A proper boaty question

[Magic_Sailor]

My boat is a Jeanneau Fantasia 27. She ain't got no anode attached to 'er 'ull. She never has had and I am not aware of any fixing point. She does have an anode on the prop shaft and another in the engine.

Whilst working on another boat the other day, I noticed a hull anode and mentioned this to the old salt working with me. He said, all boats should have a hull anode.

Anybody care to comment?

Anybody know anything specifically about Fantasia's about this point?

Many thanks

Magic

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[jimi]

I used to have a prop anode .. but it fell off alonong with the propeller .. I have now a "proper" anode wired in. Think you'll find a diversity of opinion on this but my thoughts is that it best to have a whopping great hull anode .. all IMHO

<hr width=100% size=1>O wad some Power the giftie gie us
To see oursels as ithers see us!

 

[Neal]

If you ain\'t got a problem...

with electrolysis, I think most people will say leave things alone. That's the advice I was given by MG Duff a few years ago, when I was wondering whether to fit anodes to my my 40 yo wooden boat.

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[john_morris_uk]

Re: If you ain\'t got a problem...

With respect, wooden boats cloud the issue in this case. (Built with the correct fastenings many wooden boats are better off without anodes - I've had to replace lengths of planking where anodes were fitted on a wooden boat and where the electolytic action destroys the wood etc etc)

The question should be 'Does the prop anode on your Fantasia erode each season and is the prop etc still on good nick?' If it does and the prop is ok, then the anode is doing its job and why fix somethings that isn't broke. If you do have other metals nearby that are being attacked electrolytically than you need more anodic protection with those metals connected as well.

I have one prop anode on my shaft and its replaced every year. Nothing else is bonded, and there's no problem. Follow the basic rules. Good electrical connection between the anode and the metal is protecting and the anode must 'see' the other metal ie be reasonably close.



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[john_morris_uk]

Re: If you ain\'t got a problem...

I've just remembered the advice I was given by a boat builder watching me fit the anode on the prop shaft of my Sigma a couple of years ago. When I'd done the screws/allen bolts/bolts up quite tight he came along and suggested I clouted the anode a couple of times sharply with a lump hammer. Sure enough, I could then do the screws up a couple more turns and I was confident it was in solid contact and wasn't going to fall off. Done the same ever since and never had a problem.

The p- brackets falling out of the boat , but the prop's not corroded...

Only joking. Rotate the shaft so you stress upwards towards the boat as you 'tap' it...

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[MainlySteam]

It would seem to me that you don't need one.

The general rule is that you will only get galvanic action if the two differing metals are electrically connected together, and some metals even if electrically connected together will still live happily. This means:

If you have a frp boat any metals exposed to the sea that are not electrically connected to a dissimilar metal do not need protection with an anode. Putting aside the propulsion, this is commonly the case on frp vessels unless someone has artificially bonded all the underwater metal bits together. It is not usually the case on metal vessels unless precautions are taken to electrically isolate other metals from the hull, or inert materials such as plastic seacocks, etc are used.

If your propellor shaft is made of a marine grade ss shaft material and your propellor is a proper bronze and your shaft is insulated from the engine (as is commonly the case if you have a flexible coupling) then you do not need a shaft anode either (this all obviously excludes saildrives which implicate aluminium in the circuit) - proper bronzes and proper marine ss shaft materials live happily together without significant galvanic action.

It is a common incorrect belief that if ones anode is being eroded then it is protecting something. Usually though it is only because one has created a battery that was not there before. For example, put a plate of copper in the sea and it will not erode (well not within our timescales), but put a zinc anode on it and the anode will erode very quickly and the copper will be as it was before - all that has happened is that one has created a battery and created a demand for zinc. People tend to think that if they bond all the metal bits of their frp boat together and to an anode, that because the anode starts fizzing they have done something positive. Whereas in fact they have just made themselves a battery.

While I am sure those views will stir a howl of protest, it is the way of the world, and that is the way we do it. As an example, our own boat is steel which is a much more difficult case than frp - it has no shaft anodes (the shaft is electrically isolated from the engine) and all underwater metal parts are either electrically isolated eg the rudder stock is isolated from the hull, or an inert material such as plastic is substituted. The boat, being metal, has one 8 inch teardrop anode on the keel and that easily lasts 2 years. There are no signs of galvanic action anywhere on the boat after more than 7 years continual immersion (averages about 5 days every second year out of the water). If the boat was frp the anode would probably last about as long as the same piece of zinc put in the sea without any boat attached to it.

