Our aluminum hulled Passoa is wired so that the electrical system is totally independent of the hull. House and (engine + windlass) batteries are charged by separate alternators, and there is no connection between two 12 volt systems.

The boat is equipped with an electrical leak tester each system. It shows problems if any one of the four battery terminals is connected to the hull. We have tested that
Today, I can see 12.7 volts from the positive of either battery to the hull.
Resistance form the negative terminals varies, typically 200 to 800 ohms

Surely I should see open circuit from the negative to the hull, and zero volts from the positive to the hull

I am measuring with a fairly modern, mid price electronic multimeter. A Prova CM 01 I have on info on its sensitivity

Comments please