America's Cup: Predictwind - Wind direction impacts Day 11 + Five days

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.

A breeze that is from a more southerly direction is expected to delay or even preclude racing on Day 11 of the 34th America's Cup, for Saturday and Races 14 and 15 of the 2013 America's Cup. The first race was due to start at 1.15pm on Saturday, 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

Wind graph for five days from September 21, 2013 San Francisco at 1315hrs - Sunday’s race is indicated by the yellow line. Read the actual data above the graph section. Shows that there are two days under the wind limit, before the wind exceeds the wind limit on the two following days.


Officially the breeze is being forecast as being from a southerly direction, if this remains the case at race time, it will not be possible to set a course until the wind swings into the West, blowing straight down San Francisco Bay. When the wind does swing it is expected to be in the 16-19kts range. It has also been raining heavily in San Francisco.

With the racing at its regular start time of 1315hrs the wind feeds produced by Predictwind indicate that winds will be around 16-18kts. The key point is the direction of the breeze, which one feed shows as being from the West and the other from the SW, we have regarded the latter feed as being the more accurate of the two when there is a variation between the two independent feeds.

Wind map for September 21, 2013 San Francisco at 1315hrs - Start of Race 14. The left panel shows the wind blowing across the course

The above image shows the wind speed and direction 1300hrs with breezes expected to be 16-18kts at the start of Race 14 - these are only average speeds and gusts will be higher, and may exceed the wind limit.

Wind graph for September 21, 2013 San Francisco at 1315hrs - Start of Race 14 - indicated by the vertical yellow line. Read the actual data above the graph section

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.

Wind map for September 21, 2013 San Francisco at 1415hrs - Start of Race 15. the left panel shows the wind still blowing across the course.

The top image shows the wind speed and direction at 1400hrs still with the most accurate feed showing a SW wind direction making it impossible to set a course to windward breezes are also lighter in this feed, although with increased winds on the Alcatraz end of the course. On the right hand feed (PWC) the winds shown as being stronger and from the West. The feed used to develop the wind map on the left (PWG) does not show the wind swinging West until 4.00pm - over an hour after racing is permitted to start.

Wind graph for September 21, 2013 San Francisco at 1415hrs - Start of Race 15 - indicated by the vertical yellow line. Read the actual data above the graph section

The second image (above) shows a graphic representation of the wind build - confirming the numbers for the uncertain prognosis. Reading the top line we can see the wind direction, the wind strength with winds forecast to be 15-19kts and from the SW and South.

The third chart (below) shows the wind expectations for Saturday and Sunday. 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

Windpredictions for September 21, 2013 San Francisco for Friday and Saturday

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 13 and the red dotted line shows the course recommended for Race 14. 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 can use the full width of the course due to the evenness of the 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.

Downwind course projection September 21, 2013 - faint purple line is for Race 14, Red dotted line is for Race 15

Upwind course projection September 21 2013 - faint purple line is for Race 14, Red dotted line is for Race 15

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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