Our Furuno Scanning Sonar has gotten a real workout in the Tuamotus. To put this in perspective, the following is a screen shot of our Furuno chart program showing Rangiroa – the largest and most populated of the Tuamotus. The lagoon is so large that the entire island of Tahiti could be placed within its confines.
Zooming in a bit on the northern bit of Rangiroa, you can see that the vast majority of the lagoon is unsurveyed with a few random soundings. You can also see our track down to Blue Lagoon on the bit that is charted. What you can’t see is that the first third of the trip is wall to wall pearl farms with random buoys every where.
This is pretty much typical of the lagoons here so eyeball navigation and the scanning sonar are the tools of choice. As a consequence, we have been refining out techniques when it comes to the use of the sonar, which I will get to in a moment, but first, a bit of a geometry lesson on exactly what the sonar shows.
I apologise for the quality of the drawing, but it is the best I could manage with the tools at hand. In the drawing, the top horizontal line represents the surface of the water and the bottom horizontal line represents the bottom. Our little boat is positioned at the top left. The angled line is the sonar beam. While the beam is actually more like a cone, for this illustration, we will consider the beam to be like a laser beam. The tilt angle of the beam can be adjusted with 0 degrees being horizontal and 90 degrees being vertical. In this case, the beam angle is something like 15 degrees. The range (how far out the beam extends) can also be adjusted so the combination of range and tilt angle determine how far down the beam sees. The gain control is used to adjust the intensity of the beam with the ideal being strong enough to give good returns but not so strong as to generate false echoes.
In our example, I have portrayed two coral heads. The black one is low lying and the red one extends almost to the surface. Considering the black one first, when the beam first encounters it an echo return is displayed on the screen. As the boat moves toward the black coral head, you can see that it reaches a point where the coral head lies below the beam and, as such, is no longer visible on the display. The way this plays out on the screen is that the return from the coral head gradually moves in on the screen as the boat approaches it. At the same time, the return becomes less intense as the boat approaches it and it finally disappears indicating that the boat will safely pass over it.
In the case of the red coral head, as the boat approaches the head, the return moves closer on the screen and actually becomes stronger. In this case, we are going to hit it and its time to alter course before doing so.
In terms of actually setting up the sonar, there are more adjustments – the first is the direction in which the beam is pointing (Training) which in most instances is pointing directly off the bow although any angle relative to the bow can be selected. The second adjustment is Sector Width which refers to the number of degrees left and right of the training direction the unit will scan. In most instances, we set this to 90 degrees although anything up to 360 degrees can be selected. The result is that the sonar is scanning 45 degrees either side of the bow which is what we really need to focus on when navigating in uncharted waters.
Other adjustments include tilt angle, the angle at which the beam is pointed down from horizontal, range, how far out we are looking, and gain which is the power of the beam. In reality there are practical limitations here. In shallower water (less the 20 meters) wee can only look out 200 meters under ideal circumstances with 120 meters being the norm. In shallow water the tilt angle must be kept low to see out far but the practical limit is around 5 degrees before reflections from the surface degrade the image.
Historically, I have always set the tilt angle so I could clearly see the bottom reflection like the image below. When using the sonar for long periods of time, however, this complicates interpretation of the image as one is always examining changes in the image that in reality have no real impact on safety.
We have therefore shifted to a new strategy where I do not tilt the beam down far enough to return a bottom image. Point being, we are only concerned about objects that raise to the level of a threat for impact. As such, I can safely forget about anything more then 10 meters deep and concentrate only on objects which are shallower then this. The result of this strategy is a blank screen most of the time which requires no interpretation. Objects which do result in returns are monitored to see if they are increasing in intensity or decreasing in intensity as the boat approaches. Like our simple drawing above, objects which slowly disappear as we approach them are safely ignored while those that don’t are given a wide berth. To aid with safety distances, I set a range ring that defines the distance at which we will initiate a course change if an object appears inside the ring.
When anchoring, we set the sonar in a 360 degree scan mode around the boat to look for obstructions. In the image below, you can see a three coral heads between 9 and 11 o’clock but they are well outside the range ring of 46 meters which represents our swing radius.
For the technically inclined, we have found that using short pulse length and max gain give us the best results. To insure that I have the gain set correctly, I will occasionally tilt the beam down until I get a solid bottom return and adjust the gain accordingly and then return to my shallower tilt angle. If objects do appear to be problematic, I have one of the user defined buttons set to vertical mode and when the beam sweeps over an object of concern, pushing the button does a vertical scan in that direction so I can can get a good approximation of the actual depth of the object.
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