We put them on in July. So far so good. We tested -635 millivolts with zincs and wanted to get more negative. After the change to aluminum we re-tested at -1028 millivolts. We changed out a 6" x 12" x 1/2" plate, prop and rudder zinc all to aluminum. The change over was ~$10 more. Boatzincs.com has free shipping on orders over $100. BoatZincs.com (978-841-9978) – The Online Superstore for Zinc Anodes