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Old 02-08-2014, 07:02 PM   #1
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Anodes on a steel hull

Hi there,
greetings from Tigre, Argentina!

I have recently bought a 55 ft river trawler which is currently under mayor maintenance repairs in a shipyard. After having the hull thoroughly ultrasounded, one 5 x 7 ft steel plank has been replaced at each side of the keel (were the screws and rudder shafts exit the hull). According to the contractor that did the job, the rest of the hull is ready for sandblasting and painting. The boat will be moored and sail the Rio de la Plata river, where the quality of the water is very similar to that of the Mississippi in the US. Due to the different opinions heard about the number and size of the anodes to be installed, I must recon, IŽm getting a bit paranoid about this issue. Any hints?
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Old 02-08-2014, 07:39 PM   #2
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My 33-foot-waterline steel boat has four zincs spaced along the hull bottoms, both port and starboard (total of 8), as well as one on the end of the propeller shaft and one on the rudder. So far, no electrolysis issues in saltwater.

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Old 02-08-2014, 09:11 PM   #3
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Hi, looks great! unfortunately the previous owner of my boat must really have neglected it, cause when I bought it most anodes were worn out and corrosion signs around the screws and rudders was quite important. Perhaps placing the same number it had (10 at each side) and checking them once a year may be a good alternative
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Old 02-08-2014, 09:17 PM   #4
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corrosion on rudder

This is how it looked before doing the repairs
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Old 02-08-2014, 09:34 PM   #5
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check around...pretty sure there are pretty standard formulas for how much zinc in pounds you need for a certain sized or weight steel hull...not hard to find....
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Old 02-08-2014, 09:42 PM   #6
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Perhaps there were some electrical issues in the water at the location your boat was previously docked? Or maybe it simply was lack of timely maintenance.
When I was working in the sugar industry I made many visits to Argentina usually Tucuman, Jujuy, and Salta area, but did make a side tourist trip Buenos aires to Tigre a very nice area. Beautiful country, fine people and Asados and Vino.
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Old 02-08-2014, 11:09 PM   #7
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Try this link and see if it helps.

http://www.boatzincs.com/anode_weight_calculation.html
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Old 02-09-2014, 04:43 AM   #8
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[QUOTE=neurodoc;211970]Hi, looks great! unfortunately the previous owner of my boat must really have neglected it, cause when I bought it most anodes were worn out and corrosion signs around the screws and rudders was quite important. .../QUOTE]

Haul out my boat once a year (located on 38th parallel). Replace zincs as needed, treat propeller, pressure clean bottom, touch up bottom paint as needed, with complete bottom paint every second year.
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Old 02-09-2014, 07:13 AM   #9
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Hi Steve, thank you for the nice words about my country! If you happen to show up in BUE again, you can count on sailing the delta on my boat and having a good asado and wine!
About the boat: you are right, it was not maintained at all for almost 10 years…. and there also must have been some electrical issues in the water, cause corrosion is asymmetrical (by far more important on starboard). When I bought it last Nov, all batts were dead + no electrical connection to shore so I assume a neighbour boat has been leaking some nasty currents into the water for a long time!
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Old 02-09-2014, 08:02 AM   #10
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Hi cpseudonym! Thanks for the formula. According to my first estimations, IŽll sink the boat if I load it with the zincs I calculated …. you can bet I made a mistake doing the conversion from metric to imperial:bang head: Tomorrow IŽll have it figured out by someone more experienced than me!
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Old 02-09-2014, 10:50 AM   #11
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Sorry about the math homework. It's been a long time since I've sat in calculus and algebra class for me too
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