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Yacht Safety: Oxygen Deprivation Corrosion

The propeller shaft inside the tube, where you can’t see it, is prone to crevice corrosion. Photo: OceanMedia
Yacht Safety: Oxygen deprivation corrosion: Are the keel bolts in good condition or is corrosion eating them away? Photo: OceanMedia
Are the keel bolts in good condition or is corrosion eating them away? Photo: OceanMedia

Recently a senior pilot for US Airways told me that on commercial airplanes all operational hours are meticulously logged, and how regulations dictate that essential mechanical and electronic parts are replaced after a certain number of hours regardless of whether they are broken/worn or not. It’s purely a safety issue. On charter yachts there appears to be no such ruling. Now someone will likely say that all yachts are inspected annually by qualified marine surveyors, before insurance policies are issued, and this ensures their integrity. Wrong! Some critical components on yachts are impossible to examine without removal of these parts. Examples are propeller shafts, rudder shafts, keel bolts and transmission and throttle cables. Interestingly, annual surveys are no longer required by insurance companies since some outfits decided to only require a three year survey, apparently to garner business. In order to be competitive other insurance companies mandated the same three year inspection requirement, so in some ways safety has been compromised.

According to Bill Bailey of Caribbean Marine Surveyors Ltd., there is no requirement for sailboats, including charter yachts, to be inspected for any of the above examples of potential critical component failure. Power vessels, on the other hand, are regulated by the Maritime Coastguard Agency (MCA) and more rigorous inspection is required every five years. If the primary source of propulsion is sail rather than power this relieves the sail boat owner of this responsibility. However, a sailing catamaran, when maneuvering, uses its throttle and transmission cables intensely and the potential for failure is quite high.

In the past five years this writer has had transmission cable failure three times on catamarans, once causing considerable damage but luckily with no personal injury. In the case of transmission cable failure the yacht will likely leap forward if the cable snaps when changing from forward to reverse because the engine will still be in forward while RPMs are increased.

As Mr. Bailey emphasized, a competent surveyor will notice any flaws in the outer sleeve of a cable and recommend it to be changed but cables sometimes run through inaccessible spaces and are impossible to examine with accuracy without complete removal.

Oxygen deprivation corrosion: The propeller shaft inside the tube, where you can’t see it, is prone to crevice corrosion. Photo: OceanMedia
The propeller shaft inside the tube, where you can’t see it, is prone to crevice corrosion. Photo: OceanMedia

Similarly the loss of a propeller shaft or rudder shaft could be disastrous and keel bolts that may have been compromised by grounding need to be pulled for inspection. Where a stainless steel prop shaft goes through the deadwood (dead f/glass) it becomes prone to oxygen deprivation corrosion or pitting resulting in eventual failure. This happened to me with nerve racking consequences. I was sailing my Tayana 37 and was approaching Fiji from the north. I had just located the entrance to the archipelago through a cut in the reef. I had taken down the jib and had fired up the motor and when I engaged it in forward the prop shaft snapped. It backed itself out of the boat and jammed against the rudder. In seconds I had a severe leak, no steering, and no power. I managed to jam a rag on the inside at the stuffing box and reduced the leak to a trickle. I sailed the boat away from the lee shore and hove to. Then I jumped into the water with mask and snorkel and managed to jam the broken shaft back in and tied a line from the propeller up and forward over the toe rail to a stanchion base to keep it from backing out again. Now I had steering, a small leak and no power. I sailed the boat through the reef and anchored in a reasonably sheltered spot and took a breather – a big breather!

I later discovered that the cause of the failure was oxygen deprivation corrosion, in other words, severe pitting. The message to buyers of older boats is clear:

Do yourself a favor, pull the keel bolts, rudder and propeller shaft. Don’t forget those sacrificial zincs. Change those suspect cables and look carefully for any other areas subject to corrosion. You just might not be as lucky as I was.

 

Julian Putley is the author of ‘The Drinking Man’s Guide to the BVI’, ‘Sunfun Calypso’, and ‘Sunfun Gospel’

 

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