During a recent survey on a large steel yacht, the client elected to do the sea trial himself and my presence was not required. I joined the ship at the ship lift ready for the lift and subsequent underwater inspection. Whilst waiting, I enquired how the sea trial had gone. "Great" said my client, "except we had a bit of a fright when one of the rigging bottlescrews snapped…" To say my hair stood on end was somewhat of an understatement. "What happened? What did you do?" says I, "Oh well, we had a spare on board and fitted another one on…no sweat!"

As you can imagine my attention was well and truly fixed upon the rest of these fittings during the subsequent inspection, especially when the client produced the broken bottlescrew for me to look at. The event was a common one, not at all unusual. It was a stainless steel bottlescrew with a Norseman screw down fitting to hold the stainless wire shroud. The exterior finish was rough and no load or stress numbers had been stamped or forged into the body of the screw. It was, in fact, a hybrid make of no known origin and the story became clearer as it unfolded.

The owner, a diligent young guy said, "We've been cruising for three to four years and as always money was a bit tight. We came across this great little place in Indonesia where these bottlescrews were so cheap we bought enough spares to replace the lot!" Say no more. A year later, under a blue sky and a windless day just outside the Southport entrance on a sea trial, in almost idyllic conditions they changed tack and….bang. How differently this could have turned out.

The happy ending to this tale brings forth a subject that is generally unspoken of in daylight hours. STRESS CORROSION. We are all very aware of the normal day to day corrosion that occurs on board when seawater mixes with differing materials but stress corrosion is a different animal altogether and the results tend to be a little more catastrophic as the parts under stress tend to be quite important and are more often than not something to do with the safety and security of the boat.

HOW DOES STRESS CORROSION HAPPEN?
Well, the sad fact of it is that how stress corrosion actually happens is not fully understood. What we do know is that the metal goes without warning and often without any outward signs. It is also confined to high strength metals and the softer stuff such as gunmetal, silicon bronze and mild steel is unaffected as a general rule. It is worth a mention that aluminium rivets (N6 grade) are subject to stress corrosion and can lose their heads under certain conditions.

WHAT ARE THESE CONDITIONS?
The clue here is the word 'stress'. The metal in question must be under tension and the temperature is usually high (about 150ºF, 65.5ºc…ever touched a bottlescrew left soaking up the sun on a hot day?). Finally, the metal must be regularly dosed in seawater whilst under load and ..bingo! The perfect conditions apply.

Locked in stresses during manufacture also count and injection pipes where temps are high (i.e. exhausts and elbows) are at risk.

However, back to the rigging….imagine the scenario. A badly designed bottlescrew (lots about) that has high inbuilt stresses, not 316 stainless but an 'unknown' grade, rough surfaces and tensioned to the max for the stays and off you go, sailing in hot and salty conditions. Actually, even if you just leave the boat at its mooring those sinister forces will be at work. The wind in the rig produces a resonance that causes vibrations that can actually be heard and felt. The boat rolling produces alternating stresses. A yacht can roll in three seconds, one way, then the other. The rig keel bolts and rudder is loaded up then the load is applied the opposite way. Over thirty thousand times a day, one million cycles in thirty three days. How long has your yacht been rolling on its mooring?

Engines cause vibration and stress. Pipes, lines and anywhere that doesn't have rubber hoses that interconnect with rigid pipes are prone to stress and the subsequent cracking.

WHAT CAN WE DO?
Sensible precautions are the simple answer. Check all rigging screws have a traceable origin. Specifications when buying are important. Make sure load maximums are stamped onto components that are subject to stresses. Clean salt away on a regular basis from exhausts, engines and rig with fresh water and regularly check all rigging and change the lot after ten years use. Remember, mooring time counts, inducing fatigue cycles that will weaken rigging. Never fit high tensile bolts to keels or anywhere where the heat and chlorine (salt water) will abound. DO NOT over tighten these bolts at all!

Remember, stress corrosion starts with the tiniest cracks and often the component shears without warning because the cracks are too small. I have had people laugh at me when I produce my jeweller's magnifying glass to look at rigging….but they soon stop when I show them the tiny hairline cracks I have found.

It is no laughing matter when thirty grand's worth of rig crashes down onto someone's head during a weekend away. Play it safe, do the job properly and then you can laugh…all the way to the bank.

If you liked this article and would like to have Terry Buddell write something similar for your magazine or would like to read more of the same contact www.dolphinboatplans.com or go to www.ezinearticles.com