Sacrificial anode

[corinthmistral]

Grateful for views. My boat has a Volvo Saildrive. It’s factory/professionally fitted. The boat is a high quality GRP Dutch yacht. She has a cast iron fin keel. This year due to silting up/failing to dredge the marina within which the boat is kept, she is hard aground in the mud when the tide is out. When I hauled her out this week the saildrive anode has completely burned away and the prop (New last year) has suffered heavy corrosion and now needs replacing. Better that than the saildrive leg I guess. I had her plugged into shore power for a couple of months over the winter. During the summer she is not normally plugged in.

I have a replacement anode to fit but would prefer to add an additional sacrificial anode if possible, if there is a quick / easy way to do so. I noticed the other day that the starboard shroud plate (stainless steel) has a heavy wire connected down to one of the stainless keel bolts, as a factory fitted lightning conductor. Therefore, query: if I hang a good quality anode (I’m happy to invest in as pure/low on the galvanic scale zinc as I reasonably can purchase) on a wire connected to the starboard shroud, will this assist in reducing corrosion overall and reduce wear on the saildrive anode? I don’t think that it should affect the integrity of the stainless rigging itself, provided I use a good quality anode(?) I noticed another boat nearby does the same, although his “hanging anode” is not connected electrically to anything - it just hangs on a rope. Is it preferable to connect my anode to the rigging/keel or is a free hanging anode just as effective?

I would obviously just unclip/remove this anode when I go sailing. I am time poor(!) so with the best will in the world anything more than this at the moment is not practical for me.

Thoughts gratefully received.

 

[Catalina36]

For an anode to be effective it has to be able to "see" what it's trying to protect, ie to be as close to it as possible with nothing in the way. Will that apply to what you're suggesting?

 

[VicS]

Grateful for views. My boat has a Volvo Saildrive. It’s factory/professionally fitted. The boat is a high quality GRP Dutch yacht. She has a cast iron fin keel. This year due to silting up/failing to dredge the marina within which the boat is kept, she is hard aground in the mud when the tide is out. When I hauled her out this week the saildrive anode has completely burned away and the prop (New last year) has suffered heavy corrosion and now needs replacing. Better that than the saildrive leg I guess. I had her plugged into shore power for a couple of months over the winter. During the summer she is not normally plugged in.

I have a replacement anode to fit but would prefer to add an additional sacrificial anode if possible, if there is a quick / easy way to do so. I noticed the other day that the starboard shroud plate (stainless steel) has a heavy wire connected down to one of the stainless keel bolts, as a factory fitted lightning conductor. Therefore, query: if I hang a good quality anode (I’m happy to invest in as pure/low on the galvanic scale zinc as I reasonably can purchase) on a wire connected to the starboard shroud, will this assist in reducing corrosion overall and reduce wear on the saildrive anode? I don’t think that it should affect the integrity of the stainless rigging itself, provided I use a good quality anode(?) I noticed another boat nearby does the same, although his “hanging anode” is not connected electrically to anything - it just hangs on a rope. Is it preferable to connect my anode to the rigging/keel or is a free hanging anode just as effective?

I would obviously just unclip/remove this anode when I go sailing. I am time poor(!) so with the best will in the world anything more than this at the moment is not practical for me.

Thoughts gratefully received.

You must replace the sail drive anode and be sure to replace it whenever it becomes seriously consumed. Likewise the prop anode
If you use a hanging anode it should hang as close to the saildrive as possible and it should be connected to the upper gear box casing. Hanging one on a rope will achieve absolutely nothing.... what a plonker. Connecting it to a shroud wont do any good either. There must be a good electrical connection between the anode and the sail drive as well as the anode being close and "able to see" the saildrive leg
A zinc anode should be made of a special grade of high purity zinc normally the US military spec MIL-A-18001
You should also maintain the protective paint coating on the sail drive. That's its first line of defense against corrosion.

You dont say what engine and saildrive you have but if your saildrive is electrically isolated from the engine... see your engine manual,...... make sure this isolation is never compromised
If it is not isolated the engine probably has a fully isolated electrical installation ( ie not using the block as the common negative). If this is so make sure this isolation is not compromised

I am trying to decide if your shorepower installation should have a galvanic isolator. Do you know if the shorepower earth is bonded to any part of the boat or engine, If not a GI is not necessary. If it is then a GI is probably advisable.

 

[vyv_cox]

MG Duff market hanging anodes for this purpose. I fully agree what VicS has posted.

 

[duncan99210]

As has been said above, the Volvo Saildrive has to be isolated from the engine. I believe that rapid erosion of the anode can be a symptom of a failure in the isolation, so you should check to make sure that the isolation is intact. You can do that be using a multimeter to see if there’s a link between the engine and Saildrive. Any current flow between the engine and the Saildrive must be investigated promptly as it will lead to the leg corroding quite rapidly.
Somewhat puzzled by the demise of the prop: if it’s a Volvo standard prop they’re usually pretty resistant to corrosion as they’re fitted with an isolating bush as well as being painted. If it’s a folding prop of some sort, then it serves to reinforce the concern about the drive leg isolation.

 

[pvb]

I have a replacement anode to fit but would prefer to add an additional sacrificial anode if possible, if there is a quick / easy way to do so. I noticed the other day that the starboard shroud plate (stainless steel) has a heavy wire connected down to one of the stainless keel bolts, as a factory fitted lightning conductor. Therefore, query: if I hang a good quality anode (I’m happy to invest in as pure/low on the galvanic scale zinc as I reasonably can purchase) on a wire connected to the starboard shroud, will this assist in reducing corrosion overall and reduce wear on the saildrive anode? I don’t think that it should affect the integrity of the stainless rigging itself, provided I use a good quality anode(?) I noticed another boat nearby does the same, although his “hanging anode” is not connected electrically to anything - it just hangs on a rope. Is it preferable to connect my anode to the rigging/keel or is a free hanging anode just as effective?

No, a hanging anode connected to a shroud will do nothing to protect your saildrive, and a hanging anode on a rope will do absolutely nothing at all!

I'd suggest the problem lies with the shorepower system. If at all possible, always disconnect the shorepower lead when you're not on the boat. Fitting a galvanic isolator will help prevent premature erosion of the anode.

I'm guessing your Friendship 35 was built some 30 years ago, and I don't know when Volvo started fitting electrical isolation to the joint between the engine and the saildrive. Prior to that, the engine/saildrive on many boats was totally isolated from the battery negative except for starting, using a relay system. It would be worth investigating exactly what your system is - a decent marine electrician would be able to tell you very quickly.

 

[corinthmistral]

Thanks for the replies. Agree re: hanging anode on a rope, I couldn’t quite fathom that out.
The marina electrical system is very old, the boat/saildrive sits in the soft mid, I think, most of the time and all of the boats nearby are plugged in all of the time. I will avoid plugging her in wherever possible and just run the engine on load in the winter regularly to keep the batteries topped up - in the past that’s what I did, and had no problem with the anode. What lasted 3 years when not plugged in won’t last the winter when it is plugged in.
I will also check if there is any continuity between leg and engine.
Thank you all.

 

[Bobc]

If I were you I'd have a good look at the heat exchanger to make sure that hasn't suffered corrosion aswell. If your engine has lost its isolation, it can eat the alloy in no time.