Any time your vessel is in salt water and have metal below the waterline you''ll need electrolysis protection. You''ll want to place the zinks on the most corrosive metals like aluminum or steel. Cast iron has a great resistance to corrosion but will blister and pit if not painted or coated with epoxy. If your in a marina with shore power the problem multiplies.
And make sure, if you have old zinks, that they are still grounded (making contact). Corrosion can build up between the zink and surface of the metals. It''s common in our marina to change zinks every year weather they look bad or not. Zinks are cheap compaired to out drives, props and thruhulls. The larger vessels usually bond fittings together with electrical monitoring systems. Not such a big deal with a 25'' glass boat.