Prop Corrosion

[clyst]

Hi Guys!

The last couple of years Ive experienced a chalk like growth/corrosion on the prop of my small sloop. This when scraped comes off readily and taste neutral to the tongue.It builds rapidly although not all over the prop but in places it reaches 1/4" thick. Another problem is that the sea cock "furrs"up on the inside of the hull but on the outside casting around the hull pad! I notice that sometimes the sea cock "sweats" when the engine has run for 1/2 hour or so. Both the prop and seacock when cleaned looks in good condition and ring when tapped.
The boat is kept on a drying mooring in cornish river. So no chalk!!
The engine is a Stuart Turner (What a beauty too!!!)
The hull anode is correctly connected and errodes at a rate as would be expected
Any advice would be appreciated.

 

[oldharry]

I've not come across this chalky deposit so cannot comment on that, but its entirely normal for the cold water inlet to 'sweat'. The air in the engine compartment warms up from the heat of the engine, and is very humid. The metal of the seacock remains at seawater temperature, so the humidity condenses out on it - just the same ways as windows mist up on a cold day.

Lucky man to have a 'good' Stuart! They are a bit like the old Seagull outboard - they either run perfectly or not at all! Even brand new ex-works ones could be non-starters from the word go. No one ever quite got to the bottom of it, and even Stuart Turner themselves had to admit defeat occasionally.

The joys of early 2 stroke engineering!

<P ID="edit"><FONT SIZE=-1>Edited by oldharry on 23/08/2002 08:39 (server time).</FONT></P>

 

[charles_reed]

Sounds as though your anode is working overtime - you haven't, by any chance got any aluminium below the water?
In fact the deposit is probably hydrated zinc salts, unless you have some stannous deposits coming down in the fresh water.

 

[Trevethan]

What sort of anodes do you use? If you are on fresh water most of the time, then rather than zinc, magnesium is called for I believe. Depends how high up the river you are I suppose!

You can find out what the deposit is by heating it in a gas flame. various metals will tend to colour the flame differently. Not sure what colour zinc turns a flame (its been 18 years since I took chemistry and all I remember is that copper turns a flame blue-green)

Scrape some off and blast it with a blow lamp. Come back and tell us the colour. I am sure some sciencey types here will be able to tell you what it is.

Regards,

Nick

<P ID="edit"><FONT SIZE=-1>Edited by Trevethan on 23/08/2002 12:25 (server time).</FONT></P>

 

[clyst]

Thanks for the info Charles
HYDRATED ZINC SALTS---UMMMMMMMMMMMMMM not being a chemist is that as bad as it sounds? There is no alloy fittings nor is any other boat in the river affected. do you think this needs further investigation or just live with it as it does not seem to be de-zincing the prop just soaking up H.P. from my trusty STuart!

Cheers ! Terry .

 

[matt_lake]

I get these deposits on my Broom 37. We spend on average 7 months in fresh water and 1 month in salt water and use soft anodes which have to be replaced every season. The problem is that when you clean the deposits from the props there is slight pitting underneath which will probably mean new props in the next year or so.