Accuracy of Depth/Fish finders

[Lou C]

Today while leaving our harbor in the marked channel, the reading on our Hummingbird fish finder dropped from about what it should be down to 3 feet or so. I trimmed up the drive really fast, and moved to and area of deeper water. I really could not believe that the water at one hour past high tide could be that shallow in the marked channel. Later on with the boat stationary, I measured the depth with a boat hook and compared that to the reading on the fishfinder, and they were within one foot of each other. The transducer is on the transom near the speedometer tube. I wondered if trimming up the drive just prior to this may have caused enough turbulance to give the low reading. Any ideas?

 

[FlyBoyMark]

Re: Accuracy of Depth/Fish finders

Keep in mind...the transducer reads the depth from where the transducer is and not where the bottom of the boat or outdrive is. Measure from the transducer to the water line and adjust on your sonar's offset keel setting for proper depth reading. Some people use the bottom of their outdrive as the keel offset setting. Then the reading you will get will be between your outdrive to the bottom. Or you can set it for your actual depth instead...In addition the angle that the transducer is aimed changes with diffent power settings and the angle of the hull in the water. Set the trasducer so that it is level for your particular operating needs. The beam shooting at an angle will read deeper, the more the angle, the more the error at being deeper.