Welcome to the forum Markus and congratulations on the boat
In the early days of electronics
, transducers were only available in bronze. These cannot be used on aluminium hulls. I think the complicated arrangement that has been fitted to your boat is designed to isolate a bronze transducer from the hull.
A thick aluminium ring has been welded to the hull and tapped. Bolted onto this with the six stainless steel
bolts is a plastic ring. A bronze transducer can then be fitted to the plastic ring and is not in electrical
contact with the hull. It looks a very good and well thought out system.
With modern plastic transducers it is unnecessary, as these can be bolted directly to the hull. At some stage a plastic transducer has been fitted so the complicated system is therefore overkill, but it still does have some potential advantages. It allows the transducer to fire directly downward and also means the transducer can be fitted flush or slightly indented into the hull (too much will create turbulence). This means any lifting straps used by a travel lift
will not put any pressure on the transducer itself.
I would retain the system. If using a plastic transducer you can replace the plastic plate with an aluminium one. This would be easy to fabricate and stronger than a plastic one.