www.iFourWinns.com

I would mount it where you have the oval.

Despite what I said about mounting it behind a strake, it would not be a good idea for it to be under the extended running surface. Most transducers have a feature where if it gets hit by a floating log then it pops up/back out of position. Mounting under the extended running surface would confine it so it couldn't pop up. Wire routing would be a problem also. As alex8q4 said, the wire should be routed up the transom well above the water line before drilling through. (like the speedometer hose)

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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings

Yep due to the hull design that is really the best place to put it.
When you install the bracket; put sealer (Boat Life for below the waterline or 3m 4200), in the holes and on the screws. Put the screws on most of the way; then stop and let the sealant cure. Then go back the next day and put the screws in the rest of the way. This keeps the sealer from getting squeezed out from where you want it.
Last points:
You will have to drill a pretty big hole in the transom for the connector to go through. Meadure carefully on the other side because the steering system is there. After you drill the hole seal the sides of it with epoxy resin. Then pull the cable through and seal all around it with sealer. Cover with a clamshell cover.

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

I had to replace mine and it was in the Oval area. I did all the things Lou suggested. The hole for the transducer cable was fairly large like 3/4 inch for the connector to fit thru.

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Terry
Current: 07 Crownline 255 CCR cuddy - 350 Mag MPI/B3 "Casi Cielo"

Previous:97 245 Sundowner 5.7GI/SX

Tow vehicle: 1986 GMC 2500 Camper Special 454/TH400

here's mine....over on the right side of the pic. The extra wire coming out of the transom mount is my little trick for replacing the Cobra/Volvo SX trim sender without having to break your fingers trying to snake it through the transom mount...

https://www.dropbox.com/s/f6777spyd01u8 ... a.JPG?dl=0

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II