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.
Pete