It sounds to me that your boat may have been done properly - don't muck it up.

John

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[billmacfarlane]

I had a Jeanneau for 12 years, without a hull anode and never really had a problem. There was an anode on the prop and I put an extra one on the prop shaft but that usually fell off near the end of the season. The Yanmar ones were replaced every season. When I sold the boat there were no problems with the sea cocks, prop shaft, propellor or P bracket. I couldn't tell you why but as someone's already said - if it's not broke don't try to fix it.

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[VicS]

Re: Wacking the shaft anode

I think I would hold the heaviest hammer or lump of iron possible against the anode while tapping it on the opposite side with some thing a little lighter to minimise the shock given to the p bracket or the hull.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[vyv_cox]

Re: Wacking the shaft anode

I always hit the anode simultaneously with two heavy hammers on opposite sides. In the past I did experience the occasional loss of an anode, presumably because the bolts came loose, but never since using this method.

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[dickh]

Re: If you ain\'t got a problem...

That's a good idea, I thought I had tightened my shaft anode up really well when fitted in the spring, yet in July it was loose when I had a mid season scrub, so I tighten it up again. When hauled out two weeks ago, it was still tight.
I'll try "hammering " the new one next year.

<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif

 

[DeeGee]

There seems to be too much mystery about this...

1) there is a galvanic table, and the further metals are apart, the more active will be the electrolytic action between the two due to higher galvanic potential difference. The lower metal will be eroded to 'protect' the higher (aluminium and zinc, for example are at or near the bottom).
Refer to <A target="_blank" HREF=http://www.corrosion-doctors.org/Aircraft/galvseri-table.htm>here</A>

2) any two metals connected together and immersed in an electrolyte (eg salt water, but not pure fresh water) will form a 'battery' in this electrolytic sense.

3) alloys are metals connected together - NOT molecular indivisibles - so they will have this problem. Propellors are made from an alloy, and will have this problem.

4) the relative area of the metals is a determiner in the rate of erosion, so, referring to the lower galvanic component as anode and the higher as cathode.. a v large anode area rel to cathode area will result in slow erosion of anode, and vice versa. [i.e.a big anode is better than a little anode].

5) if a prop, or other component, is made from an alloy which will thus inevitably allow elimination of the anodic metal, BOTH the anodic and cathodic elements in the alloy can be proteced by a third connected metal, lower in the scale than either.

... so, 3) suggests that any through-hull, not connected to any other component, will nevertheless galvanically corrode. Your s/s shaft and bronze prop are close together in the table, so they will erode slowly - but they will erode. If you have a shaft anode, it will 'protect' shaft and the metals in the bronze alloy.

Beyond this, what happens to an individual case is a bit misleading.. too many factors are in the equation, as the resistance of the circuit to the anode is v important, as is the salinity, as is the area of the anode vs cathode. Just consider the facts and apply them to your particular circumstances, to explain why and whatever.

end of rant

<hr width=100% size=1>Black Sugar - the sweetest of all

 

[andy_wilson]

Whacking the shaft anode.

This could hold an all time record for the number of 'hits'.

There are going to be a few internet users reading this thread who were doing a search for something else!!!!

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[MainlySteam]

Is no mystery to me and am not sure where you get the view that it might be.

From your previous and current comments (your point 3) on propeller alloys (and other fittings) failing through galvanic action when isolated I must assume that the British have not discovered how to make good propellers yet. This is NOT a problem (at least within the possible lifetime of a boat) unless a poor alloy is used - usually, among the copper based alloys that just means avoiding brasses (some also miscalled "bronze") which are alloys with zinc. Brasses should never be used for a boat's immersed fittings and if they are it is representative of extremely poor practice.

Your comment flies in the face of reality as it is plain from the evidence (including that of my own boat which has an unprotected and isolated propellor and shaft) that it is not a matter of concern, providing correct materials are used as they always should be. Your comments on this are completely at odds with the professional practice applied to the vessels we manage the design and construction of and cause unecessary concern to amateur boatowners.

