Avoiding boomers

SW WINDS 5 TO 10 KT. SEAS 1 TO 2 FT. A CHANCE OF T-STMS IN THE AFTERNOON. Also a chance that a product below will provide many many more details...…and possibly allow you to enjoy a good day’s sailing in spite of an iffy forecast. With real-time weather-tracking capability you can see rain and thunderstorms moving through the area, allowing you to make smarter decisions about when to head
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SW WINDS 5 TO 10 KT. SEAS 1 TO 2 FT. A CHANCE OF T-STMS IN THE AFTERNOON. Also a chance that a product below will provide many many more details...

…and possibly allow you to enjoy a good day’s sailing in spite of an iffy forecast. With real-time weather-tracking capability you can see rain and thunderstorms moving through the area, allowing you to make smarter decisions about when to head out for a sail and when to come back in.

Unlike the marine weather forecasts available on VHF radio, TV, or the Web, both XM and Sirius Satellite Weather and another newcomer, ClearPoint Weather, can all stream comprehensive real-time weather conditions, as well as forecasts, to an onboard display screen.

XM and Sirius, both well known for digital audio satellite-radio broadcasts, also broadcast digital weather information that can be collected and displayed by some marine chartplotters. Data reception is exceptionally trouble-free on the water all around the U.S. and generally several hundred miles offshore. User interfaces are provided by the chartplotter vendor and therefore vary considerably from one product to the next.

XM Satellite Weather is available on many Garmin plotters. XM offers slightly fewer weather features, but when run on a Garmin 545s chartplotter it uses the least amount of power (just 13 watts total) of the three products reviewed here. This is a big advantage, as real-time weather products like animated weather radar need to run continuously to be most effective. The identical XM service running on the larger Garmin 5000 Series chartplotter benefits from a multi-window touch-screen interface that vastly speeds up the selection of data (but also consumes a lot more power).

In my opinion, Sirius Satellite Weather running on a Raymarine E-80 plotter offers superior forecast and real-time data. This includes wind predictions at 3-hour intervals, rather than XM’s 12-hour predictions, more-accurate Doppler information, and more buoy data. The E-80 complements Sirius very well, but has some faults; for example, ocean-current forecasts are not yet available. In addition to a full-screen display, there is an extremely useful quarter-screen display that allows you to keep an eye out for afternoon boomers hundreds of miles away while staying zoomed in locally on a half-screen chart.

ClearPoint “High Definition Weather” runs on a PC (laptop or tablet) and provides wind forecasts at 1-hour intervals, has higher-resolution forecasts of some data (like wind), offers additional types of data, has a GRIB-file export feature, plus some weather information is available around the world, not just in the U.S. ClearPoint’s GUI can present graphic data in past, present, and future timeframes in a uniform manner and can superimpose multiple types of data at once.

ClearPoint users, however, must subscribe to a satellite or wireless Internet data provider. The program will use the boat’s position and an optional filter to limit data transfer and will manage remote dialing on an Iridium or Inmarsat satellite interface providing wireless access even from the middle of the ocean.

My Verizon EvDO wireless Internet connection, with good signal coverage across Long Island Sound, appeared to be the ideal companion to ClearPoint. As compared to Iridium or Inmarsat, this high-bandwidth cellular connection enables the entire high-resolution experience. I did, however, have trouble getting the higher-res data, especially in bad weather, when the connection proved unreliable and sometimes left me with no data at all.

When judging which product will help you most within the U.S., the choice is fairly clear. If there’s room at your helm for a ClearPoint installation, it could be an option, but the cost and effort required to separately manage a data connection, along with the limited Doppler radar features, make the other two products a better choice.

XM is a useful solution. The most important weather features, like Doppler radar, are available, but XM’s Doppler data is the least accurate and on the 545s plotter can blot out enough cartography to make navigation challenging. The 545s also lacks marine text forecasts, though its power requirements are low enough to work on even the smallest recreational boats.

I believe Sirius is the best solution because of its increased forecasting intervals and higher-quality Doppler data; it also provides sailors with the best tool for dodging thunderstorms.


Comparison: Present Conditions

All three products offer real-time wind, wave, air pressure, air temp, water temp, and other data at various cities and weather buoys. XM and Sirius are very close in the areas that count most. XM, however, provides no warning that buoy data is old (data is sometimes very old, through no fault of XM). Sirius offers the most buoy data, as well as the data’s age, and also provides high-resolution sea-surface temperature data. Almost all of ClearPoint’s data is high resolution, with the exception of some buoy data (and Doppler radar). The age of the data is meticulously reported.

Comparison: Weather Radar

This is the best reason to have a live weather service. A forecast of “isolated or scattered t-storms” won’t keep you in port, and if it’s currently raining, you can usually estimate when it will stop. But Doppler radar displays vary significantly, as each analyzes the raw data provided by the government’s radar sites differently. The first variable is position accuracy, which is dependent on how a service converts 3D Doppler data into 2D chart overlays. Comparing overlays to ground observations, I’ve encountered errors from 4 to 10 miles; XM had the most, ClearPoint was slightly better, and Sirius was considerably better.

Another variable is sensitivity. ClearPoint and XM tended to exaggerate the size of an area experiencing rain, but Sirius had it right much of the time. Finally, there’s the matter of interpreting what an animated radar overlay means to you. By moving the cursor over rain features on two or more historical images (each product provides an automatic distance-from-boat measurement), you can use simple math to determine the direction and speed of storm cells and rain boundaries.

XM on the Garmin provides 30 minutes of radar playback history, which is just enough to judge the speed of a cell or a rainfall boundary. However, the images can’t be paused, making distance calculations difficult. By contrast, the Sirius/Raymarine system provides two hours of radar history and can pause images sequentially.

A nice feature of Sirius is the appearance of an arrow vector indicating speed and direction when rainfall intensities are moderate to high. XM has a similar feature that predicts where high-intensity cells will travel in the next 15, 30, and 45 minutes, but it is offered rather infrequently.

ClearPoint falls short in the Doppler department, as data is delayed by 20-plus minutes and has irregular time intervals. By comparison, Sirius and XM typically provide data less than 5 minutes old. Combined with its poor position quality and sensitivity, ClearPoint is not good for dodging t-storms, but still allows you to determine, sometimes hours in advance, when it will start or stop raining.

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