Would love to see a picture of that! Do you happen to have some? I will go with your route.
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You aren't a real boater unless you drill a bunch of holes to add custom equipment. Routing wires over the transom looks so unfinished. Holes are sealed up easy peasy.
Installing a transducer in the bilge is OK I guess, but for me its just too much work climbing down under. Besides, the transom mount is easier to service.
The attached pic shows a transducer cable and a paddle wheel speedometer linked thru the same hole, with an over-sized clam shell. Out of view is the OEM pitot with a similar clam shell. BTW, the paddle wheel, pitot and GPS all show equal speed within 1 mph.
View attachment 371857