what type of transducer ?

[luso]

I'm installing my depth and speed sensor and I have run into a problem regarding the transducer. The unit I have is(ST50) a triducer is made of bronse. Upon reading the instalation manual it said not to use bronze on a metal hull ( my is steel), so my choices are: plastic which is weak or stainless steel which is very expensive($750 us).any suggestions please.
Thank you

[Landlubber]

Insulate the transducer from the hull with a plastic gasket, same with the fasteners.