Had Coast Guard Aux. inspect boat 1999 2520 Penta 5.7 duo prop. Past inspection but he could not hear blower but felt some air entering vent at stern. I felt it too. Coast guard boarded last season and said he could here blower. I have assumed the switch above helm turns it on. It's my second season with boat. 30 years away from boats so the is a learning curve for me. I smell nothing in bilge. But I am wondering where is blower and how can I be sure it's on? Thanks for advice.