[ReelMagic]

Not sure if this helps but I stay in SALT water 2 years between short hauls... I use 2 zinc's on each shaft, replace the zinc's on the raw water strainers/generator strainer, the trim tab zinc's and the 2 on the transom every 2 years when the boat comes out.. The shaft zinc's, strainer zincs and trim tab zincs are 90% gone but the transom zincs are ~60% gone.. I replace all during the short haul. Not sure how long your magnesium ones would last.. Just purchased all for my boat as it's coming out for a short haul early May......