# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: The mathematics of tacking downwind
From: Stephen N.G. Davies
Date: 2016 Jul 30, 12:34 +0800
True…but then again we are (and I still count myself among the ‘we’) also in override mode, assessing the numbers and the on-the-water situation as we see it, and making snap judgments, based on what other boats are doing, local knowledge that isn’t fed into the software, and Francis Upchurch’s gut sense, that now is the moment to tack whatever the numbers seem to be saying. We’re not always right, but right often enough for (bad?) habits to prevail.

I was reading yesterday that in the world of heuristics (the ‘rules of thumb’ we use in non-instrument based living (including navigating)), rules of thumb are all about marrying cerebral processing speed, available observational acuity and the decision making context. The example given was the ‘correct’ solution for a fieldsman getting under a baseball or cricket ball strike (or I suppose badminton shuttle, squash ball, etc.) compared with what skilled players actually do (called the ‘gaze heuristic’ - the reference was to Gerd Gigerenzer (2002), Calculated risks: how to know when numbers deceive you, NY: Simon & Schuster and (2014) Risk savvy: how to make good decisions, London: Allen Lane). Apparently analysis of videos of players at play shows they follow the rule of thumb: run with your eye on the ball keeping the angle between the ball and the horizon constant. The result is always a catch on the run because they have run in an ‘inefficient’ arc. However, errant wind gusts and other variables that might throw out an original ‘correct’ calculation for the optimum interception path are built into the gaze heuristic.

Not sure how the works with racing sailing, but from fifty years at the game (and, says he modestly, a lot of silverware in the cupboard), I think something like it prevails.
Stephen D

Dr Stephen Davies
c/o Department of Real Estate and Construction
EH103, Eliot Hall
University of Hong Kong

Office: (852) 2219 4089
Mobile: (852) 6683 3754

stephen.davies79@gmail.com
daiwaisi@hku.hk

On 30 Jul 2016, at 3:23 AM, Bill B <NoReply_BillB@fer3.com> wrote:

```On 7/29/2016 2:38 PM, Francis Upchurch wrote:
> The racing sailors here haven't got time for maths , slide rules or
> computers. they feel the wind on their face and see the tide and
> somehow, by majic (inbuilt algorithms based on years of experience? they
> get it right and take the most efficient and winning tack. Aren't humans
> ( and no doubt many other species) so good at this kind of thing? We
> seem to be inherently good at "lining things up", in a geometrical
> sense, e.g arrows and javelins). All majic to me.

Maybe dinghy sailors, but any *serious* racer 30 ft or over is looking
at polar diagrams, then developing target speeds for their own boat,
crew and sail suit. These are plugged into (or derived from) an on board
computer that displays speed, target speed, VMG, suggested course etc,
on one or an array of electronic display pods mounted near the base of
the mast.

Not exactly pipe smoke or wetting a finger :-)
```

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