Before I say what I did to my boat, here’re the symptoms. Trickle charged overnight both batteries on my Thunderjet. Gauge reading just over 12 volts. Launch the boat, noticed a slower crank but started, pulled the boat to shore, engine off and park my trailer. Push the boat off shore, jump in and go to crank, no start. No place to screw around floating down the Fraser so I started the kicker without issue and got back to shore. With the kicker on, gauge is reading 13.5 volts and the system has a few more amps to give so I crank the main and it starts but the crank still seems slow. I continue my boat ride with no issues, come home and slap a trickle charge on. Next trip, first crank slow again but starts… I always have my main or kicker on in the chuck so no big deal. During my trip I noticed some weird intermittent humming sounds coming from my engine wtf. Doesn’t sound mechanical, not depended on engine speed, not my speakers, trying to find it and it’s driving me crazy. Got back home and the next day cleaned/serviced every gd electrical connection I can find. Trickle charged the batteries again and back on the water. Start main, crank seems a bit better but sounds like the starter might be weak, , no big deal maybe replace the starter as preventive maintenance LoL. Back on the water and the intermittent electrical humming sounds are back, wtf I just can’t pin point the location and everytime I’m back at the launch the sound is gone. Here’s the funny part, during this entire time I know what I should do but didn’t want to do. I skipped the first basic step of load testing the batteries.
I replaced both batteries, take her out the next time, wow, starter sounds awesome and no more electrical humming sounds. Haha that’s what I get for trying to be cheap, I tried everything to not replace my batteries but in the end had to anyways.