesn't require drilling a large hole through the boat. Both types require careful locating and alignment. If you decide on the through-hull version, and need to install it in a hull cored below the waterline, you'll need to take great care to seal the core around the hole.

In real life, underwater features appear neither as clear nor as crisp as they do in the manufacturers' advertising. But here are a couple of operational tips that can help you use your machine more effectively. "Auto" is a good starting point for the novice, but eventually you'll be able interpret bottom features and machine capabiltities well enough to try your hand at the manual settings. Manual gain is available on nearly all echo sounders, and you can use it to control the amount of energy let into the machine from the transducer-sent sound wave. Without enough energy, you won't mark anything; with too much you'll mark everything. Neither extreme presents a useful picture.

There are different ways to set the gain manually; the simplest way is simply to increase gain until you begin to see "noise" in the water column, then back off the gain slightly—similar to adjusting squelch on a VHF.

Once you've mastered manual gain, you can begin to use your sounder to determine bottom composition for anchoring. This is a skill that takes some practice and experimentation to master. You'll probably find that the sounder is good at determining the relative hardness of the bottom it's painting by the varying thickness of the bottom return on the display. As long as you remain in the same depth with the same gain setting, a thicker bottom return would indicate harder bottom and a thinner return just the opposite—a softer bottom. Used in conjunction with the charted bottom composition and some experience, you should be able to tell the difference between rocky, muddy, and sandy bottom.

What We Tested
We tested six fishfinders. The Furuno LS-6100, the Garmin 240 Blue, the Humminbird Legend 2000, the Lowrance X71, and the Raymarine L470 fall into our target price range of $300 to $400 and represent the large marine electronics manufacturers. The sixth unit, the Navman 4200, sells for less than $200, but with its accompanying fuel meter option costing $150, it falls in the middle of our price range.

How We Tested
Even the best sounder in the world is of little use if you can't see the screen easily and clearly under all conditions. Each machine was given two ratings for "viewability," one for day and one for night. For the day rating, we viewed each machine on our test boat (a 25' Contender) from a normal operating distance in full sunlight, shade, and with and without polarized sunglasses. The night rating was done inside under darkened conditions. All machines were operated simultaneously side by side in simulator mode and adjusted for the best possible display during each test.

On-the-water testing was done in warm (85-90°F) salt water using the transom-mount transducer selected and supplied by each manufacturer. All testing was done using 200 kHz. Three machines had the capability to use 50 kHz as well, but we didn't test at this frequency.

Testing was accomplished by making multiple passes over known bottom structures in various depths. Our shallow-water test was done in about 7' over a small fiberglass boat wreck. Mid-depth testing was done over a large steel ship sunk in 120', and the deep-water evaluation was carried out while drifting over a steel structure in 290' of water. We set each unit to the Auto mode and adjusted the gain only as necessary.

Each unit was assigned a score for "ease of use." A unit earned a high score if our tester could operate most functions easily without reference to the manual. If even simple functions required constant manual review, scores were lower.

The warranty on each machine is listed in the accompanying table for both parts and labor costs for repairs.

We didn't test the fuel monitor option for the Navman unit, nor did we test the basic navigation capability of the Furuno unit. Our testing stuck with sounder performance only.

We tested only at low speeds, where the performance of any sounder is best. On a sailboat at planing speed, as on a powerboat at high speed, one should expect some decrease in performance even with a good installation.

We didn't rate installation difficulty, as this would be different boat-to- boat. Installation factors include the locations of transducer and screen, and the wiring runs.

Power consumption as stated by the manufacturers of all units tested was one amp or less.

Furuno LS-6100
This is the largest sounder we tested; it has the biggest screen, and the most diverse menu of options. When purchasing this unit, you must select a transducer. Our test unit was supplied with an Airmar P66 transom- mount transducer with speed and water temperature included, which sells for $80. When added to the $279 price of the display unit, the total was $359, well within our price range.

The LS-6100 is a dual-frequency unit with 300 RMS watts of power output and the highest resolution screen of any unit tested. It performed well in all testing and carries an above-average warranty. Deep-water testing was done in Auto mode with the frequency set to 200kHz. The unit showed good detail of the wreck, and we would expect it to read the bottom in far deeper water using its 50 kHz capability.

Screen backlighting in the Furuno evenly illuminates the entire screen and has 10 levels of adjustment. Some of the features found on the Furuno include user-adjustable depth ranges, two auto modes, a reverse video option for night viewing, auto gain offset (which allows you to tweak the gain even when it's in auto mode), separate gain adjustments for each frequency, and multiple inputs for speed and temperature. This is just a sampling of some of the features most of the other machines tested do not possess.

Bottom Line: A brawny machine with all the bells and whistles of a much more expensive fishfinder. It's our top pick and Best Buy. The display is quite big, however. Space considerations could make you decide to go smaller.

Garmin 240 Blue
This mid-size unit carries a smaller-than-average screen (though with good resolution) and an intuitive menu. The Garmin fishfinder package comes with the same Airmar P66 transducer as the Furuno, but the speed and temperature sensors are supplied in a separate transom-mount unit. Screen backlighting in the Garmin is bright and evenly illuminates the entire screen. It has 11 levels of adjustment. On-the-water performance of this dual- frequency 500 RMS watt machine was equal to the Furuno in all tests, and like the Furuno we would expect this machine to be useful in much deeper water than the maximum depth we tested.

Bottom Line: The Garmin lacks some of the features found in the Furuno unit, but is a robust performer, and our second choice. If you don't have room for the Furuno, this would be a good bet.

Humminbird Legend 2000
The Legend 2000 is a middle-of-the-pack machine, size-wise, screen-wise and performance-wise. It's a single-frequency unit with 500 RMS watts of power and a smaller transducer than the other high-power machines. The speed and temp sensors are included, but supplied in a separate transom-mount unit.

