A boats Bat gauge can be defective, hence the 13V. Bat should be 12.7 unless Bat charger was just turned off.
Might have corrosion problem or loose connection. As the boats running and doing ok then the connection gets worst so the sense lead starts reading less voltage so it increases output

Measured less than 1 ohm from alt ground cable to neg batt post. Probably should measure alt ground with eng running. Dash voltmeter reads same as handheld Fluke meter. Batt did just come off charger and had a surface charge present.

I measured 16.7V on SENSE terminal with eng at 3000 RPM using handheld meter.

Just rebuilt 1 of my 13 yr old DB alternators this year. Only producing 11.7 volts and bearing was starting to squeak. Friends and I have bought many alternators from them. I usually purchase the 1 wire 105 amp. They use to be under $70 shipped to my door.

Just curious, will 16.7v hurt anything? I see on my EFI forums guys going with 16v systems.

Yes. Anything that can’t handle 16v.
Like your battery if a 12 volt battery. Light bulbs usually burn out quicker too. Will be brighter, though, until they burn out. Some electrical items like starters and fuel pumps will have better performance with no decrease in dependability. Ignition systems too. Anyway, make sure you know what something is rated for before throwing more volts at it. :thumbs

No luck getting auto parts store to spin test my alternator. If they don't have the "make and model" they can't even turn their tester on.

Years ago I made a gas powered 12v batt charger from a 5hp eng and a 105A car alternator. Haven't run it in many years. Got it going yesterday and was able to spin test my alt. Runs right at 14.7V. Think this pretty much confirms I have a grounding issue as mentioned by liberator.

Fascinating thread...great investigative, controlled, testing being done. Nice to see.

Any local alt repair shops, which know about alt's

Played around with it some more yesterday. Measured between alt gnd and batt neg post with eng running. Volt drop was very low (<0.1v). Connected a temp jumper wire across the same 2 points, but didn't help.

Moved jumper to connect alt output and pos batt post, it worked, for the most part. Max voltage now 15V at 3000 rpm. Made a real jumper with the correct terminals and some 10ga wire. Connected it to the alt output and where pos batt cable attaches to starter solenoid, no help at all. Back to 16.7V.. (BTW, there's no fuse at starter solenoid where alt output connects)

This leaves the batt cable between the batt switch and starter, the batt switch itself and the pos batt cable from batt switch to batt. All 3 are working or I wouldn't be able to crank the engine.

Still struggling to understand what's going on with this crazy thing.