{"id":150634,"date":"2024-02-26T06:00:15","date_gmt":"2024-02-26T06:00:15","guid":{"rendered":"https:\/\/www.yachtingworld.com\/?p=150634"},"modified":"2024-02-26T09:54:30","modified_gmt":"2024-02-26T09:54:30","slug":"your-expert-guide-to-routing-for-multihulls","status":"publish","type":"post","link":"https:\/\/www.yachtingworld.com\/bluewater-sailing\/your-expert-guide-to-routing-for-multihulls-150634","title":{"rendered":"Your expert guide to routing for multihulls"},"content":"While it\u2019s essential to monitor the conditions when choosing the best weather window for departure, equally important is being well-versed in obtaining and interpreting weather data while offshore \u2013 keeping up with the latest evolving weather movements enables you to position yourself to take advantage of favourable winds and avoid dangerous conditions.\r\n\r\nGetting weather data while on passage can be done via SSB radio or satellite systems like Iridium, Inmarsat, and Starlink. The crucial aspect is the device\u2019s ability to transfer meaningful data, particularly weather files in GRIB format and weather routing files. Certain satellite devices, such as Inreach, Zoleo, and Spot, cannot transfer weather data files. These SBD (short burst data) devices have limited functionality despite their popularity, and cannot handle voice communications either.\r\n\r\nReliability is paramount, especially in adverse conditions when weather updates become most critical. The connection should be available at any time, ensuring you never rely on outdated data: twice-daily weather data retrievals are sensible. Timing your analysis and planning with weather model updates will further enhance your decision-making process.\r\n\r\n[caption id=\"attachment_149041\" align=\"alignnone\" width=\"630\"]\"\" Get into a routine of monitoring long- and short-range weather at sea. Photo: Sophie Dingwall[\/caption]\r\n

Specialist software<\/h2>\r\nOffshore weather demands specialised software so you don\u2019t have to be constantly connected to the web. The software should be capable of saving data offline, reading, and compressing GRIB format files while efficiently managing your downloads based on your connection type\u2019s speed and bandwidth.\r\n\r\nThe PredictWind Offshore App allows you to select weather models, GRIB resolution, time step, and the number of days for each GRIB. You can customise parameters such as wind, pressure, rain, CAPE (Convective Available Potential Energy), wave heights, gusts, temperature, and cloud cover. Ocean and tidal currents can also be added, but their large file sizes require careful management.\r\n\r\nArticle continues below...<\/em>\r\n\r\n[collection]\r\n\r\nThese GRIBs form the foundational data for offshore weather, but to truly harness their potential, additional tools and datasets are crucial. Weather routing, wave routing, ocean data, AIS, graphical GMDSS, and associated warnings significantly enhance the interpretation of this data. You can also include parameters such as observations, satellite imagery, and tracking data.\r\n

Different scenarios<\/h2>\r\nWeather routing is at the core of weather monitoring while on passage. This gives you the information to allow you to plan and adapt your strategy. A weather routing algorithm calculates your route to your destination using the performance of your boat in any wind speed or angle.\r\n\r\nA smart weather routing tool will also use currents, along with wind waves and swell. The PredictWind weather routing tool has these features, and routes are calculated in the PredictWind cloud. This saves the user time and a massive amount of data. In the cloud, the route calculation involves billions of computations, exploring every conceivable scenario to provide you with the most optimal route based on six weather models, three wave models, and three ocean current models.\r\n\r\n[caption id=\"attachment_149044\" align=\"alignnone\" width=\"630\"]\"\" Compare models for consensus \u2013 a divergent route will also indicate where models don\u2019t all align[\/caption]\r\n\r\nThis wealth of data is derived from the latest model run, ensuring the highest resolution at all times. In contrast, attempting to download and process this vast amount of data onto the boat might consume over an hour, and still the results may not match the efficiency achieved using the cloud-based system.\r\n

