Hi Aaron I recently went through a bit of this on my boat as it was driving me nuts and I didnt want 3 antenna's.  I run 27meg, VHF and a fusion stereo and heres my thoughts.  The only way to get all three running mint and reliable is an appropriate antenna for each.  I found using the 27meg antenna on my am/fm was ok (better than no antenna) but once your 15nm out it is pretty scratchy as its not made for this and you will need an adaptor to plug into your stereo as a 27 meg should have a different screw type fitting.  So far I have been out 30nm with my new dedicated am/fm antenna and it has been crystal clear, my 27meg is spot on back to Whitfords sea rescue at that distance too, havent tested it any further yet.

other options that include splitters or small flexible antennas are ok if your inshore or up the river but if you want this for offshore fishing height is premium dont muck around

Rig,

thanks for the advice.

From what you are saying my 27mg antenna should give my stereo a signal? it sounded like it did for you.

Think i might just discard the old antennas and start with 2 new ones.

Cheers,

Its one of those things Aaron, a piece of copper wire hanging out of it will be better than nothing so yeah your old 27 will work  bit, depends how far you go out and want reception.  If you want to do it properly buy a new  FM/Am antenna

 2 pieces of leftover power cable (about a foot long) crimped onto a FM antenna plug works fine for me.  So far have only gone as far as Thompson Bay though and only listen on FM.