| During a recent survey on a large steel yacht, the | | | | The clue here is the word 'stress'. The metal in |
| client elected to do the sea trial himself and my | | | | question must be under tension and the temperature |
| presence was not required. I joined the ship at the | | | | is usually high (about 150ºF, 65.5ºc...ever |
| ship lift ready for the lift and subsequent underwater | | | | touched a bottlescrew left soaking up the sun on a |
| inspection. Whilst waiting, I enquired how the sea trial | | | | hot day?). Finally, the metal must be regularly dosed |
| had gone. "Great" said my client, "except we had a | | | | in seawater whilst under load and ..bingo! The perfect |
| bit of a fright when one of the rigging bottlescrews | | | | conditions apply. |
| snapped..." To say my hair stood on end was | | | | Locked in stresses during manufacture also count and |
| somewhat of an understatement. "What happened? | | | | injection pipes where temps are high (i.e. exhausts |
| What did you do?" says I, "Oh well, we had a spare | | | | and elbows) are at risk. |
| on board and fitted another one on...no sweat!" | | | | However, back to the rigging....imagine the scenario. A |
| As you can imagine my attention was well and truly | | | | badly designed bottlescrew (lots about) that has high |
| fixed upon the rest of these fittings during the | | | | inbuilt stresses, not 316 stainless but an 'unknown' |
| subsequent inspection, especially when the client | | | | grade, rough surfaces and tensioned to the max for |
| produced the broken bottlescrew for me to look at. | | | | the stays and off you go, sailing in hot and salty |
| The event was a common one, not at all unusual. It | | | | conditions. Actually, even if you just leave the boat |
| was a stainless steel bottlescrew with a Norseman | | | | at its mooring those sinister forces will be at work. |
| screw down fitting to hold the stainless wire shroud. | | | | The wind in the rig produces a resonance that causes |
| The exterior finish was rough and no load or stress | | | | vibrations that can actually be heard and felt. The |
| numbers had been stamped or forged into the body | | | | boat rolling produces alternating stresses. A yacht can |
| of the screw. It was, in fact, a hybrid make of no | | | | roll in three seconds, one way, then the other. The |
| known origin and the story became clearer as it | | | | rig keel bolts and rudder is loaded up then the load is |
| unfolded. | | | | applied the opposite way. Over thirty thousand times |
| The owner, a diligent young guy said, "We've been | | | | a day, one million cycles in thirty three days. How |
| cruising for three to four years and as always money | | | | long has your yacht been rolling on its mooring? |
| was a bit tight. We came across this great little place | | | | Engines cause vibration and stress. Pipes, lines and |
| in Indonesia where these bottlescrews were so | | | | anywhere that doesn't have rubber hoses that |
| cheap we bought enough spares to replace the lot!" | | | | interconnect with rigid pipes are prone to stress and |
| Say no more. A year later, under a blue sky and a | | | | the subsequent cracking. |
| windless day just outside the Southport entrance on | | | | WHAT CAN WE DO? |
| a sea trial, in almost idyllic conditions they changed | | | | Sensible precautions are the simple answer. Check all |
| tack and....bang. How differently this could have | | | | rigging screws have a traceable origin. Specifications |
| turned out. | | | | when buying are important. Make sure load |
| The happy ending to this tale brings forth a subject | | | | maximums are stamped onto components that are |
| that is generally unspoken of in daylight hours. | | | | subject to stresses. Clean salt away on a regular |
| STRESS CORROSION. We are all very aware of the | | | | basis from exhausts, engines and rig with fresh |
| normal day to day corrosion that occurs on board | | | | water and regularly check all rigging and change the |
| when seawater mixes with differing materials but | | | | lot after ten years use. Remember, mooring time |
| stress corrosion is a different animal altogether and | | | | counts, inducing fatigue cycles that will weaken |
| the results tend to be a little more catastrophic as | | | | rigging. Never fit high tensile bolts to keels or |
| the parts under stress tend to be quite important | | | | anywhere where the heat and chlorine (salt water) |
| and are more often than not something to do with | | | | will abound. DO NOT over tighten these bolts at all! |
| the safety and security of the boat. | | | | Remember, stress corrosion starts with the tiniest |
| HOW DOES STRESS CORROSION HAPPEN? | | | | cracks and often the component shears without |
| Well, the sad fact of it is that how stress corrosion | | | | warning because the cracks are too small. I have had |
| actually happens is not fully understood. What we do | | | | people laugh at me when I produce my jeweller's |
| know is that the metal goes without warning and | | | | magnifying glass to look at rigging....but they soon |
| often without any outward signs. It is also confined | | | | stop when I show them the tiny hairline cracks I |
| to high strength metals and the softer stuff such as | | | | have found. |
| gunmetal, silicon bronze and mild steel is unaffected | | | | It is no laughing matter when thirty grand's worth of |
| as a general rule. It is worth a mention that aluminium | | | | rig crashes down onto someone's head during a |
| rivets (N6 grade) are subject to stress corrosion and | | | | weekend away. Play it safe, do the job properly and |
| can lose their heads under certain conditions. | | | | then you can laugh...all the way to the bank. |
| WHAT ARE THESE CONDITIONS? | | | | |