I am tired of outside sources (JOB) influencing my life. I am gonna start fishing like I used to. Got to rig the boat again. Trolling battery, new electronics, etc. I don't want to buy a fishfinder that cost more than my boat is worth. I am looking at a Hummingbird Helix 5 DI GPS. Don't know much about units you can use a Navionics card in. Are they necessary? Anybody know how accurate the GPS is for speed? thanks DP