Anode life and sea temperature

[Ships_Cat]

Re: Interestingly

Hi Chris

It can be true and is easy to build an frp or timber boat so that it is. But agree that is not wise to disonnect them (or let them disconnect themselves) on an existing boat unless one is sure though /forums/images/graemlins/smile.gif.

Our own 12 tonne boat is steel and only carries one 200 mm long x about 60 mm average width teardrop anode on one side of the fin keel and no others such as shaft anodes, etc. The boat lives in a marina and is on shorepower for at least 2 days every week and has been also for periods of up to 9 months continuously with us living aboard.

Has been like that since new 9 years ago and no problems. We replace the anode every 2 years and look after the underwater hull coatings well. Even though steel, it would probably be ok without the anode.

John

 

[finnlou]

Re: Interestingly

[ QUOTE ]
Hi Chris

Has been like that since new 9 years ago and no problems. We replace the anode every 2 years and look after the underwater hull coatings well. Even though steel, it would probably be ok without the anode.

John

[/ QUOTE ]
yes, coating is the key /forums/images/graemlins/smile.gif, you could also test that if you had a siver/silver chloride halfcell.

although you probably need an anode for your propshaft as this will have bare metal, also, one thing of note, I have seen a few boats where the anode fixing studs were under water in the bilge, not a particularly good idea, if you can't reduce the water level then make sure you encase the connections in epoxy or something /forums/images/graemlins/smile.gif

 

[Ships_Cat]

Re: Interestingly

Yes the builder and I set the boat up after launch with a half cell (the boat actually had two anodes on when launched and one could be removed) and then rechecked with the cell again some months later and lifted the boat as well to be sure. Agree with your comment on the coatings.

The shaft and prop have proven to be fine with no anodes even though they are not isolated electrically from the hull - but we bought a prop from an already trusted manufacturer (fixed three blade manganese bronze and high performance ss shaft). The shaft is isolated from the engine though and the engine is isolated from the hull and all engine electrics are isolated from the engine.

I would have preferred to have the shaft entirely isolated from the boat but it was the builder's risk and has in the event proven to be fine. To electrically connect the shaft to the hull he replaced the Thordon bearing in the shaft seal assembly which floats on a rubber tube from the shaft log with a metal one and grounded the assembly back to the hull with a copper strap. I could easily isolate the shaft from the boat simply by removing the strap but work on the principle if it is ok now is best not to change it.

John

 

[finnlou]

Re: Interestingly

[ QUOTE ]
The shaft and prop have proven to be fine with no anodes even though they are not isolated electrically from the hull

[/ QUOTE ]
That is because it is being protected by the anode you already have since it is not isolated from your hull. In fact, if your hull coating is that good, the propshaft is probably the only thing your anode needs to protect

 

[Ships_Cat]

Re: Interestingly

Quite probably to some extent but the anode is actually around 5 or 6 m away so I think is unlikely to provide much protection and certainly not in the actual local shaft/prop contact area which is the normal main problem area assuming one hasn't a prop made from a poor material.

Although I should say that when the boat was checked with a half cell the only unpainted metal exposed to the sea would have been the shaft and prop and the interior of two ss standpipes but although I cannot now recall for sure as was quite some time ago now, I think we accepted the single anode on the basis that it gave the required shift against the assumed corrosion potential for steel not the bigger shift required against the corrosion potential for bronze.

I would not have thought that the small area of zinc we have and given its distance from the prop (which is 18 inch 3 blade so quite a big area), that it would depress the voltage when checked with the half cell enough to protect it, but that is pure assumption on my part and I may well be wrong.

But whatever is happening, as always, the best approach is if what is there is working then not to change it /forums/images/graemlins/smile.gif.

John

 

[finnlou]

Re: Interestingly

The distance from your anode does not actually have that much bearing on whether or not it is or isn't protected, the important thing is that they are bonded together as long as there is a return path, current will flow, and you would be surprised how far it will flow to get a return path. So if your hull coating is perfect it only has one return path and that is via your prop shaft, 6m of salty water will not stop it.