Short- and long-term analysis<\/h2>\r\nThere are two main types of analysis you can conduct from the data you receive during offshore sailing. The short-term focus involves examining the weather conditions for the next 6-12 hours, 12-24 hours, and up to three days in detail, covering a smaller area. This analysis is done every 12 hours to keep track of the immediate conditions.\r\n\r\nFor a longer-term outlook you focus on the next 3-10 day range covering a wider area. This provides insights into the weather trends you might encounter during this extended period.\r\n\r\nFor short-term routing, you should look for key waypoints: identify crucial points on the route, taking note of wind direction changes and trends. Add these waypoints to your plotter or navigation software for reference.\r\nThen download weather routes for all six models (always download them all). Choose the specific GRIB files and parameters you need for analysis. To manage file size, decrease the resolution to 50km or 100km and select a smaller area for downloading. The minimum GRIB parameters include wind, pressure, rain, and 3-5 days of data. If file size permits, you can also add CAPE, gust, and possibly wave features.\r\n\r\nBesides weather routes, you should also download GMDSS text and graphical data, along with AIS data to view shipping and fishing vessels within 330 miles of your position. Once the download is complete, analyse the data by quickly checking the graphs to get a rapid overview of the current situation.\r\n\r\n[caption id=\"attachment_149043\" align=\"alignnone\" width=\"630\"]\"\" Routing display shows parameters that are available to download for PC, tablet and mobile devices[\/caption]\r\n

Route conditions<\/h2>\r\nWhen assessing route conditions, first observe whether the trend in wind speed is increasing or decreasing and whether the wind direction is veering or backing. Consistent trends across most weather models provide a higher level of forecast certainty, while divergence requires further investigation on the maps.\r\n\r\nNext, quickly check the average wind speed to ensure it falls within your comfort level, taking into account the true wind angle you\u2019re sailing. Afterwards, focus on the wave data, examining metrics such as roll, vertical acceleration, and boat slamming. Ideally, you\u2019d want the roll to stay below 4\u00b0, vertical acceleration below 0.2g, and no boat slamming, as 50% slamming could damage your boat.\r\n\r\nWith this initial information in mind, you can now make informed decisions. If any levels exceed your comfort zone, consider how best to mitigate them. You might want to think about alternative courses to avoid adverse conditions, slowing down to wait for weather changes, or prepare the boat and crew for the conditions ahead.\r\n\r\nTo assess the conditions along your route, refer to the wind tab, which provides a comprehensive view of each model\u2019s conditions for your routes. Pay particular attention to warnings such as thunderstorms, lightning, gusts, wind against current, vertical acceleration, roll, slamming, and wind chill. The table displays the parameters responsible for these warnings and their timing.\r\n\r\nFor a broader perspective of the entire route, a quick look at the summary tab can be beneficial, especially for longer-term planning.\r\n\r\nMoving on to the wave tab, you\u2019ll find a more detailed breakdown of the data observed in the graphs. This section offers a clearer understanding of elevated levels of roll, vertical acceleration, and slamming. The wave heights and primary, secondary, and tertiary swell states provide additional insight into different directions, sizes, and wave periods.\r\n\r\nIn cases where you don\u2019t have a GRIB file for currents, you can use the tables to access current speed and direction at any time. Alternatively, you can tap on specific points on the weather routing map to view current details at those locations.\r\n\r\n[caption id=\"attachment_149045\" align=\"alignnone\" width=\"630\"]\"\" Diving into observations will give you a more comprehensive picture of actual conditions[\/caption]\r\n

Model consensus?<\/h2>\r\nWhen evaluating the map view with the overlaid routes, it\u2019s crucial to look for consensus across the models. You can easily spot this by examining the tracks of the different routes over the next 12 hours. If there\u2019s an outlier \u2013 when a model shows a different route \u2013 investigate the reason.\r\n\r\nThis often occurs in light winds or when a front or weather feature crosses your route. For instance, a rain band associated with a wind direction change might indicate a passing front, causing variations in routes. In such cases, choose the route that closely matches your current conditions and timing.\r\n\r\nAdditionally, keep an eye on the CAPE parameter because elevated CAPE levels suggest an intense event. That should prompt you to brief the crew to prepare for reducing sail, and consider any other safety precautions.\r\n\r\nIf you\u2019re looking at the timing of a change, then look closely at the wind, gust and rain parameters across the models. If you saw warnings in the tables, the PredictWind router shows these on the maps at the corresponding time along the routes.\r\n

Handling large files<\/h2>\r\nHandling large current GRIB files can be a challenge via offshore connections. However, there\u2019s a strategy to mitigate this issue. Download the currents GRIBs before you leave, as they don\u2019t change as rapidly as atmospheric conditions. Even if you\u2019re using slightly older data for visualising the currents overlaid with weather routes, they can still provide valuable insights.\r\n\r\nIn the map view, focus on identifying instances when wind is against the current. Weather routing warnings typically pick up on this, but the map view allows you to visualise the timings when you might enter current zones. By flicking through different current models like Mercator, Hycom, and RTOFS, you can identify any differences in positioning and strength between the models.\r\n\r\n\"south-africa-europe-sailing-navigation-hero-credit-kraken-yachts\"\r\n

