How do I plan my passages?

posted 5 Jul 2015, 07:53 by Dominic Thwaites

You might think it’s just a matter of choosing the next harbour about 40 miles on, getting up at 8 for breakfast, away by 9 to be there in time for tea.

Not so. Passage planning is a big part of this challenge and it’s something I really enjoy doing. I spend a lot of time on the task. Sometimes it may only take 15 minutes but most times it’s much longer and on occasion I’m sitting in my boat spending hours analysing the options.

There are many variables and it’s always a compromise. In effect, you are looking for the most parsimonious solution. An optimisation process, really. It’s fascinating. There are passage planning computer programs, except no where near sophisticated enough to take into account everything I'd really need, besides, it would spoil the fun. For my passages, there are a lot of considerations to take into account and that’s what would be difficult to automate - the breadth and depth of the “inputs” is considerable.

So, starting from the top I need to:

  • Make progress towards my ultimate goal of arriving back where I started! (recognise that? You always need to have a clearly stated ultimate objective before you can break down how you are going to achieve it)
  • Ensure the plan fits within any medium term constraints or objectives (for example, I had an arrival objective at Craobh on a Monday to get the masthead arial fixed)
  • Take as much advantage of the tide as possible
  • Take as much advantage of the wind as possible
  • Choose my stop over points to be suitable for the weather conditions predicted
  • Choose my stop over points to be “explorable” and enjoyable
  • Consider my current physical condition and appetite for challenge
  • Consider or avoid the navigational hurldles (tidal races, overfalls, obstructions, depths, bad weather etc)
  • Understand how conditions could change and what plan B’s I may have

To do all this I have:

  • My personal expertise as a sailor and as a problem solver
  • A Nautical Almanac that tersely tells you all about tides, navigation, harbours and much more technical and reference info; A yachtsman’s bible
  • Tidal stream atlases that tell you about the direction and strength of tidal water movements every hour
  • Charts - maps of the sea and coastline, specifically depths.
  • Electronic charts which for me give more zoomable detail (as a phone app as well as a proper boat “chart plotter”)
  • Pilot books that narrate more about the harbours, how to get to them and what you can find in them
  • Weather reports from HM Coastguard on VHF and I get them electronically on a device called a NAVTEXT
  • Various weather forecasting apps and web sites giving short, medium and long term forecasts (up to a week)

Taking all the above into account, one comes up with a place to head for and a time to start with an ETA. That’s what I tell the coast guard before I set off. What makes it more complicated is that if your passage is going to be many hours (as mine usually are) you have to take into account the changing weather and tidal situations over that period based on where you think you will be.

For example, for passage 24, I could (unusually) totally compromise on the wind and tide direction. Because the other factors overwhelmed their disadvantage (enjoyable weather, short passage). Of course, If I had wind and tide with me for passage 24 I’d be laughing, but then I’d probably plan to get farther (because it would have taken me a third of the time to do the passage). then again, I would probably have had a completely different medium term objective if I knew the winds were not against me.

Apart from the obvious objective of getting further around, the wind strength and direction is the single most determining factor of what I do.