Antifouling a Prop

[ribbony]

Welding need not be a problem, but when the conditions suit then it can be a good way to move metals around the area. Off one vessel and on to another. Anode/cathode principle.

The likely cause at the welders end is that there was not a good earth on the job and the current strayed via the salt water (I am assuming you are not in fresh water).

I heard of a situation where some welding was being done on a vessel with the welder on an adjacent barge. The earth on the vessel was not consistent and the current went via the salt water to blast off the anti-foul on its way. One worker noticed it floating in the water and stopped the process. The welder was wondering why he had to crank up the current constantly.

So, the welder should ensure that they have a good earthing between the work and the welder. that minimises the stray currents. besides it is for their sake as well, the current will be doing damage to both vessels in some manner.

Having a GRP vessel your points of connection electrically will be things like you drive train. have a good look around to see if anything else has suffered.

Best of luck !

[impi]

Thanks for the advice .... I think that's exactly what has happened here - How does the welder insure (or how do I check the guys in future) proper earthing?
Would it help if on my vessel I dangle a cable into the water with an 'anode'?

[osirissail]

Answer is Yes - See: Stray Current Corrosion . . .

Galvanic Isolator

for a discussion of the subject. Basically it amounts to "isolating" your vessels electrical system from the shore power system. There are many ways of doing this varying from cheap to expensive.

[Wotname]

My old thread is due for an update / feedback.

After relaunching the boat with the copper-platted bronze prop fitted, it stayed completely barnacle free for 12+ months. After about 18 months, there was small build up of barnacles and by 2 years it was covered to the point of being unuseable.

So yes, it did work for me. Over a year of zero barnacles. It was clear that the copper was no longer evident after 2 years. I am not sure it was cost effective and at least here, it is difficult to get copper platting done "while you wait". Lead times were weeks not days.

[MagnusS]

Originally Posted by Jim Cate

I would like to point out one discouraging fact:

Copper sheathing on old wooden ships was not applied as antifouling in the modern sense, ie to stop barnies, tube worm, coral and soft growth, but to prevent the dreaded shipworm (terado (sp)) from infesting their hulls. If you read Cook's journals, you will note that they frequently were forced to careen their ships to remove -- you guessed it -- barnies, weed and all the gunk that we worry about... off of their copper sheathing.

So, my friends, I fear that copper plating on our props will not solve the vexing problems of fouling. Might look nice for a while, tho...

Cheers,

Jim and Ann s/v Insatiable II lying Gladstone Qld Oz

Actually, cupper only prevents from barnacles where there is NO ZINC connected.

The English navy had tremenous corrosion in their ships iron parts under water due to the copper plating being more noble than the iron, and therefore they bolted masses of Zinc anodes to prevent that. Now, the zink stops the cupper ions from getting loose (a miniature form of corrosion) from the cupper surface, and makes the barnacles love to grow on it.

In this picture the propeller and the shaft was painted with a strong 50% Cupper antifouling, as well as the rest of the bottom. The shaft support metal bearing housing is electrically isolated from the zinc anode by the rubber bearing. Subsequently the propeller and the shaft becomes unprotected for the barnacles to attack, while the bearing housing just close to the zinc is protected, beacuse the zinc is electrically isolated from it.

In Sweden, the SXK (a Cruising yacht club) have made trials and put cupper pipes into sea water, half of them with zinc anodes boted, half without. The result reminds about this propeller picture. Without zinc anodes, no barnacles.

[Cheechako]

Great info and demonstration. (That is what your boat bottom looks like after 5 weeks in Luperon! With fairly fresh bottom paint.)

[Nicholson58]

[QUOTE=MagnusS;879258]Actually, cupper only prevents from barnacles where there is NO ZINC connected.


In this picture the propeller and the shaft was painted with a strong 50% Cupper antifouling, as well as the rest of the bottom. The shaft support metal bearing housing is electrically isolated from the zinc anode by the rubber bearing. Subsequently the propeller and the shaft becomes unprotected for the barnacles to attack, while the bearing housing just close to the zinc is protected, beacuse the zinc is electrically isolated from it.


WOW - what a difference fresh water makes. Here is our bottom after two seasons in Lake Michigan; no coatings on the prop or shaft; VIVID antifouling.

[MagnusS]

Originally Posted by Cheechako

Great info and demonstration. (That is what your boat bottom looks like after 5 weeks in Luperon! With fairly fresh bottom paint.)

NOTE:

NEVER remove the zinc anode from a propeller unless you have got a verification from the propeller maker that it is really made of marine grade yellow metal. Sadly, some makers use de-zinking yellow metal grade, and those MUST be protected by Zink anodes.

Also make it 100% shure by Ohm measurement when om land, that the propeller and shaft is NOT connected electrically to the shore power safety ground. That is a precaution against corrosion from potential differences injected by the shore protected ground and the potential of the seawater surrounding the yacht.

Saildrives from Yanmar and VOlvo must have propellers where the propeller hub outer part is electrically separated from the hub inner part. Otherwise the bronze/Brass propeller will damage the Aluminium casting of the Saildrave. Good quality folding propellers for these saildrives are made like that. This is also easy to measure by a Ohm meter when on land.

[billr]

Interesting. The Walters Keel Cooler on my boat is a copper-nickel alloy. It has a zinc, and some fouling on the bare metal, but not much even after 20 years.