The longer term view<\/h2>\r\nTypically, the longer-term view only requires attention every 24 to 36 hours, unless specific weather features are forming and warrant closer monitoring. For this planning, start by enlarging your GRIB area well beyond your passage length and consider which side of your route the weather is approaching from. Choose a 100km resolution and download just one model \u2013 ECMWF is my recommended choice for longer-term analysis.\r\n\r\nNext, narrow down the parameters to wind and pressure only, while obtaining more days of data. Ensure the download also includes GMDSS text and graphical data.\r\n\r\nWeather routes are still important, as they allow you to track any low-pressure systems or extreme weather events that might develop over time. This broader view provides insight into the bigger picture of weather patterns forming.\r\n\r\nTo identify potential weather events more accurately, refer to the GMDSS graph, which indicates everything from troughs and fronts to gale areas and hurricanes.\r\n\r\nFor a more in-depth analysis, interpret the GMDSS text, which has been professionally crafted by a meteorologist, to investigate any specific events of interest.\r\n\r\nThis longer term analysis will help you make better informed decisions for a more efficient and more comfortable passage, and either avoid or prepare for heavy weather.\r\n\r\nJust bear in mind that this isn\u2019t an exhaustive methodology and there are various ways to utilise this weather data. With practice, you\u2019ll likely develop your own methods and processes over time.\r\n\r\n
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Over recent years there has been a big increase in the number of cruising catamarans making the transatlantic crossing. Of the 250 yachts crossing the Atlantic with the ARC and ARC+ this year, 60 (24%) were multihulls, and in some Caribbean anchorages monohulls are now the minority with private and charter catamarans dominating. I’m fortunate in having raced and cruised multihulls on four transatlantics, along with half a dozen passage speed records on Playstation, the late Steve Fossett\u2019s maxi cat. As a meteorologist and weather router I also help a number of catamarans which are cruising around the world. Routing fundamentals Weather routing is all about getting a boat from point A to point B in the optimum way. This is usually considered to be in the fastest possible time, but when considering a transatlantic or ocean passage I’ll often be asked for the most comfortable or safest route. Looking at the ARC, the most direct route from the Canaries is a northerly route which \u2013 for around two out of every three years \u2013 will likely be faster than the traditional route of heading south towards the Cape Verde Islands, then west. This was not the case last year as a large wind hole covered the mid-Atlantic northerly route for the 2023 ARC. In a more usual year, some of the racing boats will take the northerly route while the more cruising oriented boats will head south. There are a number of reasons for this; averages suggest that a southerly route will give more consistent tradewinds and more comfortable sea states while the northerly route will often include a period of beating to pass through a trough mid-Atlantic \u2013 which is fine if you are racing, but it does tend to spoil the dinner if in cruising mode. It also holds an advantage for the faster boats as the weather, for the first part at least, is likely to be close to the forecast but for a slower yacht two weeks into a passage the forecast may have changed significantly, and the northerly route may become less attractive. For accurate routing \u2013 either by a skipper\/navigator on board or using an onshore router \u2013 it’s very important to match the boat\u2019s performance polars to the expected weather and sea state. Professional crews will spend inordinate amounts of time working on performance polars but for cruising yachts we usually make do with the design polars (or slightly modified ones) and when offshore we would drop them to assume the boat is sailing at about 80% of polars. Article continues below… It is easy to buy expensive routing software and, with a little bit of work, a skipper\/navigator can download the forecast, add the boat\u2019s polars, press a button and get the optimum route. Some forecast providers (Predictwind being the best known) will offer cloud routing where, by entering your boat polars and route, routing is done ashore using different weather models and the results are sent to the boat. This …Continue reading »<\/a><\/p>\n","protected":false},"author":447,"featured_media":150638,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[767],"tags":[2447,1633],"review_manufacturer":[],"acf":[],"introduction":null,"_links":{"self":[{"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/posts\/150634"}],"collection":[{"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/users\/447"}],"replies":[{"embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/comments?post=150634"}],"version-history":[{"count":3,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/posts\/150634\/revisions"}],"predecessor-version":[{"id":150641,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/posts\/150634\/revisions\/150641"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/media\/150638"}],"wp:attachment":[{"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/media?parent=150634"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/categories?post=150634"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/tags?post=150634"},{"taxonomy":"review_manufacturer","embeddable":true,"href":"https:\/\/www.yachtingworld.com\/wp-json\/wp\/v2\/review_manufacturer?post=150634"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}