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Predictwind - Iridium

America's Cup- Predictwind - Winds expected to co-operate on Day 12

by Richard Gladwell on 22 Sep 2013
The America’s Cup Race Course, framed by the Golden Gate Bridge - today the wind blows from right to left, and the ebbing tide flows from left to right against the windflow. The blue sea that can be seen behind the City to the right, is not ocean but the greater expanse of the San Francisco Bay - which exits through the relatively narrow channel under the Bridge. © Richard Gladwell www.photosport.co.nz

The regular San Fransisco breeze is expected to return on Day 12 of the 34th America's Cup, for Sunday and Races 14 and 15 of the 2013 America's Cup. The first race is due to start at 1.15pm on Sunday, local time with the second race scheduled for 1415hrs.

Towards the bottom of this story we feature Predictwind's course routing optimised for wind and tide on the AC race course, and using an AC72 performance profile. It is included to show that the effect of the tide on the race course and race strategies. See bottom of story for the course variations.

For the official line on how the current factor is determined click here Today the tide is flooding (coming in through the Golden Gate) and will negate the previous advantage that exists with an ebbing tide.

With the racing at its regular start time of 1315hrs the wind feeds produced by Predictwind indicate that winds will be around 12-18kts. The discrepancy being the rate of speed of the increase in strength of the breeze which is expected to be be around 5kts an hour or two before the race start.

The more reliable of the two feeds shows lighter winds peaking around the time of the second race at 18kts.


The above image shows the wind speed and direction 1300hrs with breezes expected to be 12-18kts at the start of Race 14 - these are only average speeds and gusts will be higher, but it is not expected to exceed the wind limit.


The second image (above) shows a graphic representation of the wind build - confirming the numbers for the prognosis. The significant feature is the variation between the two feeds - making for a less certain prognosis.


The top image shows the wind speed and direction at 1400hrs still with the most accurate feed showing a West to SW wind direction with the two models being in much closer agreement with the breeze expected to be 18-20kts


The second image (above) shows a graphic representation of the wind build - confirming the numbers for the prognosis. Reading the top line we can see the wind direction, the wind strength with winds forecast to be 18-20kts and from the SW and West.

The third chart (below) shows the wind expectations for Sunday and Monday. Typically the forecast for Sunday is revised upwards closer to the time of the race start. Note that sea conditions described in this chart are based on open water and are not descriptive of an enclosed harbour


The courses optimised by Predictwind for an AC72 show that the optimum course only has no real variation between races. The faint purple line shows the recommended course for Race 14 and the red dotted line shows the course recommended for Race 15. The mark positions used in this projection are only very approximate - and will vary on the course set for the day. At the bottom of the upwind leg the model shows that the boats cannot use full width of the course because on the adverse, incoming tide.

The point here is that downwind is there is little variation in the options. And upwind in both models the fastest course is to the south, or cityside shore, aside from allowing a short leg to Alcatraz in the first race before heading for shore. The tide is expected to be in flood (incoming) for this race, and boats will need to avoid its effects upwind by taking the city shore.



Two weather feeds are used by Predictwind, the key to the accuracy of the forecast lies with the graphs, as when the two lines for the two feeds are quite close together and following each other in the trend, then the likelihood of the forecast is high. If there is separation to any marked degree, then there still some issues to be resolved but usually 24 hours from the race they are well aligned.

The Predictwind model takes various weather feeds and then polishes these, part of this process is factoring in the effect of local topography on the breeze - well demonstrated by the images with the funneling of the breeze.

Developed by former Olympian, Jon Bilger (470, 1992), www.predictwind.com!PredictWind application was used by the Swiss team Alinghi to win the 2003 and 2007 America's Cups, and has become the leading wind system used by racing and recreational sailors around the world. It is available on a subscription basis and also with a free forecast option.

A mobile app for Android and iPhone is available and www.predictwind.com!PredictWind is probably the most used app on the Sail-World iPhone.

Real time local wind observations have been added as further functionality.

You can check the weather anywhere in the world, and get a five day forecast, plus a variety of other features and functions of www.predictwind.com!PredictWind. The app is synchronised with your PC, so that your latest set of locations are always available without resetting each time you change a device.

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