If you believe good bronzes not containing zinc will fail I would be very interested in your telling me exactly which of the alloyed metals is the anodic one to copper causing the problem.

John

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[DeeGee]

Sorry if I suggested that YOU found it mysterious, it was really meant to be more general, and your point on the thread was more or less accidental. I suppose it was the fact that you suggested no anode needed for prop and shaft.

The finest bronzes for shafts and or props are described as 'highly resistant to saltwater' - cos they do corrode, but SLOWLY, as I said before, as the alloy is from closeby metals in the table. The bronze alloy components will be lower in the table than s/s, by a short hair, and thus will SLOWLY corrode due to anodic behaviour. I do not want to row with you over this, so please don't take umbrage. <font color=blue> "Your s/s shaft and bronze prop are close together in the table, so they will erode slowly" </font color=blue>- I stand by my original words.

Agreed that zincy props are a whole different ball-game, and there are plenty of such things out there. A de-zinc'd prop just falls to peices eventually. I have had one. As an electrical engineer, I should have known better, but my prop was not connected to the anode, and it was a stock prop, I assumed (!) that the prop manufacturer knew his business,and the boat-builder his, but I was wrong on both counts.

Through-hulls, with no connection to any other part of (grp) boats (not mine, in this case) fail regularly due to de-zincing, as far less fuss is made of their material.

What sort of bronze and s/s do you use for your props and shafts, and I will tell you the anode-cathode relationship as well as the galvanic potential driving it, but I guess you already know....


<hr width=100% size=1>Black Sugar - the sweetest of all

 

[MainlySteam]

It may be that the shaft and propeller materials will erode slowly but it is not going to be at a rate of importance in the lifetime of the boat - they will also erode whether protected or not. All metals will erode slowly in seawater except for the very noblest, and there is no argument for protecting connected metals if the rate of erosion is similarly slow.

My first comment is that if one has underwater fittings upon which the watertight integrity of the boat relies eg thru hulls and seacocks, that need protecting in a plastic or timber boat, then one best replace them. Protected or not they will prove to be heartbreakers sooner or later. If one does bond them, then if one also does not also distribute zinc around the outside of the boat the protection will mainly be in ones imagination.

If one has a propellor that is eroding from dezincification, then one best replace it as introducing zinc for protection is only an elastoplast fix - one has a brass propeller and because of current distribution issues it can never be fully protected in any event. One may, of course, need to protect other things eg an aluminium sail or out-drive casing, from a propellor of proper and more noble material but that is an entirely different issue. If the shaft is not electrically isolated from the rest of the boat, then one may also need to protect other less noble metals on the boat from the propeller if there is a metal path between them.

Unnecessary protection only turns ones boat into a shorted battery, and the fact that ones anodes are eroding says nothing at all about the level of protection that they are providing (as I pointed out with the copper plate with a zinc anode on it example). If one placed an anode on an unprotected but of suitable material propeller/shaft combination (for example as on my own boat) then it would erode but only because of the creation of a battery, not from protecting it. Start doing that to the whole of ones boat, implicating seacocks, etc then the more zinc one loads on, for unecessary or "just in case" reasons, the smaller the internal resistance of the battery one has created becomes and the greater the potential for unwanted electrolysis issues to arise. Typically, for one reason or another, the boats with most problems are the ones loaded up with the most zinc.

In the end, professional operators pay me for managing the design and construction of high quality vessels for them (in fact I have had a North American client with me this afternoon regarding new builds we are managing for him outside of NZ), so if when I give freely another considers it wrong I am not fussed. My only concern is that others are not led into "solutions" that are ill founded on unnecessary fears and which may lead them into other unsuspected problems.

John

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[DeeGee]

I see no use in furthering this disagreement, but since you are citing your occupation to give weight to your professional opinion - I am a chartered Electrical Engineer, entitled to follow my name with CEng. MIEE with a special interest in marine corrosion. There is no doubt that sacrificial anodes will protect alloys of all kinds, and most boat owners will have no idea of what sort of alloy their prop is made. Protecting it with an anode, either on the shaft or hull, is not only the well-accepted method, it is the only method. In In the case where well matched metals have been used in the alloys, the protection will be less needed than where cheaper alloys are concerned.

<hr width=100% size=1>Black Sugar - the sweetest of all