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heightened risk

Image courtesy ORCV

Heightened Risk

The risk of a yacht being knocked down by wind and waves, so that the mast is horizontal to the water, is so well known that the yachting authorities write rules that encourage yacht design to survive this occurrence.

In the first instance it is necessary that the weight of the keel on the one side exceeds the weight of the rig and the down force on the sails on the other so that the yacht has a tendency to want to come upright again. This is called resistance to capsize and is a requirement of all keel boats racing under Australian Sailing (formally Yachting Australia) Rules. The way a yacht can demonstrate this is either to present detailed designer calculations or to do a physical haul down test. With a one design class it is usual only to test one boat and to assume all the others will be the same. Passing the requirements of either of these methods is not a guarantee that a yacht will, in fact, resist capsize or self right in all conditions at sea.

The two principal reasons why passing the knockdown tests is no guarantee yacht will self right, is that firstly both assume calm seas, which is unlikely in the sorts of wind that would initiate a knockdown, and secondly the risk of down-flooding. Down-flooding is when water starts flowing into the hull of a boat over cockpit coamings or through improperly sealed hatches. When water enters the hull it moves the center of floatation (the fulcrum for the balance of righting and tipping forces) usually in favour of ‘the tipping’ as opposed to ‘the righting’. For this reason Australian Sailing have a rule that when a yacht is tipped to 90 degrees all openings to the interior of the hull shall be above the water. Again this assumes no waves. Now we are faced with an issue of seamanship. That is, the maintenance of the watertight integrity of the hull. It is rare to find a hatch or opening that is 100% watertight but if a hatch is not properly secured and flies open, allowing litres of seawater per second to enter the hull, and then the chances of recovery from a knockdown are significantly reduced.

As yachtsmen we should now be well acquainted with the concept of heightened risk when it comes to wearing life jackets. When the wind and waves spring up, and we put lifejackets on because it is prudent to do so, we should also be: locking in our cockpit washboards, ensuring our hatches are secure, and keeping our bilges dry as best security against a knock-down.