In-hull depth transducer fails to read shallow depths

[tenacity31]

I installed a Garmin Airmar P79 in-hull transducer. The transducer reads depths >15FT very accurately to the chart but anything 15FT or less, the depth indication starts flashing and does not continue to update until going into deeper water. The transducer was mounted in accordance with all directions. Hull is fiberglass (not certain if it is cored below the waterline)

Any ideas on why this may be the case?

 

[dlochner]

What is the bottom like? Depth sounders like nice hard bottoms. Softer bottoms absorb the pings rather than reflect them. Weeds, lots of decaying vegetation, really soft Chesapeake mud are poor bottoms for the sound to reflect from.

 

[Davidasailor26]

There may be a Gain adjustment on the sounder. When the gain is too high it will pick up false echos at low depths and get confused.

 

[tenacity31]

What is the bottom like? Depth sounders like nice hard bottoms. Softer bottoms absorb the pings rather than reflect them. Weeds, lots of decaying vegetation, really soft Chesapeake mud are poor bottoms for the sound to reflect from.

i have only used it in the muddy chesapeake bay. But I am still able to get good readings in the mud as long as long as it’s deeper than 15FT

 

[tenacity31]

There may be a Gain adjustment on the sounder. When the gain is too high it will pick up false echos at low depths and get confused.

No way to adjust gain on the P79 discussed in the manual. thanks though

 

[dlochner]

i have only used it in the muddy chesapeake bay. But I am still able to get good readings in the mud as long as long as it’s deeper than 15FT

In the shallow water the mud can be thinner, softer, or deeper. Weeds are also less of an issue in water over 15 feet deep.

 

[Tally Ho]

I installed a Garmin Airmar P79 in-hull transducer. The transducer reads depths >15FT very accurately to the chart but anything 15FT or less, the depth indication starts flashing and does not continue to update until going into deeper water. The transducer was mounted in accordance with all directions. Hull is fiberglass (not certain if it is cored below the waterline)

Any ideas on why this may be the case?

In-hull meaning mounted inside the hull (no hole)?

Not working in <15 feet water (when you really want it) is no bueno.

Greg

 

[Crusty Ol Salt]

How do you have it mounted?
I used the toilet seal wax ring trick on mine. It took a couple of tries and if there are any air gaps or air bubbles it would give me false readings or no readings.

 

[tenacity31]

How do you have it mounted?
I used the toilet seal wax ring trick on mine. It took a couple of tries and if there are any air gaps or air bubbles it would give me false readings or no readings.

mounted to fiberglass with approximately 19 degree deadrise angle sanded smooth with 80,120,220 grit. cleaned area with isopropyl then adhered the mounting frame with silicone sealant. filled with propylene glycol (i used marine antifreeze). Top of the transducer lies about 4 degrees from level (the manual says up to 5 degrees is acceptable)

Not familiar familiar with the toilet seal wax trick.

 

[JBP-PA]

Mine does the same thing on certain bottoms. Below 6ft in soft mud or heavy weeds it get confused, either blanking out or obviously wrong. On the same bottom in deeper water it works fine. On hard bottoms in shallow water it works fine. I had assumed that close to soft bottoms the signal to noise gets so bad it can't resolve a depth, in deeper water, the depth where the noise is the worst is reported as the the depth.

 

[Crusty Ol Salt]

Not familiar familiar with the toilet seal wax trick.

Here is one of many posts about using toilet seal wax.

Transducer question

I really like the idea of melting the wax and forming a puck in a small bowl

 

[jssailem]

Here is the data from Airmar.

The P79 includes a plastic mounting base which is aligned to the centerline and secured inside the hull. Then it is filled with nontoxic antifreeze. The transducer is inserted into the base and adjusted for the proper deadrise offset, insuring the beam is shooting straight down for maximum echo returns. The coolant provides the best medium to transmit through the solid fiberglass and into the surrounding water.

I would check the fluid level in the unit. You have a two signal system designed to work in deep and shallow water.
One of your sides may me be not functional
Choosing Frequencies
The frequency of the sound wave is chosen based on how far the wave has to travel. Lower frequency waves can travel farther in a medium, such as water, because the motion of these waves is closer to a straight line.

You reported the depth was good but shallow was not. Perhaps the 200hz frequency side of the 50/200 hz transducer has an issue.

 

[shemandr]

Mine did that too. I guess it was the soft mud in the creek where my dock was. Otherwise it was pretty good. In the creek I pretty much knew the route to my slip. In other creeks not so much. Still it was better than no sender and another hole in the hull.