unfortunately even wood is subject to electrolysis which leaves it all weak and wispy- this is what happens when wooden boats get "nail sick'" . The tube is immersed in salt water right next to metal that is a different nobility so right there you have a little battery happening and so the less noble alloys in my rudder tube, along with my prop and the barb on the seacock for my cooling water inlet(which fell off on about my third sail leaving me with no engine- nice way to learn how to sail all the way back into my new marina berth!!)all ended up elsewhere!!! obviously if there is no metal whatsoever under the water on your wooden boat(and you are not floating in 240V like a lot of bloody marinas) you don't need an anode on a wooden boat- you can also put too MUCH zinc on but that is another story- it can all be checked quite easily with a multimeter!!
I should really return Rob's thread to him now!!! btw Rob, Wind and Tide was a great read- there is a little bit ofJerome in me alas lol so I agree with most of his philosophies!!