Anodes - prop damage

[Guest]

A lesson learned here guys.. check that shaft anode.

I lifted out recently and found that the shaft anode was missing..
There was my lovely maxprop looking like someone had attacked it with a welding torch!

Estimate for repair is £300 plus... although some say that it should not be repaired as dezincification will have weakened it.. ? any expert views?

Also the bronze in the cutlas bearing was gone..
I think I took it out just in time..
It will be going back with a fixed prop and an anode over the side clipped to the shaft for the winter.
Not sure about longer term but snorkel and mask will be donned in the summer to make sure the anode is still working.

 

[charles_reed]

Sorry to hear of the damage.

However it sounds as though you've got a major galvanic leakage problem on your berth - you'll probably find you need to replace your ss propshaft as well.

Your standard hull anodes should have held the fort - are they properly connected? have you got A/C power leakage going on close at hand? are you in an old mining area with high concentrations of metallic salts?

Apart from the shaft anode (they always come off before the end of a season) I have two big hull anodes and a separate anode on the boss of my prop.

 

[Guest]

Only goes to prove that a proper system with a through hull anode bonded in to all the relevent metalwork can't be beaten.

If you've just got the shaft anode, have a look at the P bracket as well, could be disappointing, espcially if it hasn't been bonded in to the circuit via the stern gear.

You have my sympathies, I had to by a new prop last season because of inadequete bonding. Then the boat fell apart, structurally, but thats another story! and nothing to do with anodes!

 

[Guest]

There are no other anodes on the outside of the boat.. the prop is insulated from the engine by the coupling.. ( some anodes in the engine).. shaft and P bracket both SS and look ok .. saved by the prop I guess..

Seems strange as boat is ten year old.. never any other anodes fitted just one on the shaft.

Regards

 

[Guest]

Robert, I suggest you fit at leaast 2 anodes (the big ones) to the outside of the hull. Then wire them all up together. My wire starts at the chainplate goes down to the top of a keel bolt, tee's off to each stainless water tank, goes on to the engine then to "P" bracket and finally to the rudder stock. I also have a connection through the flexable shaft coupling. Each time the boat is taken from the water I check the continuity on each of the anodes, prop shaft, keel etc. Also check condition of anodes and replace if they look to bad.

 

[charles_reed]

You need 2 external anodes 500gm ones will last 1 season so I'd suggest 1kg ones.

These should be wired with at least 3mm2 cable to connect with your engine and RF earth plane.

Additionally you will have pencil anodes on your raw water cooling - these will probably need changing every 2nd year.

Do check ALL your thro' hull fittings - unless plastic they'll all have dezincified and will probably need replacement.

You've actually come off very lightly, the natural outcome was sunk at berth - mend the Maxprop it can only shed blades.
Draw the shaft - any pitting will be the part in the tube not the exposed shaft. If the P-bracket is of 316 A2 it should be OK

Suggest using phenolic cutless (note spelling) expert info from Countrose Bearings, Keith Reddings or Andy Markham on 0121 356 7220.

Look very critically at the berth, someone, somewhere is acting the kathode - and it's probably not you.

The lack of external anodes can only point to very careless or naive builders.

Suggest the £40 or so spent fitting anodic protection will be a very worthwhile investment.

 

[Guest]

You might also consider adding a "galvanic isolator" to your shore power ground (earth) if you have a shore power hook-up. It will prevent stray AC from mingling with your DC and cut down on galvanic corrosion. You might also want to check out Nigel Calders book or Don Casey's to see how to do the tests for stray AC or DC current to see if the problem is in your marina or berth.

I would do like the other gentleman said and rebuild the prop. A proper rebuild will make it as good as new.

 

[ccscott49]

I can only add to what has been said, check your berth, fit big anodes, incidently the backing of your cutlass cant be bronze. it wouldnt have corroded away.

 

[HaraldS]

I recommend you check out this URL from Westmarine Advisor, especially the part about "Hot Marinas". http://www.westmarine.com/webapp/co...dvisor.d2w/show_advisor?fn=50.htm&store_num=9

You have got a lot of good advice here, but I think the common thinking that bonding all underwater metal and connecting it to big through hull anodes is probably not going to help for long in a real hot marina, the big anodes just buy you time.

In the US it's quite common to bond every bronze seacock and it was exactly those that got dezinced.

You mention that your shaft is isolated from the engine by the flex coupling, if that's really true, then your prop was eaten by just the electrolysis between the prop and the shaft, after your shaft anode was eaten away. This means some neighbor must have a severe problem and you might not have any defense gainst that.

Next thing I'll install on my boat will be a hot marina alarm...

 

[charles_reed]

I have to agree with HaraldS - big anodes only buy you time.

The problem is that your boat has been in a bubbling galvanic cell, probably nothing to do with you or your boat.
If you're in a marina rattle the bars around the management's cage - it's a 90% probability the problem is down to a shorepower leak.

 

[HaraldS]

Here's another suggestion: Take a digital voltmeter and drop the two leads into the water about 3 feet apart. Try a couple of directions. Since your isolated shaft is probably only about that long you should see something near or more one volt and increasing with the spacing in the direction of the current flow through the water.

I tried this once when I was anchored in Norway near DC power terminal for an underwater power cable. - I changed into another corner quickly.

DC power transmission is getting more and more popular since it saves one wire, they run large ground grids on the terminals to transfer the large currents to the water. No good thing to be near to.