I am looking to shed some weight in my 16 ft Riverhawk Pro-Caster. The itself boat weighs  only 220 lbs. I now have about 150 lbs of batteries. I have two group 27 batteries that power my Minn Kota 80 lb Terrova. When I upgraded to a new Humminbird Helix 10 Mega SI , I added a group 24 (46 lbs) instead of the small tractor battery I had always used. I feel like the group 24 is too heavy and overkill just to run a fish finder and crank my 20 HP Tohatsu motor. From my reasearch, it appears that a battery like the one below (12 lbs) would work. Would it?

https://www.amazon.com/gp/product/B00KC39BE6/ref=ask_ql_qh_dp_hza

This may come in handy.  

https://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html

 

I looked up the specs, your unit draws 1.2amps.  I plugged the following into the calculator above, 1.2 amps at 5 hour(fishing trip length), would require a 18ah battery to not drop below the 50% discharge level.  This is not taking into account cranking the engine, only running your fish finder.