Well if the battery is able to start your big motor when the finder cuts off or goes dim, it should have enough voltage for your F/F. I'd start looking at electrical connections, both positive and negative, and also fuses. Ideally, a dedicated and fused wire directly to the battery for each finder is best. Sometimes connecting to fuse panels doesn't supply enough voltage due to dirty connections and long runs. I had a G-3 boat that was only a couple years old, and my finder was dropping off when connected to the fuse panel. I wired it direct and it was fine after that.