Why not mount transducer well below waterline?

[wild.coast]

So all the install videos for transducers say you have to have it just barely below the boat hull, like 3/8".
I get that having it too high would mean the transducer ends up out of the water once you're on plane. But what's wrong with having it too deep??
I mean, when the boat is moving slowly, say 2-3 knots, it's full submerged anyway and that's mostly when you'd be using it.

 

[Chris1956]

You want the transducer to be just under water when the boat is on plane, if you can. Depth gauges are real handy at high speed, also, however, you do not want any drag from them to rob performance.

If you have a spot in the bilge with solid fiberglass, you can mount it there, flat against the hull. It will work pretty good, and not need to be covered in antifouling paint. It will need to be wet, however, to function.

 

[dingbat]

The primary reason for a “skimming” transducer is to protect from impact damage.

Forward looking (Live) and down/side scanning transducers can not be mounted in a “shoot thru” hull installation

 

[Scott Danforth]

not to mention, too far below the hull and the hydrodynamic forces when on-plane push the transducer up and make it useless.

 

[wild.coast]

Ah interesting. My challenge is that it's a zodiac with somewhat complex/crammed transom!

I have what I believe are called 'ride plates' installed off the back of the transom to extend the hull further back around the outboard and make it easier to get on plane

There wasn't any spot to mount the transducer on the transom itself so I made a plastic plate off the arms on the ride plates and mounted it in line with these:


Now, I'm not sure this is all coming together in the water at speed

The tubes extend back past the transom as will all RIBs. There is a fair bit of water spray from the inside edge of the tube. The white plate I installed also seems to be catching the water and making a big old water spray. All this means that there is a fair bit of spray/water turbulence around there!

The plates are more or less in line with the hull but actually ever so slightly up, so I'm thinking my transducer ends up out of the water when I'm on plane right now.

Shot at speed:


Any advice??

Right now all I get is noise and zero depth reading when the boat is moving faster than 5 knots

 

[dingbat]

Shot at speed:
View attachment 399589

Any advice??

Right now all I get is noise and zero depth reading when the boat is moving faster than 5 knots

Wayyyy to much turbulence........the water under my transducer is dead flat, smooth, no bubbles whatsoever.

Anything that protrudes from the bottom of the hull is a potential problem. You have brackets, screw heads, rough surfaces, etc. going on.

Some hulls just generate too much turbulence to be effect at speed. Looks like you have one of them

 

[Chris1956]

You could mount it next to the drain hole in the transom. There looks to be enough room to either side.

 

[ejnichol]

The head has to plow the water surface when on plane with minimum immersion but enough to get 100% bubble free water flowing across head/sensor surface.

Your head looks like its tilted backward a bit.

The sensor head needs a slight forward tilt to direction of water flow. Not this steep but keyboard has limits ----->\.

Parasitic drag is unavoidable if you want usable data while on plane.

 

[rhedayi]

You could mount it next to the drain hole in the transom. There looks to be enough room to either side.

I believe garmin suggests 18 inches away from prop to the side

 

[dingbat]

I believe garmin suggests 18 inches away from prop to the side

My thru hull transducer is on center line, 18” in front of the transom. My transom mount side scan transducer is on starboard side, 12” off center line.

Generally speaking, conventional sonar is mounted outside the radius of the prop on the starboard side. Mounts on center line with twin motor installations

Side viewing installations require the transducer to be mounted in specific locations depending on the deadrise of the hull.

https://static.garmin.com/pumac/Transducer_Placement_Guide_EN-US.pdf