I took the boat out today. I was looking at the voltage input on the fish finder. when the boat is off or even idle it reads 13.4 volts.

When I take off to full speed it is reading 16.8 volts.

Is this normal or does it have a alternator or something. I can't think of any reason why the volts would go up with the motor running. Anyone know what a boat alternator looks like ;D Going to check next time I get it out but forgot to today after I dropped it off where I park it.

Its a 1989 69ho Johnson 3 cyl. outboard. Not 100% of the year but this is what the title says. There is no numbers one it that I can see and the guy who works on it for me said her thought it was another year but oh well.

The motor does not have an alternator per say. The charging system on a 89, 60 consist of the stator and rectifier. I does not have a voltage regulator so it's very possible it could be showing over 16 volts. A lot has to do with the condition of the battery and the batteries state of charge. A battery that is on it's last leg or one that's fully charged will not load the charging system so without a regulator the voltages can run high. Dirty/bad connections will also cause high readings.

I would also have to question the accuracy of the sonar's voltage reading if you say it show's 13.4 volts when the motor is off, unless you are refering to when you have just shut the motor off. It's not possible for a battery to hold that high of a voltage for a prelonged period, so after 24 hours, the battery should only show 12.6 - 12.8 volts, depending on the make of battery and condition.

Yea I checked into it it does show 12.6 volts when it sat over night. I double checked with my meter and with the motor running it is putting out 16v and when I shut it off it shows 13.6v. after a while it drops to 12.6. So all is normal.

I was confused cause I never knew the motor had any kind of charging at all.  I was always checking it cause of the radio I added and some extra lights. but seems to be keeping it charger itself.