Bob,

A couple of points.
1. Since you are in the GL and the water is fresh and cool galvanic corrosion will be less than in warm high sediment, bracish or salt water. Your readings will tend to be lower in fresh than salt.

2. The non-isolated rudders will defelt your readings down and will add the disimular metal to the bonding grid.

3. Since your rudders have a strong barrier coat there is very little SS surface area exposed to electron transfer.
If your rudder coatings fail then your readings will drop further (often rapidly) and will expose your rudders will become anodes.

4. Your packing glands are isolated to some extent. Since the bronze glands may be grounded through the steering cable the effect would be minimum if any since they are not submerged. I would pay attention at the point of any bronze to SS contacts for evidence of corrosion.

Since you haul every fall and have not observed corrosion of the hull..in my opinion I would not be concerned.

safe boating,
dave