Welcome to the C&C 27 world!
In my MkV, I still have the probably original Standard Horizon Deep Sounder. Regarding the speed meter, before changing it, I made sure that the new Thru-Hull Transducer was going to fit exactly in the same hole as the old one, to avoid dealing with your problem. Both thru-hulls are near the engine and the sink compartments. The location is not optimal. The speed paddle can be affected potentially by turbulence from the keel and when I am heeling on port side, the speed meter barely measures something. I believe that the manual of the ST40 suggests installing it in from of the keel. The deep sounder location shows me that it is too shallow when it is too late. Another sailor, that has the deep sounder in front of the keel, told me that it does not make any difference to know it is too shallow, a fraction of second before it is too late.
You can find some other responses to your question in the “black arts” section http://www.cc27association.com/fixes/fixes.html or searching in the forum for the keyword “transducer” clicking "search" in the menu above
Enjoy the site!
Pablo
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