'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 ©
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A fresh San Fransisco breeze is expected on Day 14 of the 34th America's Cup, for Tuesday and Races 17 and 18 of the 2013 America's Cup. The first race is due to start at 1.15pm on Tuesday, 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.
The official forecast is 'Windy all day, building through the afternoon. 20-25 knots.'
With the first race of the day set for its regular start time of 1315hrs the wind feeds produced by Predictwind indicate that winds will be around 21-22kts. The feeds produced by Predictwind are not in agreement on the strength at the race start time, as with yesterday the significant feature of the forecast is that just two hours before the start the winds were predicted to be near calm. The two Predictwind feeds vary on the timing of the building breeze, with one feed saying it will not start until 11.00am. Both feeds shows the winds reaching 21kts at race start time for Race 17.
Officials will add the strength of the tide to set the new wind limit, which will take it just over 24kts. However the winds would be expected to exceed that level with the current forecast, and delayed or cancelled races are a distinct possibility.
The breeze is expected to quickly build in the late morning and early afternoon to reach a strength of 22kts for the second scheduled race of the day.
The forecast does not take account of the influence of fog, which can have several effects on the race course winds.
The above image shows the wind speed and direction 1300hrs with breezes expected to be 21-22kts at the start of Race 17 - these are only average speeds and gusts will be higher.
The second image (above) shows a graphic representation of the wind build - confirming the numbers for the prognosis. The significant feature is the feed are is that after the race start both feeds seem the wind easing off it's current level.
The above image shows the wind speed and direction at 1400hrs still with the stronger breeze in the middle of the course indicated by the yellow wind arrows.
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 20-21kts and from the SW.
The third chart (below) shows the wind expectations for Tuesday and Wednesday. Typically the forecast for Wednesday 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
Wind-predictions for September 24, 2013 San Francisco for Tuesday and Wednesday - PredictWind.com
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 17 and the red dotted line shows the course recommended for Race 18. 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, aside from the recommendation to gybe early and get into the incoming tide. 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 24, 2013 - faint purple line is for Race 17, Red dotted line is for Race 18 - PredictWind.com
Upwind course projection September 24 2013 - faint purple line is for Race 17, Red dotted line is for Race 18 - PredictWind.com
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), 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 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 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.
by Richard Gladwell
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10:23 AM Tue 24 Sep 2013GMT
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