Sharper Pictures
Marine electronics is an exciting area for anyone with an interest in technology, and the underwater images now available on a MFD are astounding. To get optimum results, though, you’ll need a hot transducer.
What would previously have been just a ‘fishfinder’ is now a Multi-function Display (MFD), and usually includes multiple fishfinding and sonar capabilities as well as GPS with chartplotting, engine management, radar and autopilot integrations.
Most exciting for fishermen, though, is the range of ultrasonic capabilities available. Rather than just a single sonar beam pointing downwards, there are now numerous technologies available. Conventional two-dimensional fishfinding displays have been enhanced with Broadband and CHIRP (Compressed High Intensity Radar Pulse) technologies.
Both technologies take the same idea of a downward pulse of ultrasound, but by varying the intensity, frequency and duration of that pulse the clever electronics are now able to produce a much clearer picture of what lies below.
Broadband is a solution that varies the pulse width according to the conditions and depth to produce an optimal signal. CHIRP transducers, by comparison, transmit a longer duration pulse that changes frequency, resulting in greater resolution and depth capability.
In addition, side-scanning and down-scanning sonar provides a 3D view of structures below and to either side of the boat. This allows the skipper to not only see what is directly below the boat but also potentially interesting structures to either side. For scuba divers this is also a great feature, since points of interest like caves can be identified before jumping overboard.
The trouble, though, is that each of these technologies requires different transducer capabilities. While broadband and CHIRP can be supported by the same conventional type transducer (provided it supports the required frequencies), the side-scanning option requires a pair of long transducer elements mounted longitudinally with the hull.
Downscanning, which produces a 3D image of what is directly below the boat rather than to the side, also requires a special transducer which can fire a sonar beam in a specific pattern.
Depending on the brand of MFD you’ve fitted, your display may already support the additional technology. You may have the option to add an additional transducer, or replace your
Depending on the brand of MFD you’ve fitted, your display may already support the additional technology.
existing one with a multi-function transducer which provides two or more technologies in one unit. In some cases an extra interface is required, such as the Sonarhub module for older Lowrance displays, or a software update may be required.
If you have one of the latest-generation Lowrance or Simrad MFDS, you can get all four main functions in one single ‘Totalscan’ transducer. This means that instead of two or more transducers, a single unit can provide all four functions as well as measuring water temperature.
The only downside is that the combined unit comes in one size (and thus power range), which is ideal for recreational boaties but may not suit those who like to fish really deep. In that case a high-power transducer, such as the 1kw units from Airmar, may be more appropriate.
INSTALLATION Having decided you need a new transducer, the next step is installing it. This is absolutely the most critical part of the entire fishfinder solution, and getting it wrong will lead to poor results and frustration.
To function well a transducer requires undisturbed water, and while this is easy when the boat’s at rest, many boats produce aerated water along the hull when under way. The traditional transom-mount transducer is the easiest to install on most vessels, but may also give the poorest performance.
At high speed the transom-mount transducer may not even be fully in the water, and many boats suffer from this problem. If you have noticed that your existing fishfinder won’t give a picture when the boat is moving fast, this is the probable cause. For larger vessels, an in-hull or through-hull installation may be better than a transom mount.
Shoot-through transducers (mounted inside the hull) can be used on solid hulls, either solid glass or aluminium, although not on timber hulls or those with soft-cored fibreglass construction. Properly installed, this transducer type is arguably the best option since the transducer itself
does not disturb the water flow, and it can be located forward of the engine and so it’s always in clear water.
Also, since they do not require a hole to be cut into the hull, there is no potential for a water leak at the mounting point. Angled units can be purchased to match the hull profile, and they can be installed off-centre so long as the keel is not in the line of the sonar beam.
When choosing a placement, make sure the unit is not trying to shoot through a chine or is close to a hull fixing, both of which will also affect the resulting image.
Through-hull units are the next option, and these can either be flush-mounted on a suitable horizontal surface, or fitted into a fairing block to correct for the angle of the vee on the hull.
A hole needs to be drilled through the hull for the transducer mounting screw, and the cable is fed through this with appropriate sealant. Many high-power transducers are available as through-hull mountings, since their target market is often larger commercial vessels.
The final option, which generally applies to the large, flat side-scanning transducers, is to surface-mount the unit. Again, this is only an option if a perfectly horizontal surface is available since the side-scanning capabilities requires a clear view to either side of the boat.
A surface mount does leave the transducer exposed and liable to be damaged if that part of the hull ever touches an object such as a trailer, or if the boat is ever beached, but that shouldn’t concern a launch owner.
Our project boat was coming out the water for new antifoul – a perfect opportunity to replace the aging fishfinder which came with the boat.
We spent some time deciding on a unit, eventually selecting the latest Simrad XSE package, which included its Totalscan transducer. Space allowed us the luxury of a larger display, so we opted for the largest model in this range, the GO9 with a 9-inch touch screen.
Because the boat is a glass-over-ply construction, a shootthrough installation was not an option. Also, the very narrow catamaran hull meant that the only transom mount options were very close to the propeller, so clear water while underway could not be guaranteed.
On the other hand, each hull has a perfectly flat bottom which runs horizontally while underway, so either a throughhull or a surface mount were the best options.
So we chose to surface-mount the Totalscan transducer, ordering the optional mounting bracket with the package. This transducer is not yet available in a through-hull version, although the separate Structurescan unit provides similar capabilities with a through-hull mount option. With a relatively thin plywood hull the potential for transducer damage would be the least of our worries if we ever beached the boat!
The bracket could therefore be screwed to the hull using short stainless screws that did not penetrate through the plywood skin. Sikaflex sealant was applied to each screw hole and the unit was attached.
Because the Totalscan transducer can be mounted with the cable exiting either end, we positioned it with the cable to the rear so it could be run up the transom the same way the previous transom-mount transducer had been. It was then a fairly easy job to feed the cable through to the helm and display unit.
Job almost done. It is important not to apply a solvent-based antifoul to the face of the transducer, since the chemicals can attack the plastic. Also, a soft (ablative antifoul) surface may absorb some of the signal and reduce effectiveness.
West Marine produce a special water-based transducer antifouling, which is reasonably priced and works well. We applied this to all exposed parts of the transducer surface. We used normal antifoul over the steel bracket and on the cable.
Now we can’t wait to get her back in the water and see our gorgeous new display showing 3D colour images of everything around the boat. BNZ
Don’t apply a solvent-based antifoul to the face of the transducer – the chemicals can attack the plastic.