If you are departing on a long ocean passage there will not be reliable forecast data available which covers the passage. This is just how it is. We are more fortunate than all previous generations of sailors, in that we have access to a wide range of tools to help plan passages. For long, and short, ocean passages, just as for all of recorded history, your experience, judgment and knowledge is important.
How would you plan for a 50 day passage, such as the sail between New Zealand and Hawaii?
Two of the routes between New Zealand and Hawaii, that you will discover from reading cruising guides are roughly shown:
The two routes are, very approximately:
Its useful to remember that there are two types of decisions:
For long passages, weather routing is not very useful in choosing a strategy. The long term strategy is chosen based on climate data, cruising guides, talking to others, etc.
Once you have chosen a strategy, in this case, choosing the first of the two routes outlined above, weather routing becomes useful again. Weather routing helps with tactical decisions. However, when used for a long passage, you need to use the tools a little differently than for shorter passages.
There is more information on this topic, in the manual.
Normally, when planning a departure, you have a destination in mind that you are heading toward. However, for long ocean passages, in the context of the short term tactics, this is not the case. Consider the route from the image above. The goal is not to follow the route indicated by the red line.
The best we can do is to choose a favorable departure time, and then, while underway, constantly plan to position ourselves such that we take maximum advantage of the weather forecasts, over and over.
The first step in the departure planning is to generate a routing solution with no target point. For example, this is a solution using weather data from several weeks before I departed NZ:
The colors are true wind speeds. The yellow dots represent violations of the maximum wind conditions I had set for the solver (30 knots apparent wind.)
The solver was able to find paths to all of the points in the isochrone solution space shown, while respecting my comfort settings. However, when I examine the GRIB file, there is 40 knots of wind south of the path at 4 ½ days into the forecast. This would be generating larger seas north. (When you see those yellow dots, be careful and examine the GRIB forecasts for those regions of time.)
It seemed imprudent to leave on a long passage, only to be faced with these conditions after 4 ½ days. We could stay farther north of the path, but there is uncertainty on which way the weather system generating those winds would move, it could easily trend north as it moves east which would clobber us if we were in that area. We want enough wind to sail but not so much as to break things.
Here is a second image, from the day before my eventual departure.
I have turned on the track of my sailboat, Luckness. In this image, I had also placed the cursor at the point where the last isochrone hit the actual route I sailed. (This dynamic path shown follows the cursor, allowing you to explore the paths in the solution space.)
Notice that the isochrones make the furthest progress east, south of the red route indicated, at around 40°S. This is 2° further south than the planned route, which is close enough to consider heading toward that area.
There is nothing worrying in this forecast. At this point, you can choose a target point for the router to head toward, re-run the solver and have it generate a path for you.
You go through this process at least daily:
Note from the image above, that the weather route shown, and the actual route I sailed, are a very close match for the first 2 ½ days. At that point the weather route suggested it was better to continue east, but instead I jibed and headed more south. Lets examine that decision in more detail.
I’ve repositioned the start point and time to where I actually was at that jibe. Using the weather forecast data I had access to at the time, here is the updated solution space:
In this image, I’ve created a target point on the actual track I sailed, and also positioned the cursor on the rough outline of the route I was using as a guide.
Note that the WR path to the target point, and the dynamic path to the route are the same for the first 24-28 hours. This is ideal, as before you need to make that decision (jibe and stay north of the route or continue south and follow the weather router path) you will have downloaded fresh weather data and can reanalyze this problem based on where you actually are at the time.
When choosing intermediate, tactical, target points for the weather router to follow, there is no need to restrict yourself by only considering the weather data you download and analyze.
For example, if you are sailing a passage which is heading into reliable winds, such as the trade winds can be, you should use that knowledge to place the target point so that you can take advantage of those winds.
If some area of winds is more or less consistent, perhaps before leaving on your passage, you can study the area and run many routes through it. This should give you a good sense for how you want the initial portion of your passage to join this portion. Of course, you will be reevaluating your initial guess as you approach that decision point.