Transom transducer location? Mounting Pad?

Pat310

Family got me a new Garmin chat plotter that came with a transom mounted transducer. Anyone use one of these on twin engine cruiser and can help me with ideal mounting location? From my online research so far, between the drive seems to be the spot. Also has anyone used a stern saver mounting pad to attach the transducer? Hate the idea of putting more holes back there. Great reviews on amazon.
Stern Saver Glue-on transducer mounting System (Black) https://a.co/d/0XmsRTp 

Ian

Pat - we have the same vessel. I was told to keep away from drives as much as possible so I mounted mine on the port side with the bottom about 1’ under the surface and set an offset in the plotter. The horizontal surface is flat, vertical a slight curve but pulled flat to the hull. Didn’t drill through, just deep enough to hold well and filled with 5200 then screwed in. Ran cable up above waterline and drilled through just under swim platform. Mine is a Lowrance with a good size plug so the hole was bigger than I liked but filling with 4200 as well as a clamshell over the entry and filled keep it all dry.

LaRea

Turbulent water is your enemy.  Don't mount the sensor downstream of obstructions such as a strake or generator intake.  And if you have an existing thru-hull depth sounder, put the new sensor on the opposite side of the boat.  

I used a transom-mounted fishfinder for a couple years.  It worked great at low speeds.  At planing speeds, it was less reliable than the thru-hull I have now -- meaning, it would have dropouts and other artifacts at high speeds.  

Pat310

Thanks  @Ian  I don’t suppose you have a picture of the exact location?  Also do you have any problem with it reading on plane?

Ian

@Pat310 this is all I have, zoom in and you should get the idea. Like @LaRea said, Lowrance suggested as far away from the factory throughhull unit as possible and this was the best place. I only use for depth and on plane no issues at all; I see artifacts on the scope but when on plane I get plenty of depth beneath me.

Lake_Bum

To further expand on what LaRea mentioned, if you have a factory depth finder with a transducer already on your transom, mount the new one on the opposite side of that. The reason is, the beams/signals will cross each other, and cause some crazy readings.  

MarkB

Through hull transducers solve all these problems.

Ian

MarkB said:

Through hull transducers solve all these problems.

Until you have one already as a factory thru hull and you need to add the transom mount. It was recommended not to use a second shoot thru/thru hull due to signal interference.

Pat310

Was a gift and came as a bundle

LaRea

I have two thru-hulls happily co-existing:  the OEM depth sounder (Airmar P79) and a fishfinder (Airmar B60).  

Willhound

LaRea said:

I have two thru-hulls happily co-existing:  the OEM depth sounder (Airmar P79) and a fishfinder (Airmar B60).  

Very possible if on different frequencies.

LaRea

@Willhound -- yes, different frequencies.  The OEM P79 runs at 235 kHz.  The B60 runs at 200 or 50 kHz depending on depth. 

But I have used two sensors at the same frequency.  I had the P79 plus a transom-mounted P39, which I added for redundancy (and water temperature).  I don't recall having severe interference problems.  Eventually, I replaced it with the B60 because I wanted a fishfinder.  

YYZRC

I didn’t want to drill my transom so I installed my ducer on the trim tab. It works fine up to about 25mph and then no longer reads, but I’m not looking for bottom structure at that speed anyway. 

MarkB

@YYZRC as long as it's level when you need a true reading.

YYZRC

@MarkB my ducer reads accurate for depth despite the 18 degree angle of the trim tab. I would never recommend this setup for a proper fishing boat but for my needs it’s great. 

MarkB

YYZRC said:

@MarkB my ducer reads accurate for depth despite the 18 degree angle of the trim tab. I would never recommend this setup for a proper fishing boat but for my needs it’s great. 

Oh for hard bottom, it's probably okay ... the 18 degree isn't too far off. At 200 kHz, it will be a bit off, because the cone angle is 15 degrees ... so you'd be out by 3 degrees from the widest part of the cone ... not sure how much that adds to depth in % ... I'm too lazy to do the math right now ... lol. At 50 kHz you should be fine as the cone angle is 50 degrees.