Garmin Sonar for Cruising

[AnsleyS]

Garmin devotes most of their sonar marketing to fishermen but we have put together a Garmin system on our Krogen 42 and are evaluating it for cruising. We bought a 1983 Krogen and removed the old electronics and installed a Garmin system with three multifunction screens, a GPSMap 8616xsv, GPSMap 8612xsv, and a GPSMap 7407xsv. The “xsv” suffix indicates that the screen can receive and display high definition sonar signals. The system also includes a GMR Fantom 24-inch radar dome, a AIS 800 Transceiver, an autopilot with the Reactor 40 compass, the GHP Smart Pump v2 and the GHC20 control head in the pilothouse. These are connected with the NMEA200 hub and a high speed network through the GMS-10 Network Port Expander. There is an AIS antenna on the mast and an external GPS antenna for the AIS. There is a GC200 IP camera covering the stern and two VHF radios with position data vis our only NMEA0183 connection. The sonar transducers are the GTS1M-THP which are a pair mounted on either side of the keel and are an all-in-one transducer set for (1) mid-range CHIRP traditional, (2) higher frequency “ClearVu” and (3) a flexible frequency side view. Also mounted is a forward scanning sonar with the Panotix PS51-TH transducer. This system has so many data paths that I needed to map it for clarity.

As we started to use this system, we started to experiment with the different views and their use and then we started to vary the frequency of the transmission and found some interesting things. We started to be able to scan an anchorage to look at the bottom before setting the hook and we started to wonder if we could judge the density of the bottom using different frequencies. This is an ongoing project so I welcome your thoughts along the way.

[Kettlewell]

I would imagine this could work since we get a similar but limited functionality using a dedicated fishfinder, an old Raymarine b&w unit. I do find the bottom information very useful. For example, I can tell when it is weedy, when the bottom is rocky, and also see the general depth trend. All very useful when anchoring or in shallows. It can also be disconcerting, because it reads the tops of dense weed as the bottom sometimes, indicating shallower depths than we have, but I can tell by the texture that it is weed.

[AnsleyS]

The transducers that we installed included the Garmin GT51M-THP Port and Starboard pair which are all-in-one, Traditional, CHIRP, “SideVü”, and “ClearVü” variable frequency transducers that contain a fast response water temperature sensor. The variable frequencies enable different views of the bottom. Generally, the lower the frequency the more penetrating power in the beam and the higher the frequency the more detail is shown. CHIRP (an acronym for Compressed High-Intensity Radiated Pulse) is a system that sends out a multi-frequency pulse over time which results in an improvement of the image you see.

The Garmin GT51M-THP transducers offer the “Traditional” CHIRP 80-160 kHz transmission in a cone pattern which is 24° forward/aft by 13° wide; the “SideVu” CHIRP 260-455 kHz, oval pattern 2.0° forward/aft x 51° wide; and the “ClearVu” CHIRP 455 kHz, oval pattern 1.4° forward/aft x 29° wide. These give different views when in use.

An example was when we were cruising down the Ottawa River from Ottawa by the Petrie Islands we encountered a rippled sand bottom contour that changed depth by four to six feet as we passed over the ripples. These big ripples are similar to smaller ones that we saw on the beach in Prince Edward Island. The Traditional CHIRP picture shows the bottom variation directly under the boat with the 80-160 kHz transmission and the “SideVu” shows a 51-degree fan at 455 kHz. The higher frequency reveals more definition.

[sv_pelagia]

Hmmm... Those sonar transducers might not last long out here in British Columbia... abundant flotsam (think logs) would take then out.

[wrwakefield]

I don’t have experience with the Garmin equivalent, but did install a Navionics Forward looking SONAR transducer in 2017. [2D vs. the 3D offered by the more expensive systems.]

I find it quite handy as yet another aid to navigation. It helps with hints at bottom type [a learning curve since it requires physical comparison with bottom samples on the anchor] and is quite handy in shallower water- if one goes slow… [it only looks ahead 4-6x water depth…]

I also use it to map the bottom when anchoring, and can often do so by just turning 360° in place [back and fill under power] where I plan to drop the hook in deeper waters.

The Navionics forward scanning transducer is designed to shear off flush with the fairing block [which has a 3/8” through bolt on its leading edge to help keep it in place] if it encounters enough resistance- purportedly leaving the plug in the through-hull intact to avoid water intrusion after shearing. [As yet untested- despite plenty of opportunities…]

I would love to install the CHIRP transducers. Our hull shape will require a pair of them.

In case this is useful.

Cheers, Bill

PS: No idea why the images are presented upside down…