Depthfinder loses bottom at slow speed

Electrical and electronic topics for small boats
Posts: 21
Joined: Thu Oct 29, 2015 10:07 pm

Depthfinder loses bottom at slow speed

Postby Rascal » Tue Jul 30, 2019 6:12 pm

I installed a Garmin echoMAP CHIRP 54cv with CV23M transducer on my 2015 170 Montauk. The transom mount transducer is the ridiculously long type that sticks way out from the boat. The transducer is mounted where Boston Whaler shows it in their parts diagram, which is the starboard side chine flat. This is the same location I've seen transducers mounted on many previous generation Montauks and it's where my 150 Montauk had it. The alternate location seems to be closer to the centerline, but I wanted to stay as far as possible from the prop (and follow Whaler's diagram).

It works fine at planing speed. However, it almost always loses the bottom when stopped or at idle speed. I started with the transducer parallel to the bottom and 1/8" below the bottom. I tried again at 3/8" below the bottom and parallel. I also tried 3/8" below the bottom and angling the aft end of the transducer down. The angle was based on the assumption that it works better at planing speed when the stern is lower. That may have helped a little, but it still loses bottom most of the time at slow speeds or a stop. I've read plenty of topics where people have the opposite problem - a good reading at slow speeds, but losing bottom at higher speeds due to turbulent water. The Garmin instructions and website don't offer much help. Any ideas? I'm hoping to get some input before adjusting the height yet again or moving it and having to re-caulk each time.
Rascal (Scott)
2015 170 Montauk

Posts: 7852
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

Re: Depthfinder loses bottom at slow speed

Postby jimh » Thu Aug 01, 2019 11:52 am

As a test, when the bottom echo is lost at low boat speed, try tilting the engine up so the gear case is out of the water. I suggest this on the speculation that perhaps the sounder sees a big echo off the engine gear case and decides to adjust the depth range to be about 2-feet.

Also, many sounders have a huge number of settings and options. The sounder may have become stuck in some manual mode where depth expected and gain to use have been preset to values that are not good for general use. I mention this because last week my sounder went haywire, and getting it back to working condition took me 30-minutes of fiddling with gain, depth, palette, and color line settings.

I agree: your problem with loss of bottom echo at low speed is unusual,