We set up an outdoor "lab" for our tests that could be quickly sheltered from this year's rainy New England spring.

Propellers, shafts, struts, outboard and sterndrive engine housings, and through-hull fittings, composed variously of bronze, stainless, and aluminum, are notoriously difficult to keep clean underwater—not to mention the hulls of metal boats themselves. All these metals (assuming the bronze is high-quality, not a brassy bronze) are quite corrosion-resistant by themselves underwater. Unfortunately, since they don't resist fouling, they have to be coated with something to ward off growth.

If you paint them with a standard copper (cuprous oxide) antifouling paint, you commit the sin of placing dissimilar metals against each other in an electrolyte, and inviting corrosion. The effect is especially pronounced with aluminum: Copper, a relatively noble metal, essentially…