Ground issue?

[ShoestringMariner]

Hey guys, I think I read somewhere that nothing should ever be grounded to aluminum boat hull. In leaning over the gunnel trying to sort out a bit of a rats nest, I have discovered there is voltage getting to the hull. I kept getting a tingle in my arm. I checked with a multimeter and found that while the motor engine leads are connected or the CMC tilt is connected, I can get a full voltage reading between the positive terminal of the battery and the hull itself.

should I assume that this should not be? This boat had a considerable amount of corrosion around the transom and rear corner of the boat up to the waterline.

If this should not be the case, I must assume that there is a ground fault issue in either the motor or power trim.
and in which case, I’m wondering where I should look for the typical culprit(S) so I don’t have to dissect this whole thing. I want to go fishing tomorrow !!!

I’ve got a 1975 mercury 650. Well known for deteriorating wiring issues.

or is this generally acceptable as long as there’s a good positive and negative connection coming from the source of connection. ( battery leads going to and from the motor are in good condition. Therefore all current will travel through those as opposed to the hull. (The hull can still transfer current however when the amps are flowing they are going through the intended cables)

Edit, ALL wiring on this boat goes back to negative terminal on the battery.

 

[ShoestringMariner]

Edit; MODS PLEASE DELETE. I now realize that an outboard bolted to an aluminum hull will automatically be grounded to it. But I see all major current returns directly via. battery cable.

Thanks