Left: Figure A is a simple plot of overall length against range of positive stability – crudely the ‘heel’ at and beyond which each yacht will naturally remain upside down. Given the fortunate rarity of such incidents this is most of the data recorded that is available for designers and rule managers to work from. Grey triangles represent popular yacht types; purple triangles are instances of yachts that inverted and recovered; red dots are instances of a yacht failing to self-right (many with loss of life). Above: Figure B illustrates the range of positive stability for a generic 46-footer – in this case to a heel angle of 136.5 degrees
Length-Capsize angle plot. The ORC rule requires that the yacht’s hull and appendage geometries are measured, and the freeboard measured to calculate the displacement. Also the yacht is given an inclining test to measure its righting moment. Combining the calculated displacement, the hull geometry and the righting moment the ORC software calculates the position of the yacht’s centre of gravity and the stability curve. The boat shown in Fig B (top right) is 14m LOA and with a range
of positive stability of 136.5° it sits well above the safety boundaries. That said, the ORC righting moment curve is not the same as you would get from a full ISO-certified inclining test. For historic reasons the righting arm curve assumes the boat
has a flush deck and no cockpit. This means that a large cockpit that floods as the boat heels and thereby reduces the range of sta- bility will not be captured; also a large coachroof that increases the immersed volume and increases the range of stability will be missed. But, these considerations aside, the ORC methodology is accu-
rate and consistent. If you want to get a stability curve that includes cockpit volumes, coachroof and so on, the inclining test data does not need to be repeated – you just need to update the geometry file. Also, with careful weight management the test does not need to be repeated very often; as weights are taken on and off the boat the vertical centre of gravity of the boat can be recalculated and the stability curve revised based on the new VCG.
Inclining Discussions about the value of the inclining test often revolve around two topics: ‘The inclining test is difficult, and anyway it’s not accurate
because…’ and ‘Why bother with this when I can get data on a boat just like mine
and use that?’ Neither of these objections holds water. The inclining test has
been part of ship stability calculations for more than a century. It works for vessels of all sizes from cruise liners to small yachts.
It’s a simple test to do: pick a calm day and use a shifting weight from side to side, induce a heel angle, measure the heel angle for each weight shift, measure the weights, and that’s it. Today you need to have a CAD file of the shape of the boat, and you need to be able to fix the position of the waterplane at the time of the test in that file. Then it is just maths. Back in the day when the naval architect was working with a
planimeter, Simpson’s Rule and log tables you had to be very careful about where the heeling weights were positioned so that you didn’t introduce errors due to changing trim. Now modern hull design programs just need to know where the weights were at each stage
of the process. It doesn’t matter if you use the boom, or a spinnaker pole, or weights on deck, you just have to heel the boat. Also, there is nonsense talked about how much heel angle you
need to achieve for an accurate result – the answer is as little as you can get away with. A cruise ship inclining will see the boat heel by a few tenths of a degree, but because you can use a pendulum that’s metres long the deflection can easily be measured. A 10m pendulum hung in a stairwell deflects 17mm for each 1/10th of a degree – inclining more does not improve the accuracy of the result. So make life easy for yourself, measure the heel angle accurately, collect plenty of data points and make sure you know the flotation waterplane. Then close your ears to those who may say ‘I wouldn’t do it like that’. And the ‘Why bother, there’s lots of typical data’ objection? Yes,
there is a lot of data for boats ‘just like yours’, but how do you know for sure? A quick check through the ORC database shows that for many well-known production boat types there are many variants (for example, deep keels, shoal draft keels etc), so displacement and range of stability can vary by 10% or even more. If you are serious about safety knowing your boat’s range of
positive stability is just as important as having non-expired flares, secure keel bolts and so on. Once an inclining test is done the results are of lasting value. Even if you change the mast or ballasting arrangements, provided you keep track of the changes in terms of weight and centre of gravity position a new righting arm curve can, and should, be calculated. This is exactly how the ORCi system works: during the flotation
and inclining test the weight inventory of movable items is recorded; thereafter the certificate can be updated to a new loading condition without the need for a new inclining test. The OSR acknowledges that not all entrants for offshore races
will be able to put their spot on the graph, because only ORCi mandates an inclining test to get a certificate. In these cases they require the application of parametric screening systems based on weights and dimensions. These include the ISO STIX calculation and the IRC SSS. These screening tests were developed over time by experienced designers and safety experts and offer an alternative if an inclining test cannot be done. However, what they don’t do is generate a righting moment curve,
which is the key element of accurately assessing the range of positive stability. If a yacht is close to failing these screening tests, then the first step to fixing the problem is getting an inclining test done. The investment is worth the peace of mind. Not to mention complying with the rules. Andy Claughton, International Technical Committee * ISO 12217-2:2017
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