OSA Routing Series

These articles are designed to help sailors build a practical framework for making better routing decisions offshore.

We have published a series of articles on the fundamentals of sailboat routing, using the Newport Bermuda Race as a primary example while highlighting lessons that are equally relevant to other offshore races, including events such as the Sydney to Hobart.

The series begins with an introduction to the four core elements of routing: routing software, weather GRIB files, ocean currents, and boat performance as defined by the polar file.

From there, the articles take a deeper look at one of the most important and often least understood aspects of offshore routing: interpreting Gulf Stream behavior. In particular, we emphasize the value of comparing at least three different current-related data sets when planning a Gulf Stream crossing. These include sea surface temperature (SST), which helps identify the northern wall of the Gulf Stream; currents derived from sea surface height (SSH) measured by satellite altimetry; and forecast current models that use these observations to predict how the Gulf Stream may evolve during the race.

A central message of the series is that routing software is a powerful tool, but it is not always right. The navigator still plays a critical role in interpreting the data and making sound decisions. Do the RTOFS and Mercator current models agree? If not, which is more believable, and why? Answering those questions requires comparison with real observations such as SST and SSH.

The articles also address an often overlooked challenge in offshore racing: eddies that spin off from the Gulf Stream. These features can be difficult to identify in SST and are not always well resolved in current models. In many cases, currents derived from SSH provide the clearest picture. In the second article, we compare these different data sources in a real-world example to show how navigators approach route planning and decision-making in practice.

Because SSH data is so valuable, the series also includes step-by-step guidance on how to obtain it from NOAA and incorporate it into routing software.

Sailor Takeaway

• Routing software is a tool—not a decision-maker. Always validate outputs.
• Compare multiple data sources: SST, SSH-derived currents, and model forecasts.
• Look for agreement between RTOFS and Mercator—but investigate when they differ.
• Use SST to identify major structure (e.g., Gulf Stream walls), but rely on SSH for finer detail, especially eddies.
• Expect eddies to impact performance and routing decisions more than models often suggest.
• The best routing decisions come from integrating data, not trusting a single source.