Little plastic fin with cable on back of boat?

[Planking]

What is that little plastic fin on the back bottom of a boat that as a chord connected to it. Im getting ready to install a transducer and that thing is right where i need to put my transducer and i don't know what it is for. lol
Thanks.

 

[TennSportsman]

I think it's the speedometer reader.

 

[Planking]

Thats it. Thank you!!