Before you go drilling holes, pop the transom tranducer in a plastic bag with some water
in it and lay it on the inside of the hull
(avoid cored areas). Connect it up and see what sort of signal you get, it will probably be good enough, unless you want speed and temperature.
If this works then just glue the transom tranducer in with sealant
, despite what it is written in the manufacturers instructions it works very well.
Our dive club rib
has a transom tranducer glued in that I stuck down as a temporary measure just to use it for an afternoon in 1997. It's still their and working fine.
If it doesn't give a good enough signal then consider buying
the shoot through hull type which has a round plastic fitting glued to the hull, filled with oil
and the transducer sits inside it. Lots on e bay etc, but do get the Garmin
one as the Raymarine
ones have different connections. they are all made by an outside company called Arimar btw.