Forget the forward looking sonar. I have an Interphase Probe forward looking on my big cruiser. It is a nice toy, and it can be useful if you are poking around looking for a distinct spot for fishing/diving. It is useless for avoiding objects if you are going at any significant speed. The basic problem is that it takes it several seconds to make a sweep. In that several seconds, you will have moved the distance that you are looking ahead. If you are going at 20 MPH it wouldn't even have time to sound an alarm before you were on what it might have seen. At 5 MPH the alarm might sound first but it would still be too late. If you are interested, I can give you a detailed write up on the use of such a device. I wrote this several years ago and it includes the times it takes to make a scan at several different depths and resolution. Note that there isn't anything wrong with Interphase per se. What is wrong is that sound only travels so fast and it takes time to send a sweep of pulses out and wait for the return. A military grade unit might be able to send many pulses out and track each one independantly, but a recreational priced unit can't do that. Any scanning sonar for recreational use will have this limitation.
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