What is a common draw?
 

Do you older formula owners also see all the lights dim when you hit the trim tabs at night or use other electricals? Put cabin lights on etc.. When I hit my trim tabs all the lights dim out and my voltmeter slams down... (Although the voltmeter is another issue, just replaced my vdo and it still reads 11 which can't be because the boat starts right up)

Is this a sign of a weak alternator?

1989 Formula 242ss

You could check the output on your alternator with a handheld voltmeter to confirm output, but it sounds more like a ground issue to me. I would check and clean them all and see if it still happens.

Good luck.
Billy

Not uncommon, the trim pumps pull a lot of power and will drag down the gauges, lights, etc. Usually the wiring is on the small size at around 10awg anyhow. Add on corrosion in the hot and ground from an older boat and you get what you are seeing.

Every boat I have owned or still own has done that.
And that would be six boats an astro fish and ski a VIP two Donzis a Regal and a Formula.

fixx
 

nstall bigger ground wire to he battery,or you have corrosion somewhere..

I had that problem, and it got progressively worse. Turned out to be a bad battery switch. It would carry the small loads, but not heavy current.