Cruising comments

We had a dodgy Autumn dodging typhoons, but found a weather window for the COA National Day Rally re-routed from Macau, which took us on a three-day tour of the Sokos, Silvermine Bay and Po Toi Island. We enjoyed some lovely winds, sunshine, food and great company to ameliorate the disappointment of not being able to go to Macau.

Repairs to the dock are under way at Fisherman’s Wharf, but may not be completed by Chinese New Year. We are monitoring the situation carefully, so please keep an eye on our website : www.coahk.org

Unfortunately the COA Emerald Bay Race to the VRC in Sai Kung had to be cancelled due to Typhoon Khanun, but it will be back on the calendar next year.

A fresh breeze for the inaugural COA Douglas Cup, sped the fleet towards the Lamma Fun Day Music Festival, raising HK$5,450 for the Child Welfare Scheme in Nepal (www.cwsn.org). What a party . . . see you there in 2018!

By the time you read this, the Commodore’s Christmas Cup, will have concluded 2017, with four simultaneous staggered start races finishing at RHKYC Middle Island for a prizegiving party.

I recommend starting 2018 with HHYC’s New Year’s Day Beach BBQ and don’t miss the HHYC Kowloon Cup, the first race in 2018 COA Championship Series. Check out our Cruising Calender at www.coahk.org for details and follow us on Facebook. If you interested in COA events, do join us! You can download the membership application form from our website.

My technical subject this month is ‘crevice corrosion’. “What’s that?” I hear you ask. Well, it’s the common mode of failure in stainless steel and the reason people refer to stainless as ‘unreliable’.

The mechanism is a difference in the electrical potential between the top and bottom of a crevice, pit or crack, which causes National Day Rally On the beach at Lamma Fun Day Douglas Cup prizegiving Lamma Fun Day beach party Crevice corrosion in stainless steel shackle Crevice corrosion in chain plate galvanic corrosion. The corrosion products (eg. rust) expand in the crack adding stress and extending the crack. ‘Stress corrosion cracking’ is broadly the same mechanism, accelerated by sustained high operating stresses, untreated stresses frozen in during manufacturing and high temperatures.

It can happen in many materials, but stainless steel alloys in the presence of chlorides are particularly vulnerable. This is important to yachtsmen as many of our key structural parts are made of stainless steel, immersed in a concentrated solution of sodium chloride, better known as salt water! Chloride concentration levels are highest in splash zones due to water evaporation, ie. deck level.

316 stainless steel is commonly used in marine environments. Its 2% molybdenum content makes it more resistant than 304 and other common grades, but it’s not immune. Modern marine fittings are designed to minimize crevices and blind holes and are highly polished to eliminate surface pits when the process can start.

Therefore, keep an eye open for tell tale rust stains and then inspect these areas closely for cracks. For key areas like chain plates, you can carry out dye penetrant crack detection using a special two-can aerosol spray kit. Your boatyard should have one. For mooring shackles, inspect them annually and replace them regularly, it’s cheaper than a new boat.

One of the best saltwaterresistant materials is galvanized mild steel, as the hot-dipped zinc metal coating fills surface defects, preventing the process starting. This is why we use it for mooring chains and anchors.

However, the sacrificial zinc coating eventually corrodes away leaving the steel unprotected. That said, mild steel is a ductile material, less susceptible to cracking than the more brittle stainless steel. Anyone who has cut through a hopelessly rusty mild steel shackle will have seen that the corrosion is on the outside and there is still clean, strong steel is left inside, hence its reputation for reliability.

Many shackles available in Hong Kong are zinc plated, which is too thin to provide long term protection, so best to seek out the best quality green pin shackles for your ground tackle. Hope to see you on the water in 2018, happy cruising !

 
       
 
Widget is loading comments...
 
COPYRIGHT © 2018 FRAGRANT HARBOUR PUBLICATIONS LTD