Underside rubber hull plug.

[remaxpat]

Just bought a 2005 Maxum 2100 SC with Mercruiser 5.7 and just noticed a black rubber plug in the underside of the hull about 1-1/2" in diameter, located 35" from the transom, just off center. Did not notice earlier as it was obstructed by the trailer pads. Anyone seen this before?

 

[GA_Boater]

Welcome aboard.

Most likely a thru-hull transducer of some sort for a depth gauge or fish finder. It probably isn't rubber, but hard plastic.

 

[82rude]

Yes, that sounds like a thru-hull transducer.