What does the depth finder measure?

[jhnstniii]

Does anyone know if the depth finder is reporting distance from the transducer to the sea floor or is it reporting distance from the water surface to the sea floor? I assume the former. We have a 2019 R27. Thank you.

 

[Nwdiver]

That depends, in the garmin mfd you can set the offset to whatever you want

Zero offset measures the sea floor to the transducer location. The location of the transducer on my boat is glued to the inside of the hull near the head

Ed
Old bold diver

 

[Todd Tugnut]

My preference is to set the offset so I am seeing the distance under the lowest part of our boat. If you want to check what you are measuring, when docked, tie a weight to a line and lower it over the side near your transducer. Then put a tape measure to the wet part of the line.

 

[SJI Sailor]

+1 to both, and also set ours like Todd notes.

BTW if you look closely at the sonar screen on Garmin it shows the depth both ways, if I recall correctly -- using and showing the offset (if applicable) as part if the "big" number, and showing the direct reading to the transducer in the small graph numbers.

 

[William Sanborn]

Not a big problem in our Ranger Tug 27, as we go into the same areas we traveled with our Sailboat and a 6 ft keel. Of interest in some area with a grassy bottom the sounder will measure the tops of the plants, rather than the mud. Need to know where you are navigating.

 

[Someday*]

That depends, in the garmin mfd you can set the offset to whatever you want

Zero offset measures the sea floor to the transducer location. The location of the transducer on my boat is glued to the inside of the hull near the head

Ed
Old bold diver

My transducer is the one where it sets in a "cup" of liquid...problem is this liquid keeps leaking out...can this cup be removed and just caulk/glue the transducer to the hull and it work that way?

 

[Maggie Anne]

The reason for the cup is that the transducer itself is not actually bonded to the hull. It sends a signal through the liquid in the cup, through the fiberglass hull, and into the water. This works because the fiberglass and antifreeze have similar acoustic properties so sound is not strongly reflected at the boundary between the two but instead passes through with little attenuation. If you tried to glue the transducer directly to the hull, any air or glue between the transducer and the hull would cause degradation of the signal both on transmission and detecting the return echo from the bottom.

 

[Someday*]

The reason for the cup is that the transducer itself is not actually bonded to the hull. It sends a signal through the liquid in the cup, through the fiberglass hull, and into the water. This works because the fiberglass and antifreeze have similar acoustic properties so sound is not strongly reflected at the boundary between the two but instead passes through with little attenuation. If you tried to glue the transducer directly to the hull, any air or glue between the transducer and the hull would cause degradation of the signal both on transmission and detecting the return echo from the bottom.

I understand...even if the signal is not perfect all I need is depth...not using it as a fishfinder etc so don't need detailed readings....I'm just tired of having to fill the cup even after having someone work to get it sealed to the hull better (grinded glass and smoothed area then sealed the cup)

 

[Maggie Anne]

I believe Garmin makes another transducer the GT8 that has a somewhat similar idea. You glue a foam ring (that they supply) to the hull, then fill the ring with epoxy to ~1/2 inch depth then press the transducer into that. The GT8 has a 250W rating compared to the 600W rating of our GT15 transducers. I don't know if the GT15 would work well using that method but you might try that although if it didn't work, then not sure you could recover the transducer to try another approach. By the way, I'm curious as to where the leak occurs, I've heard other tugnuts have some complaints regarding leakage, is it around the threads where the transducer connects to the cup or at the base where the cup is bonded to the hull.

 

[Someday*]

I believe Garmin makes another transducer the GT8 that has a somewhat similar idea. You glue a foam ring (that they supply) to the hull, then fill the ring with epoxy to ~1/2 inch depth then press the transducer into that. The GT8 has a 250W rating compared to the 600W rating of our GT15 transducers. I don't know if the GT15 would work well using that method but you might try that although if it didn't work, then not sure you could recover the transducer to try another approach. By the way, I'm curious as to where the leak occurs, I've heard other tugnuts have some complaints regarding leakage, is it around the threads where the transducer connects to the cup or at the base where the cup is bonded to the hull.

Thank you...I will look into that option.

As for where the leak is...I don't know exactly...next fill of the cup I'll use a pink antifreeze to see if can locate where it is leaking.(clear mineral oil in it now)

 

[bob daily]

To answer the original post question, the depth finder is measuring the distance between point A - the transducer and point B - the bottom or any object that will supply a return echo. The monitor will allow for the value to be changed ( + or - ) to get the results the user wants, IE - distance from surface to bottom or distance from bottom to the boats keel.
As to the problems with in-hull transducers there have been numerous posts about pros and cons and the (fix), most in hull transducers require the mount to be installed over solid fiberglass (not recommended for cored hulls), with a thickness of 16mm (5/8 in) or less.
My boat a 2009 R21-EC, has a cored hull and the location of the original in hull transducer was over 5/8 in thick. I'm not saying they will not work if not installed as per the manufactures recommendations, just not as designed. Bob