If you're in a sw slip, no doubt about it... put zincs on them. You'd be surprised how badly they can erode, and unless you take a really good look at them, you might not notice it, (hidden by paint). I've seen the stainless mounting screws, (on the hinge) erode to the point where the heads broke off and then the plane popped off. This happened while the boat was under way. :shock: The boat was pulled and after inspection, the heads on more than half of the screws were gone. The same thing was happening on the other tab, (it would have only been a matter of time) all of the mounting screws had severe erosion. Also, after wire-wheeling the paint off of the remaining plane, hinges and back plates, they all had bad erosion. Ended up replacing everything, (metal).
I prefer the rudder style over the "kits" that attach to the bottom of the actuator. When those zincs start to deteriorate, the mounting screws can become loose. I've actually discussed this with someone at Bennett, and they agreed - go with rudder type. I know there's a bushing in there, but I still don't trust it. What I do is... use only 1 side, (half) of the zinc and mount it on the top side of the plane. That way you don't have anything bulky on the bottom of the plane to interfere or disturb normal tab performance, plus, imo, it would be under constant force which might accelerate erosion, or even loosen the zinc/hardware.
It's not just a matter of having your boat up to snuff, but other near-by boats in your marina can affect your boat too. One year, you might be fine, then next year, someone keeps their charger going 24/7, or has inadequate protection on their boat, and it robs other near-by boats. There's really no way to get control of it, except to insure you have zincs everywhere you could possibly need them, and keep an eye on them.