Sonar
Sonar or so(und) na(vigation) r(anging) works in a manner similar to radar, except sonar uses pulses of sound waves underwater to find the distance to a sound-reflecting target. Since the speed of sound is about 196,000 times slower than light (in sea water), the response time for sonar is proportionately longer.
Fir was last refit with a fathometer that used sonar to report the depth. The system does not work and is not worth repairing. A factor in our deliberations is the safest way to install new sonar systems. It would be necessary to haul the boat out in order to drill a new hole in the bottom of the hull. Haul out alone is more than $38,000 therefore, we want to use haul out as an opportunity to install all the transducers anticipated for our future use. There have been tremendous advances since 1988.
Depth
A fathometer is a critical piece of equipment for Fir. She draws 11.5 feet. The continental shelf drops off quickly in the Pacific which means that deep water is relatively close to land. In contrast, on the East Coast the slope is gradual. Outside commercial shipping channels, there is a lot of east coast water less than 15 feet in depth. We intend to operate in the Chesapeake Bay with conditions of less than 20 feet of water.
Fish Finders
Most recreational sonars began as fish finders. Fish have an air bladder to regulate their buoyancy, and early recreational sonars were designed to identify this void (air pocket). Rudimentary systems showed the bottom of the water column and red blobs indicating the position of air bladders. These displays were difficult to read for the uninitiated. Fortunately, modern-day recreational sonars are much more sophisticated. CHIRP systems send signals at a number of frequencies.
Forward Looking
Forward looking sonar sounds great giving one the impression that the system will "look forward" to identify reefs, submerged and floating objects for the ship's operator to avoid. The fact is that systems aimed at the yacht market (e.g. EchoPilot, Garmin, etc) are not really designed for this task. For physical reasons, most systems are limited to 8-10 times water depth. Therefore, in 15 feet of water, the system will see obstructions 150 feet in advance of the boat. Fir can't alter her course in this short span so the information only allows you to brace for impact. In addition, these systems have trouble near the water line where partially submerged shipping containers and floating logs will go undetected.
There is a UK company that produces forward looking radar for the commercial shipping industry. The systems purportedly solve a number of the limitations listed above. The cost of this system is prohibitively expensive making no sense for Fir's intended activities. Our understanding is that the company is trying to develop a less expensive system aimed at the yacht market, but it is not for sale today.
CrowdSourcing
Soundings on charts can be dangerously incorrect. Not only is the information old, but the areas where data is most needed often will have shifting shoals. The idea of crowdsourcing is that numerous participating boats will upload their soundings to the internet so that it is available to other boats. In theory, this will produce up to date soundings. The participating boats use their GPS and depth gauges to create a more accurate map of the area under their boats. The problem with crowdsourcing is that you are relying on the quality and installation of a system and a source you know nothing about. If the adjustment between the transducer depth and waterline is incorrect, then the crowdsource data may be less accurate than the official chart.
For the above reasons, we are not interested in traditional crowdsourcing. However, we are very interested in utilizing the technology internally. We intend to add a high end CHIRP sonar unit to our tender. As we near shallow waters we will dispatch the tender to map the bottom of our route and upload the results to Fir using the crowdsourcing software. This will allow Fir to operate on the East Coast, the Caribbean, and in rivers without fear of running aground due to outdated charts.
Choices
The thinking process begins with the high cost of hauling the boat out of water. To be cost effective we should install the most sophisticated transducers/systems available.
Fir will be installed with a simple depth/temperature transducer as a backup system. The depth transducer will report directly to the N2K network. We will also install a modern CHIRP transducer able to map the bottom at much greater depths. We intend to contact _________ and see if their newer forward looking sonars are reasonably priced.
For reasons stated above, the more sophisticated sonar system will be installed on our tender. We will install a forward looking sonar as well as a full CHIRP