Newport- Bermuda Race Strategy:
Navigational Cons Professor of Oceanography Emeritus, Department of Marine Sciences at the University of Connecticut, 17 Newport Bermuda Races (winning navigator 1986), author “Gulf Stream Tutorials”on Bermuda Race website By W. Frank Bohlen
eveloping a winning Newport-Bermuda race strategy that accommodates variability in weather, ocean current, and sea state requires skippers and navigators to consider a number of factors. Among them are the boat, the crew, the competition, and also the probable tide, weather, and Gulf Stream conditions. The process of analyzing these factors should begin well before the week of the start of the race, which this year is on June 20.We will take as a given that the boat is structurally sound and reasonably well equipped with a suite of instruments that includes GPS, a barometer, sensors that track wind speed and direction, inwater boat-speed sensors, and a means to measure surface water temperature. Strategic planning begins with consideration of the boat’s performance characteristics using the polar diagrams (Fig.1) and associated tables and data files in US Sailing’s Performance Package (http://offshore.
Fig. 1. ORR 2012 True Wind Polar Diagram (US Sailing Performance Predictions Package)
ussailing.org/ORR/Performance_Products.htm). Maximizing the utility of these diagrams requires accurate calibration of all instruments.
One concern is the height of the wind speed sensor. The wind speeds indicated in the diagrams are assumed to be measured at 10 meters (about 33 feet) above the water surface. Masthead heights for the majority of offshore boats are greater. Since wind speeds increase progressively with distance above the surface due to decreasing frictional effects, winds at higher elevations will be stronger than those at 10 meters. If the masthead sensor indicates 6 knots of wind, the boat speed may be noticeably less than indicated on the polars because the wind
speed at the 10-meter elevation will be less than 6 knots. A means to correct wind speeds for actual sensor height is provided in U.S. Sailing’s Performance Package.
Fig. 2. 500 mb chart before start of 2010 Newport Bermuda Race
nsiderations After you gain some confidence with performance predictions, your next concern is sails. Decisions about sail selection and use can be assisted by US Sailing’s Race Optimization Package, which allows evaluations of different combinations of sails under a variety of wind conditions and their effect on ORR rating. The program can answer such questions as, “Should I carry a combination of asymmetrical and symmetrical spinnakers? Should they be tacked to the centerline or to a sprit?” This extremely powerful utility also forces you to engage in careful consideration of the wind and sea conditions likely to be encountered on the way to Bermuda.
Predicting the Weather
The weather along the Newport to Bermuda rhumb line is typically dominated by the BermudaAzores offshore high, and by the migration of high and low pressure systems across and along the continental United States. This pattern may be complicated by sea breeze conditions at the start in Newport and by island-induced thunderstorms near Bermuda. Tropical developments may introduce further “complications.” Given this variety of factors, it’s not surprising that each Newport Bermuda Race is unique in terms of winds and weather. This necessarily complicates sail selection and optimization. Reviews of past Race conditions will help in this process. Analysis of historic weather conditions can
include discussions with past race participants, and also examination of the National Weather Service archived forecasts and analyses available at http://nomads.ncdc.noaa.gov/ncep/NCEP. Special emphasis should be placed on two types of NWS analysis: the surface wind/wave analysis, and the 500 millibar (mb) chart. The latter depicts the conditions in the vicinity of 18,000 feet aloft that typically affect the distribution of the high and low pressure systems at the earth’s surface, and their associated winds. The Newport to Bermuda course typically has a predominance of winds from the south and west, with speeds ranging from light and variable to a solid 30-plus knots. The usual race has a limited amount of reaching and running, but there are exceptions to this (the 2012 Newport Bermuda Race, for instance). Study of the 500 mb patterns over the weeks before the race start should indicate the probability of unusual conditions. In a typical year, May to June is a time of rapid change as spring transitions to summer. Northern hemisphere warming favors a northerly migration of the core of the flows at 18,000 ft. This core is indicated by the 5640 meter contour (shown as 564) on the 500 mb charts. Locations in close proximity to New England often favor a coastal ridge aloft resulting in high pressure at the surface offshore and south and west winds. As the the core proceeds farther to the north conditions typically become more benign and less well defined often resulting in winds differing substantially from the historical norm.
500 mb chart, day after start of 2012 Newport Bermuda Race
A comparison of the 500 mb charts during the 2010 and 2012 Newport Bermuda Races provides a graphic illustration of what can happen (Figs. 2 and 3). This offers at least a partial explanation of why the predominantly reaching 2012 race differed so much from the historical norm.
