Installing an in the hull transducer

[Mike Oldak]

Our install of a P79 transducer faced several problems. First the kit enables you to adjust the device for up to 22 degree in a single direction. Our hull had tilted both fore and aft, and port to starboard. To compensate we used JB weld marine to build a small circle of compound.
First we drew the outline of the tank onto the hull. Then mixed the JB weld and placed it on the hull with a higher wall where needed to create a somewhat level base. The JB weld is like clay and allows significant adjustments as it slowly hardened. We placed wax paper on the top and then used a flat square to level the top. It took two applications to create the level base.
Then (a little messy) put the transducer into the tank and used a level to level it both fore and aft, and port to starboard

 

[jssailem]

Nice. Does it work?
If yes, that is when you know you got all the air out of the mix. You can sit back. Give out a big sigh and have earned your beverage of choice.

 

[Mike Oldak]

Nice. Does it work?
If yes, that is when you know you got all the air out of the mix. You can sit back. Give out a big sigh and have earned your beverage of choice.

I just made a circular dam and then filled it with rv antifreeze so no air bubbles below transducer. Structure just provided a round hollow circle for the transducer tank to sit on.

Yes it works

 

[jssailem]

I’ve heard of owners using mineral oil. Antifreeze is a new one for me.
Well done.

 

[Mike Oldak]

I’ve heard of owners using mineral oil. Antifreeze is a new one for me.
Well done.

Me too! But that is what the directions suggested.

 

[Ralph Johnstone]

Structure just provided a round hollow circle for the transducer tank to sit on.

Hi @Mike Oldak . This is certainly not meant as a criticissm of you, as your only following the instructions, but the idea of having to submerge the transducer face in some form of liquid was proven as pure unadulturated several decades ago. What's so incredible is that this fairy tale is still floating around. Check the archive of posts here to see how most people mount the transducers properly.

My suggestion would be to remove everything you have done to date and install it using the modern practice.

Me too! But that is what the directions suggested.

Could you please post a copy of these instructions as I find it a real head scratcher that this sort of is still being distributed after being found to be useless so many years ago ?

 

[Mike Oldak]

I've install several in hull transducers and they all worked. Have both a sonar and depth reading thru the hull
Both garmin and raymarine sell and support such installations.

Dont have the manual with me but google says--.

For an in-hull transducer like the Airmar P79 (often referred to as the T79), you should use non-toxic propylene glycol [index:1.1.3] (commonly sold as marine/RV antifreeze) [index:1.1.3] to fill the mounting base [index:1.2.6]. [1]

Recommended Fluid

• Propylene Glycol (Marine/RV Antifreeze): This is the official manufacturer-recommended fluid [index:1.1.3]. It provides an excellent acoustic path [index:1.2.6], will not freeze [index:1.2.4], does not evaporate quickly [index:1.1.6], and its bright color makes it easy to spot leaks [index:1.3.6]. [1, 2]

 

[Ralph Johnstone]

I've install several in hull transducers and they all worked. Have both a sonar and depth reading thru the hull

I'm sure that using a carefully installed silicone base, you would have had the same readings but without the hassle and problems of fluid leakage in the future.

From my personal experience of installing an in-hull transducer in a friends 40' Catalina, when we took out his boat and my boat (H-310) into 40-50 ft. waters in an anchorage, we always had the same readings +/- a few feet. Sometimes he was higher sometimes I was but nothing worth considering. Discrepencies were most likely due to the bottom contours. My transducer was a through the hull from the factory.

I'm sure the Airmar's data shows a what ? a 5% greater accuracy with liquid immersion as opposed to solid hull contact.

You never know what crap the sales department is pushing at any time but for practical purposes, the liquid immersion is a waste of time. Definitely not worth it for the extra trouble.

If that supposed extra few % accuracy is that important to you, go with a through the hull transducer and eliminate any possible source of error, regardless of how small.

 

[Ralph Johnstone]

Have both a sonar and depth reading thru the hull

Not clear here. Sonar are the transmitted sound waves which measure the depth. Can you please clarify what I'm missing when you say "both" ? Thanks.

 

[Hayden Watson]

I’ve heard of owners using mineral oil. Antifreeze is a new one for me.
Well done.

I have been using the same Peak Sierra pet safe antifreeze in my transducer that I use in my engine and hydronic heater. I build a transducer sea-chest for my transom mount transducer. I then stuck it to the hull with G-flex epoxy.

 

[Ralph Johnstone]

I build a transducer sea-chest for my transom mount transducer. I then stuck it to the hull with G-flex epoxy.

Quick question for you Hayden. Why go to all that trouble when the solid inter-layer method (silicone, etc.) is so simple and has no potential leakage problems ?

 

[Mike Oldak]

Not clear here. Sonar are the transmitted sound waves which measure the depth. Can you please clarify what I'm missing when you say "both" ? Thanks.

The T79 transducer measures depth and provides a contour of the ground. Probably analyzes the sonar reflections. Look up axiom for pictures of the sonar in use.

 

[Mike Oldak]

Quick question for you Hayden. Why go to all that trouble when the solid inter-layer method (silicone, etc.) is so simple and has no potential leakage problems ?

