Shifty Business – The Gains of Tacking on Wind Oscillations

It has to be said that I have sailed with some folk who give little if any real notice to wind shifts, or are even aware that there may be a pattern or some weather signs which they could take advantage of in their upwind progress (and off wind when gybing a lot say in an assymetric type boat) They tend to go along with the fleet or as they kind of feel they should tack over towards the layline rather than end up on a left hand flyer.  Other helms are a little dismissive of a 5 degree wind-shift and tend to only tack in bigger shifts, or panic and tack when they sail into an ‘apparent’ wind shift, ie a hole in the wind when the boat suddenly backs its sails under its’ own speed.

How much is there to gain by tacking on the shifts as little as 5 degrees and how can you plan for a wind ‘strategy’ to mix into your game play on the water?

In the first half of this blog we will look at typical oscillating wind conditions influenced by low pressure systems (in the Northern Hemisphere)

I was about to sit down of a wet sunday, brush up on my trigonometry and draw some geometric diagrams, but dear reader of course Googling is so much easier these days and hey presto, surprise surprise there are plenty people who have trodden this path and created nicer diagrams, and different ways of thinking about the gains to be made.

Big Gains!

I had seen plenty of diagrams and explained in sailing school to my pupils with a set square that windshifts on the beat are worth tacking on and you should use your compass or check your heading bow to land features to judge if you are being headed or lifted. However I never have qauntified the gains because well frankly the last time I used Trigonometry was thirty one years ago!

Others have and tacking even on a five degree windshift is well worth it – the gain is in fact 12% in terms of velocity made good versus a boat which continues to sail on what seems just a slight header. Another source quotes this as being equivalent of 1 knot of boat speed, which is even more maybe 20% but I am inclined to think that a 12% gain is about right looking at various diagrams which show  a la new americas cup ‘leading line’ how much further ahead you travel VMG wise upwind.

 

Let us quantify and qaulify this further – 12% means over a windward leg of a nautical mile when you make say 4 knts made good, if you tack on all the headers you will reach the windward mark a full minute ahead of your opposition, or in other words 200 to 400 meters ahead, or 20 to 40 boat lengths. Now of course a following boat may either tack a bit randomly up the recommended 60 cone towards the mark until they lay it on stb say, and get some advantage by sheer monkeys-with-type-writers chance, or they may spot a couple of shifts and tack or follow other boats who do so. If they do this then you have with all else being equal,  maybe ten boat lengths on them on the first beat given  you came off the start line even with them and were free to tack on the shifts.

Shift Patterns and Weather Signs

Wind shifts have quite complex reasons behind them, but here so far we are talking about wind ‘oscillations’ around a mean compass direction. Luckily we can use local weather signs to pick out the shifty nature and see if there is a timing to the pattern or if the signs are obvious

If you ever wondered which boats are out early and up at the windward mark while you are still organising sails and having a last coffee then you can be pretty sure that these are the boats who are going to beat you and get top placings. They have been up the beat trying to look for a timing pattern to shifts or signs that they are coming. Also they may be looking for the general trend of wind direction over the course, and stregnth/lifts from the land when Coreolis effects are happening. They will be noting the average course on the compass on each tack, and how big the shifts are and how loing they last.

In the northern hemisphere (reverse port/stb for antipodean climes down under) there tends to be a general wind stregnth related effect which drives an oscillating wind at the surface. Firstly when the wind lulls it tends to back, ie anticlockwise and this is a header on a starboard tack, lift on port. Often there is a larger lull after the stronger, right side wind pattern is shifting to a lighter and the same can be true of the reverse – in a gentle header for a few minutes the wind dies further on the left hand side and then suddenly you are confronted by what seem massively lifting gusts from the right if you are lieing on starb oard tack.

If you can’t really spot of timing in the pattern of shifts then you can usually spot at least ripples on the water. Wavelets over the wave train which appear to point from nearer the bow are a header while those which are nearer the beam are a lift. If you get an aerial view up a hill or by using a drone over your local race course during typical ‘gradient’ breeze ie low pressure isobar influenced wind, then you can see this effect when the water is fairly flat at least.