In the deep-water test it really ran out of gas; we had to use full gain to mark the wreck. For sailors, this may make less of a difference than it does for deep-water fishermen.

The screen on the Legend 2000 is not as bright and easy to see as some of the other units. In addition, the backlighting comes from the right-hand side of the screen and illuminates it unevenly.

Unlike the others, the Legend 2000 is controlled by knobs. This is preferable to pushbuttons, in our opinion, especially when adjusting gain.

Bottom Line: The machine has potential, but in our opinion it needs a better transducer and some tweaking of the screen to be competitive in this price range.

Lowrance X71
This is another middle-of-the-pack machine, both in size and performance. It's a single-frequency machine with 188 RMS watts of power. The supplied transducer is on the smallish side with a temperature sensor included. The speed sensor is optional. Screen resolution is the lowest in its class at 160 x 160, producing a somewhat blocky appearance. As with the Humminbird, the screen's illumination originates from the right, and was uneven. It has no level adjustment— just on or off. Tonal quality is not on par with the better units in this group, and the screen really has just two tones, black and white. It achieves a middle gray tone by rapidly turning selected pixels on and off.

Even with the small transducer and relatively low power output, this unit did mark the wrecks, even in the deep-water test. But the display is not nearly as good as the Furuno's or the Garmin's.

Bottom Line: We think this sounder is overpriced for what you get.

Navman 4200
This is the smallest and least expensive unit tested. The 4200 is packaged with a small transducer, which includes both a speed and temp sensor. It's a single-frequency unit with 150 RMS watts of power.

It performed adequately for a machine in its price range, but had a glitch. Whenever the machine painted hard bottom structure, like the test wrecks, it would blank out part of the display beneath the structure, causing a loss of detail in the picture. This can be seen in the accompanying photo. The screen also has a major flaw that led to the poor ratings on both day and night view testing—it flickers. This did not occur all the time, but was a problem during testing. We don’t know if the machine picked up interference from the test boat’s electrical system, but we do know none of the other units hooked up to the same power supply had the problem. The backlighting comes from one side of the screen, illuminates unevenly, and is extremely dim.

Rus Graham, V.P. of Marketing at Navman USA, Inc. reviewed our comments on the 4200, and after talking with Navman engineers, had these comments on the three main issues we raised. Regarding the screen blankout he said, "Bottom blanking was developed to reduce entry-level user confusion with unnecessary data. Based on user tests, this feature was not embraced by more seasoned users and engineering scheduled the removal of the feature." He addressed the backlighting and screen flicker issues by saying, "At the same time we cut over to the new version of BIOS [Basic Input/Output System], we will introduce our second generation of backlighting. It will incorporate a new diffuser to more evenly spread light across the screen and increase its overall brightness. The screen flicker you experienced is less straightforward. Based on our tests and observations, engineering believes that our display performs comparably to competitors displays at a similar price point."

Bottom Line: We would wait until Navman makes the changes outlined by Mr. Graham before considering the purchase of this sounder.

Raymarine L470
This is the most expensive unit tested. It's about average size, but has one of the larger screens. Too bad the resolution at 240 x 128 is well below the best units. The L470 came equipped with an Airmar P65 transducer, which includes speed and temperature sensing. It is a dual-frequency unit with 500 RMS watts of power and an abundance of features.

Like the best units tested, the backlighting evenly illuminates the complete screen and has six levels of adjustment—but even at its brightest it failed to equal the Garmin and Furuno.

Two quirky things about the machine: First, the mounting bracket is not secure enough and picks up any boat vibration that may be present. Granted, this would be a non-issue if you flush-mount the display. Second, the cable connections are the push-in types that use an O-ring lock. We'd rather see a positive mechanical lock, as featured on most of the other units. On the plus side, the warranty is the best of the six tested products.

We asked Raymarine to review our comments. Product Manager Morton Andreason responded, "Although I don't disagree with most of your conclusions, the product is obviously getting a little dated. I do feel the review focused too much on the negative aspect of the product, and ignores many of the unique strengths of the L470." (He mentioned the 500W RMS of power at both frequencies and PVDF Sidelooker transducer option, which he said "offers the best Sidelooker option in the industry."

Bottom Line: At this price, we'd expect better attention to details like the bracket, and better screen resolution. This unit only has about half the total pixels of the Garmin and about 40% of the Furuno.

In fishfinders, the little things one unit does, and another doesn't, can make a world of difference. Among the sounders we looked at, shallow-water performance was acceptable across the board, but displays varied widely in viewability. This can be difficult to judge in a store when initially viewing them under fluorescent lighting.

The Furuno LS-6100 is our top pick. Its superior screen, good performance, and depth of features put it over the top. If you don't have the room for the Furuno, we'd recommend the Garmin 240 Blue.


Also With This Article
Click here to view "Value Guide: Fishfinders."
Click here to view "Performance: Fishfinders."
Click here to view "Cruz-Pro's PC-Based Sounder."

Contact- Airmar Technology, 603/673-9570; www.airmar.com. CruzPro Ltd., 011-64-9-833-5065, www.cruzpro.com. Furuno USA, 360/834-9300, www.furuno.com. Humminbird, 334/687-6613, www.humminbird.com. Navman USA, 866/628-6261, www.navman.com. Raymarine, 603/881-5200; www.raymarine.com. Garmin International, 913/397-8200; www.garmin.com. Lowrance Electronics, 800/324-1356; www.lowrance.com.

Comments (0)

Be the first to comment on this post using the section below.

New to Practical Sailor?
Register for Free!

Already Registered?
Log In