Have any of you had experience with silver chloride electrodes for determining the correct amount of anodes you should have on your boat. I have ordered one of the silver chloride electrodes and will be going down this path shortly. It would be good to hear of other peoples experience with them.
I have heard of people not having any fixed anodes on their boat and when they anchor somewhere or go into a marina, they get out the meter and electrode and just dangle anodes on a wire (the wire needs to be connected the the boats bonding network) over the side until they get the correct voltage reading on the meter.
The table below is only a guide as I have also read that the desired voltage readings for glass and aluminium boats falls into a slightly different bracket.
Hello datawiz, The article below is what got me started down this path. I have been having trouble with paint coming off my hull and the metal is still shiny underneath (no rust). I asked the guys in the slip yard if it could be too many anodes but they didn't seem to think so. When I slipped my boat a week ago some of the repairs I did 12 months ago were peeling again. The guy who wrote the story below had similar issues as what I am having but the paint damage I experienced is nowhere near as bad as the photos in the article. The guy ended up getting on to Seabis and installing one of their monitors.
For the last couple of day I have been isolating all my electrical system from the hull. I installed an isolator on the negative of the engine start battery and removed the common negative between the house and engine start batteries. My HF radio has to be left electrically disconnected all the time and only reconnected when required for use. The only other negative path to the hull was on a screen for the radar so I removed it. I tested the radar and it is still operating ok. In my past life a screened cable should only be earthed at one end other wise you may get circulating earth currents. So at this point I don't thing the screen for the radar will be an issue.
My battery chargers are internally isolated between the 240Vac supply and the batteries. The 240vac supply to my boat only feeds 1 x special elcb GPO and I checked to make sure that nothing that was plugged into it was proving an 'earth' path to the hull.
I also have a 2000w inverter on board which has the frame earthed to the hull of the boat but the negative from the supply batteries is internally isolated from it luckily.
I have also been fitting earth straps to the drive shaft and rudder shaft. Brought these a while back and never got around to installing them. The test electrode I ordered should turn up about Wednesday so it will be interesting to see what reading I get.
www.fsc.com.au
www.syc.com.au/wp-content/uploads/2013/05/Marine-Galvanic-Corrosion-Made-Easy-SYC.pdf
The second article is very good. Most boats do have too many anodes.
So I'm guessing this is a steel boat in a marina. How close are the other boats? What antifouling are your neighbors using? How much water under the keel at low tide and is there any rubbish there? Friends raft their boats up?
Years ago I had an alloy fishing vessel moored up at a wharf. I worked nearly every day and I had anodes on the stern. I was lucky to get 3 months from them. Low tide I had about 1 metre under the stern and one day when the water was clear, I noticed a tractor wheel right under the stern! I can only presume it was placed there intentionally.
I heard a story a long time ago (30 years back) where the tug boat used at Wiepa had aluminum hulls and they used steel cable to attach them to the steel pylons on the wharf. Goodbye tug boat hull. 
There was a large alloy boat on the slips here that had sheets of alloy replaced at the bow. Anchor chain and anchor was not isolated from the hull.
Found another couple of webs site that have more information on Anode requirements. My tester turned up so I'm hoping to check the hull voltages over the next few days.
Some of the stuff I am learning, I had a bit of an idea on, but it's good to have it spelled out.
1. Things like the importance of Anode distribution.
2. Size/weight has more to do with how long the anodes will last.
3. Number of/surface area, of anodes has more effect on the current/voltage produced.
4. The hull of the boat (and other wetted metal surfaces) is an anode already and needs to be converted to a cathode by bolting Zinc (steel hull in salt water) onto it.
5. The quality of the hull paint (and prop paint) effects the required number of anodes.
6. Anodes are for Galvanic corrosion and are not for the protection of electrolysis, even though Anodes can be chewed up by an electrolysis problem.
7. You need to regularly check your hull voltage and don't assume it'll be okay till you slip the boat next time.
8. It appears you don't need to buy a super duper silver chloride electrode to test the hull (wetted metal) voltage. This is still under investigation.
9. Too many anodes will blister your paint (on a metal hull) due to the build up of hydrogen on the metal surface.
10. Too many anodes will also encourage marine growth.
11. For Anodes to work, your hull (wetted metal) voltage need to be around -250mv below that of your protected metal (in my case mild steel). Mild steel has a potentilal difference of -650mV (in salt water) when checked with a silver chloride electrode, so I need the amount of Zinc anodes attached to my hull to pull this voltage down to around -900mV.
12. Electrolysis is really about electrical issues your boat has and these can be a 1000 times more damaging to your wetted metal surfaces than galvanic corrosion. The 2 problems should be address separately.
www.galvatest.com/
www.boatzincs.com/faqs.html
