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Isochrones – the BWR topic

In this post I will talk about the routing process. I will use BWR (Bluewater Racing) to generate isochrones. Isochrones are to me the major reason to use routing software.

A little disclaimer on BWR
First, one absolutely cannot steer using the BWR output. I can use BWR to estimate VMC targets. BWR + Optimum Angles is a winning combination.
Second, BWR just happens to be good enough for my needs. While I have no desire to switch at this point, I suspect that qtvlm is the better free software.

The setting
We are in a RTW race, with our boat somewhere in the South Atlantic (34S 23W). Goal is ‘east’, Leeuwin etc. I set an arbitrary WP south of Capetown (43S 20E). The route will be a little under 1 week. I would never trust the forecast longer that 3-4 days anyways.

BWR_options [1]

I start by downloading the grib. I will only move south or east, with the goal to stay in the forties. Thus grib boundaries are set by the start/end, with an ice-limit at 45S.

Next I set up the BWR route. The important variables are ‘Grid Size’ and ‘Max Range’. A larger Grid Size is faster, I use 20-30nm for long races. Max Range is a multiple of Grid Size, a factor of 5-6 works well. With a setup like this, I can have the first route done within a minute of the wx update.

BWR_isochrones [2]

The isochrones (red lines) tell me where I could be in x hours. I don’t know yet which the best route is, so any boat that sails to the isochrones can claim to be ‘ahead’. If I were to change plans multiple times, I would fall behind. My normal approach is to pick a spot on the 72h or 96h lines and route there with better resolution.

Picking the best spot is a matter of experience. One has to look at the predicted wx picture. Best position vs the weather systems. And understanding the skill of the forecast model is important. One can get a good feel for the skill by running the router for every wx. And generally light winds = low skill.

The VMC target is orthogonal to the isochrone. That is unfortunately not an accurate way for calculating targets, but you can see how there is change… for the proposed route we start around VMC100, then shift to VMC135 by 72h, then shift back to VMC90 toward the end.

We can see a big split. One could dive south and stick to the boundary. For the proposed WP this is about even. Decisions, decisions*.

BWR_closeup [3]

For the first 48h, the isochrones turn out very pointy. That makes life very easy, if I take the north option, I get a narrow track, no decisions for the next 36h at last. Just aim for the pointy bit. (max VMG most likely). Much of the time isochrones are very round, so finding the best spot is less obvious.

*In this example it’s a bit late to turn south. I think south looks a bit faster on paper. On the other hand north keeps the boat out of the stronger winds (>30kn). Toss up, really. But I like north.