My installation had about a 22 degree hull angle port to starboard and a 17 degree Fore to aft. While I never heard of that method but saw it on a recent YouTube I needed about a 1 inch wall on one side going down to almost no wall on the other. Trying to get that much silicon to stay in place I believe would be very difficult. The YouTube video showed a silicon attachment flush with a very tilted hull, resulting in a very skewed sonar path and incorrect readings.
Someone also mentioned the issue of whether the silicon could be used without any air bubbles
But never really investigated the use of silicon since it wasn't in the factory instructions. If I had a perfectly flat hull that was perfectly horizontal then a little silicon would probably work fine. Just need to provide a medium that allows the sonar pulses to penetrate and then be heard.

I actually did think about making a dam' like I did and then filling it with a resin mix

 

[Ralph Johnstone]

Trying to get that much silicon to stay in place I believe would be very difficult.

Gotcha.

 

[njlarry]

I just bed mine in plumbers putty. Dries out every few seasons and easy to replace. G flex would be more permanent but make transducer changes harder.

 

[Hayden Watson]

Quick question for you Hayden. Why go to all that trouble when the solid inter-layer method (silicone, etc.) is so simple and has no potential leakage problems ?

If you get a single tiny bubble in the silicone, it will screw up the signal. This also lets me change out the transducer for another one if needed.

 

[Hayden Watson]

My installation had about a 22 degree hull angle port to starboard and a 17 degree Fore to aft. While I never heard of that method but saw it on a recent YouTube I needed about a 1 inch wall on one side going down to almost no wall on the other. Trying to get that much silicon to stay in place I believe would be very difficult. The YouTube video showed a silicon attachment flush with a very tilted hull, resulting in a very skewed sonar path and incorrect readings.
Someone also mentioned the issue of whether the silicon could be used without any air bubbles
But never really investigated the use of silicon since it wasn't in the factory instructions. If I had a perfectly flat hull that was perfectly horizontal then a little silicon would probably work fine. Just need to provide a medium that allows the sonar pulses to penetrate and then be heard.

I actually did think about making a dam' like I did and then filling it with a resin mix

This is exactly correct and why I built my sea chest to mount my trandsucer. The transducer I have has a listed beam angle and if you do not provide a solid layer of something that extends beyond that angle, it will "see air" and you will get no return. That is why the sides of my sea chest are splayed out. It also lets me install it level to the world even thogh it is on a sloping hull. I installed mine right on centerline in the "V" of the bottom in front of the keel.

The liquid method also eliminates all possibility of air bubbles in the silicone ruining your day.

 

[Ralph Johnstone]

If you get a single tiny bubble in the silicone, it will screw up the signal.

I've had 100% success with the silicone method of installation. I did it once and it worked. I'll rest on my laurels .

 

[Mike Oldak]

Does mounting a transducer at an angle REALLY MATTER?? Maybe not much. AI says .6 feet at 10 foot depth. So I'll already know the bottom is coming up. But depends on how accurate you really need the depth to be somewhat exact.
QUOTE from AI
what will be the depth error on a transducer mounted at a 20° angle
A transducer mounted at a 20° angle will over-report the actual depth, creating a 6.42% error relative to your true vertical depth (or a 6.03% error relative to what is displayed on your screen). Because the sonar beam travels along a slanted path instead of straight down, it records a longer distance to the seafloor. [1]
The Mathematical Breakdown
When a transducer is tilted, the path of the sonar beam forms a right triangle where:

• • Measured Range (\(R\)): The distance displayed on your marine electronics.
  • True Vertical Depth (\(D\)): The actual distance straight down to the bottom.

\(\text{True\ Depth\ }(D)=\text{Measured\ Range\ }(R)\times \cos (20^{\circ })\)
Since \(\cos(20^\circ) \approx 0.9397\):

• • If you look at your screen reading: The actual depth is only 93.97% of what is displayed (a 6.03% reduction).
  • If you look at your actual location: Your screen will display a depth that is 106.42% of reality (a 6.42% increase).

Real-World Error Examples

True Vertical Depth	Screen Display Reading	Discrepancy (Over-reported Depth)
10 feet	10.6 feet	+0.6 feet
30 feet	31.9 feet	+1.9 feet
50 feet	53.2 feet	+3.2 feet
100 feet	106.4 feet	+6.4 feet

Crucial Practical Factors

• • First Echo Returns: Standard digital depth sounders often look for the closest return. If your transducer has a wide cone angle (e.g., 20° or 24°), the edge of the beam pointing closest to the vertical axis may still catch the bottom directly below the boat first. This can sometimes mask the mathematical slant error in shallow water. [1]
  • Fish Arch Distortion: Fish directly beneath the boat will not appear in the center of the cone. They will be marked late or look asymmetric because the transducer is aiming slightly to the side or backward.
  • Correction: If you cannot physically adjust the mount to make it level with the waterline, check your fish finder's installation settings. Many advanced units (like Garmin LiveScope or Humminbird systems) feature a "Pitch Angle" or "Installation Tilt Offset" configuration that allows you to digitally correct for mounting angles. [1]

To help fix this issue, what brand or model of fish finder are you currently using? Knowing if it is a transom mount or a thru-hull installation would also help determine the easiest way to square it up. [1]
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