|CiCi plotting a fix on the ocean chart, which gave a good "big picture" view of our progress. Click on the picture to see a bigger version; use your browser's "back" command to return here.|
When planning a sailing trip, the overall navigation depends on the expected winds. For our trip from Portsmouth to Tortola, we will be starting in a mid-latitude zone of variable weather (though the weather systems generally move from west to east). As we sail towards the tropics and the Caribbean, we will enter a different weather zone with northeast or east trade winds (the zone usually starts around 25° North latitude).
The sailing directions typically say to sail more easterly while you're in the variable weather zone, then more southerly in the trade wind zone. This is because the trade winds can have a significant easterly component, making it difficult to sail to the east once you're far enough south to be in the trade winds. At the weather briefing before we left, the forecaster told us that the easterly trade winds were not expected to significantly affect the navigation, and suggested that we follow a rhumb line to Tortola
A rhumb line is a straight line when plotted on a Mercator chart, which is a very common chart format. Because the rhumb line intersects all meridians and parallels at a constant angle, the rhumb line has a constant course, which makes for very simple navigation (for example, "steer 151 degrees until we reach Tortola"). Although the navigation is simple, the distance to Tortola via the rhumb line is not the shortest possible distance.
To travel the shortest distance, you have to follow a great circle route, which is the shortest distance between two points on a sphere like the Earth. Although the distance is the shortest, the navigation becomes more complicated, since the "course to steer" continuously changes during the voyage. In our case, this wouldn't be a problem, since we steered to follow a route displayed on the chartplotter. Most of the time the autopilot steered, and we would occasionally have to adjust its heading by a few degrees, depending on winds and/or currents. It wouldn't be any more effort to adjust our heading to follow a great circle route, than to adjust the heading to compensate for winds and currents. In fact, the autopilot on Night Heron can be commanded to automatically follow the chartplotter route, but we didn't use that feature.
Here's a comparison of using a rhumb line versus great circle route for our voyage to Tortola. For both cases, we start at the Chesapeake Bay entrance buoy "CH" (36° 56.137' N, 75° 57.426' W) and end at a BVI approach waypoint (18° 49.6' N, 64° 37.9' W). You can see that for this trip, the difference is not significant:
Instead of following the rhumb line, Jeff plotted an optimum route by using the MaxSea navigation software and its weather-routing capability. Using this feature, the software adjusts the route to make best use of the present and forecast winds over the next few days. The resulting route can be well off the rhumb line, but in theory you should reach your destination faster by sailing the adjusted route. Unfortunately, MaxSea used a weather forecast that wasn't current, so our initial route wound up being less than optimum (too far east). Also, the weather-routing feature is not able to anticipate when you'll decide to motor, which can greatly affect the overall route-planning strategy. Later on, we stopped using the weather-routing feature.
In practice, we couldn't always sail the desired course anyway. For example, during squalls we would frequently sail to the wind (on a comfortable reach) rather than trying to sail a specific course. This reduced the stress and strain on the boat and its gear, as well as reducing the risk of an accidental jibe. Sometimes during gusty conditions the autopilot would have trouble holding a particular course, and we'd either have to hand-steer or change course. We would also have trouble sailing a desired course during periods of adverse winds, when we'd have to steer away from the course line just to fill the sails. During adverse winds, we could be blown farther off course due to excessive leeway if we pinched too close to the wind. Late in the voyage (after our sails had been damaged), we wound up motoring a lot, which allowed us to follow a direct route towards Tortola.
There are more navigation details on the next two pages.
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