Predicting the Gulf Stream
With boat preparation and sail selection reasonably in hand, and the crew testing the accuracy of predicted target boat speeds in trials and races, attention during the last two to three months before the race generally turns to probable Gulf Stream and weather characteristics. Although tidal currents affect set and drift along the entire rhumb line, they tend to be weak primarily affecting near-shore navigation, particularly during periods of near calm. These currents warrant attention during the departure from Newport and arrival at Bermuda. In contrast, there is the Gulf Stream. The Stream typically crosses the rhumb line approximately 200 nautical miles from Newport and displays maximum currents of 5 knots, with directions varying from perpendicular to the rhumb line to parallel to it. Eddies and similar water masses shed from the main body of the Stream can affect a large part of the continental shelf to the north, and the Sargasso Sea south to Bermuda. Knowledge of the Stream, its location, and its structure is essential to a successful Bermuda Race strategy. Several websites provide details of sea surface temperature (SST) useful for the determination of the location and structure of the Gulf Stream and its evolution. A list of the principal sites is provided on the race website (BermudaRace.com) in the Resources tab. There you will also find a short Primer and an archive of my Gulf Stream tutorials prepared for previous races. While there is an abundance of direct observations of SST (satellites, aerial overflights, etc.) no website provides measurements of Gulf Stream currents, which is the parameter of primary interest. Temperature serves
Fig. 4. Sea Surface Temperature chart four days before the start of 2012 Newport Bermuda Race
as an indicator of flow intensity, yet the actual location of the flow may be significantly offset from the indicated thermal boundaries. This deficiency is best overcome using a combination of direct SST data and modeled Gulf Stream flows. In the past three races and other events, the most accurate representation of the flow was NOAA’s altimetry-based model found at http://www.aoml.noaa.gov/phod/dataphod/work/ trinanes/INTERFACE/index.html. This model is often of particular value in early June because it uses radar, which sees through the cloud cover that often appears then. Figs, 4 and 5 provide a comparison of SST and model results just prior to the 2012 Newport Bermuda Race. Study of SST satellite imagery during at least one month (and preferably two months) before the start will reveal Gulf Stream structure, including meanders, the location and number of eddies north and south of the main body of the Stream, and the rate at which all these features evolve. Attempts to obtain data during the last two weeks before the race are often frustrated by cloud cover. Starting early is the best way to achieve success.
Understanding the “Weather Breeder”
Throughout the race area, surface current conditions are often affected by wind. Conversely, warm water currents such as the Gulf Stream can directly affect regional winds and wind patterns. Many racers have noticed that the Stream is a “weather breeder” (See McKinley-Ocean Navigator, 2013). Such small-area phenomena are difficult to capture in weather models. As with the Gulf Stream, understanding the capabilities of weather models is best obtained by comparing them with direct observation. Proceeding with your analysis over the last couple of weeks before the race, include the NOAA satellite observation or OSCAT data available at http://manati.star.nesdis.noaa.gov/datasets/ OSCATDate.php . These data should be compared
Altimetry Based Model Results Showing Currents and Sea Surface Heights
to GRIB formatted data showing raw wind strength and direction data without interpretation. GRIBs are at http://passageweather.com/ and on the National Weather Service website. Evaluations of the accuracy of the GRIB results are particularly valuable if these strictly model-based data are to be used in routing software such as Expedition, Deckman, and MaxSea. Beyond developing confidence in the models (and knowledge of their weaknesses), this pre-start exercise is the best way to become familiar with the weather conditions most likely to prevail during the race. This knowledge adds greatly to the value of information provided by professional weather services, and it facilitates onboard decision-making should conditions turn out to differ from the forecast.
selected race strategy and the reasons for it before the start, and be informed of changes during the race. The navigator’s instructions must be clear, in easily understood language, and discussed at each change of watch, and possibly more often. This is not a time to be a “riddler!”
Predicting Sea State
In addition to their individual value, the wind and current data you acquire and analyze will help you make accurate estimates of sea state, which is a very important factor in race strategy. The amplitude and steepness of ocean wind-driven waves can be altered by interactions between wind and current. Wind blowing against the current results in a significantly larger wave amplitude and shorter wavelength than what appears when wind blows with current or when there is no current. Steep conditions with breaking waves are often observed in the Gulf Stream and in areas of strong tidal currents. US Sailing’s Performance Package does not include consideration of the wind-current interaction. There are, however, several graphical methods that can be used to correct wind/wave estimates to account for this effect. A good aid here is William G. Van Dorn’s book Oceanography and Seamanship.
Communicate ClearlY Acquisition and analysis of these data allows the development of an initial strategic plan. By combining the probable position of the Gulf Stream and associated eddies with anticipated wind conditions with an estimate of the boat’s and crew’s capabilities, the skipper and navigator can select one or more routes to take. Some navigators use a routing program to assist in this process, but others find this unnecessary given the short duration of the race and the sometimes less than adequate model representations of winds and currents, particularly in the vicinity of the Gulf Stream. Whichever method you employ, your strategy should be continually refined right up to the start, and to some extent during the race. The success of the process depends on clear communications 23 among the entire crew, who should understand the
Race Strategy, Suggested Timeline Fall-Winter Before Race Boat Preparations - Check of hull, rig, sails and systems (including all navigational instruments) January - February Review past race conditions, see http://nomads.ncdc.noaa. gov/ncep/NCEP Obtain polars and consider optimization http://offshore.ussailing.org/ORR/Performance_Products.htm March - April Launch Boat Initial tests of polar predictions Finalize Instrument calibrations and Finalize Sail Selection May Crew training trials and races Begin analyses of Gulf Stream conditions see http://bermudarace.com for Tutorials June Continue Gulf Stream analysis Begin analysis of probable race weather conditions. http:// www.nws.noaa.gov/om/marine/home.htm Develop initial routing strategy - revise as forecasts improve within 96hrs of race start Finalize strategy following Skipper’s Meeting and Weather/ GS briefings - present to crew with details June 20 - Race Start
Published on May 19, 2014
Published on May 19, 2014
Professor of Oceanography Emeritus, Department of Marine Sciences at the University of Connecticut, 17 Newport Ber- muda Races (winning navi...