ORC Double trouble
I’m afraid last month’s ORC column suffered somewhat due to last- minute pressure on space, leading to the omission of some of the graphics… this no doubt made a tough read into an impossible one. So thank you for bearing with us. My previous thread was that rating boats with a single number
TCF is easy to understand and administer, but inevitably different boats with the same TCF do not go the same speed all the time. Using the ORC VPP and the Sailor Services tools I have had a peek behind the curtain of the single number TCF, to see how good good days might be and how bad bad days could become. A little bit of ‘know your enemy’. In the last column I used a rather extreme example, comparing
a Farr 40 one-design with a Class40 ocean racer to show how their relative performance changes with wind speed and point of sail. The ORC VPP calculates a polar speed table and this has seven wind speeds and 10 points of sail (True Wind Angles). The polar table is typical of those used by navigators and routers. The software also calculates a single number All Purpose Handicap (APH) based on a mix of wind speeds and points of sail typical of an offshore race. The speed difference can be expressed as TCF at each point on
the polar curve. For example, at 52° TWA the Farr 40 is going at 6.54kt, the Class40 at 6.41kt. So our Farr 40 is going 1.020 times the speed of the Class40, ie 2% faster. The TCF for the Class40 to the Farr 40 is 1.020, ie 20 clicks. The table of comparisons at the bottom of this column shows
how many clicks the boats would owe each other if they competed on that exact point of sail in that wind strength. As you can see the deltas run to hundreds of clicks in stronger winds on a reach, which is more than loose change in the TCF currency. The table shows mainly red cells – which indicate where the
Class40 is faster than the Farr 40. This is to be expected as the ratio of the APH is 476/455 =
1.046 – the Class40 therefore being 46 clicks quicker on APH. This APH difference can be folded into the table below to give
a new table (below right) that shows how much the handicaps at each point on the polar table differ from the APH TCF. Where the cells are positive (green) the Farr 40 has bonus clicks that make it easier to save her time on the Class40. Where the cells are negative (red) the Farr 40 has penalty clicks that make it harder
Farr 40 Polar table – All Purpose Handicap = 455 sec/mile
for her to save her time if the APH is used. So depending on the wind strength and course mix each boat will feel the relative bonus or penalty clicks when a race is sailed using the single number TCF. Upwind in 6kt the Farr 40 could sail 7.9% below target and still
save its time on the Class40. On a downwind leg in 20kt of wind the Class40 could sail 3.6%
below its polars and save its time. This is an extreme example of every dog having its day. But if
you do this kind of analysis on any pair of boats there will always be differences between the polar point TCFs and the single number APH… some good, some bad depending on where you are sitting! Of course over a season of offshore racing these bonus and
penalty clicks should even out, it’s worked that way for 100 years. Single number is a very sensible way to go, no problem there, and Mrs Claughton agrees. But be aware that for any pair of boats there will be a distribution of bonus and penalty clicks through the wind and course range. This is where the ORC VPP can tell you where and why they are there… While sailors are happy to deal with the click lottery when racing
offshore there are times when keeping it simple can be overdone. The 2022 Sydney Hobart Race highlighted an anomaly over what
sail area a boat should be rated with. Traditionally rating rules measured the area of the largest sails (jib, mainsail, spinnaker) and that was used to calculate your rated sail area, main+jib, main+spinnaker. Then other rules, eg the Green Book, might restrict the number of other, smaller, sails you could carry onboard. But there was never much consideration of how many of your largest sails you might fly simultaneously. Currently IRC and ORR (in the USA) are silent on the number of
sails that can be carried onboard, how many can be flown together, and where they are tacked and hoisted. The rating is calculated on the basis of the area of the largest jib – which in the picture on the next page is the conventional No1. However, increasingly modern bowsprit designs now carry several headsails of very similar area so that for close reaching they can sail triple-headed – a very efficient configuration especially for white-sail reaching. ‘But so what,’ you say. For offshore races run under rules that
rate only the largest headsail (IRC, ORR), and restrict the use of conventional Code Zero sails, the triple-head rig has become the
Class40 Polar table – All Purpose Handicap = 476 sec/mile
TCF difference in Clicks at each polar speed curve point. Green Farr 40 faster, Red Class40 faster
TCF difference in Clicks relative to All Purpose Handicap at each polar speed curve point. Green Farr 40 faster, Red Class40 faster
SEAHORSE 39
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