Another weather sign which is  a key driver of wind stregnth and coreolis effects are clouds, cumulus typically which are present in low pressure weather before and after frontal systems which have more consolidated banks of cloud. In many sailing areas in the temperate areas of the world this is very typically good sailing weather wind winds between force 2 and force 5 or even more. Clouds are both cause and symptom of vertical wind, bringing down the true wind from above which is veered on the back edge of the cloud, while there is most often a lull and a backed wind on the leading edge. These puffy cumulus clouds may seem random and a bit down to lady luck as to weather you get a fortuitous set over you to tack on, but there is often a longer periodicity between skies peppered with clouds and clearer skies (in the cold sector often after two fronts have gone over) It obviously pays to tack onto starboard when you are under the trailing edge of a cloud, however many get fooled by the stronger wind giving them an apparent lift on port. Which brings us to –

The Lovely Lifts Phallacy and Wrong Side of the Course

Still sailing then in our oscillating shifts, we note already that shifts are very often both in direction and velocity as more or less of the true, higher altitude isobaric wind stretches down and across the race course. The trouble with this concurrent pair of factors is that the one can fool the helm into thinking they are on great lift or a terrible header due to the effect of apparent wind on a boats heading. (google it actually for good diagrams of youtube vids) As the wind stregnth suddenly increases in a shift, so does the boat’s ability to point which ever way you are oriented on the course. However you would in the northern hemisphere, be better placed to get both this apparent wind lift AND the real directional lift if you tacked on to starboard before the gust or band of stronger wind.

Another aspect here is line and Windward Leeward course bias. If the start line is not truly perpendicular to the wind and if the W-L course is not parallell to the mean wind direction then there is a favoured side to sail more of the time on and of course get off the start line on,  because you will sail less far to reach the mark. You will also in one or both events, be able to use shifts, but you will be  at a natural advantage sailing less far and holding more to the favoured tack ie the tack which your bow points most towards the windward mark.

Another little phallacy here is ‘lovely first beat off the start line, messed up the second by going the wrong way’ . This is due to bias and just to note it, the line bias can be different from the W-L: bias or the wind may have changed average direction in a more permanent shift in the course of your first two legs of the course. More on longer duration or  permanent shifts later

Wind Shifts Also Favour a Side

Even with a perfectly laid course for the average wind direction, wind shifts will favour one side or the other of the course from their commencement. This is because they are off centre from the aeverage, low oscialltion wind which is maybe plus or minus 2.5 degrees around a mean, or if the shift pattern is predictable then the committee have lain a course as a median wind direction.

So if there is a large cross course shift to a veered wind, then it will hit the whole course at a skewed angle from the rhs and therefore boats on that side of the course will get into the lift/header first and take advantage of it first. They then get a double advantage. Given no major shore interaction effects, then this is what many sailors talk about when they say going the right side of the course for a given beat.

How then could you judge this? Well wind stregnth is one method as noted above, a lull is very often associated with a backing wind, which favours the  left hand side first, where as stronger wind is veered and favours RHS. In weather where it is the cumulus clouds which influence shifts most, then sailing mostly on starboard on the side of the course with most clouds will pay, that is the right side given an even wind under blue skies.

Now this is all fine, but if you dash off to the LHS in a lull only for it to change back you may find yourself on a big header if the wind shifts back RHS as the new wind builds. So that is why you take a stop watch, to see if the pattern is likely to last a whole beat once established or if you will need to avoid ‘corner of the pitch fliers’ . Most sailors who know their salt will choose a side of the course to sail mostly on, but not go outside a 60 cone towards the winward mark. They can then dart back over to the other side of the course in order not to loose out too much if they wind shifts over. The 60 cone as a rule of thumb means tackign with the windward mark somewhere around your shrouds to beam as you look towards it, and less towards looking over your shoulder as it is only on when you sit on the lay line.

Longer Lasting Shifts

Some oscillating shift patterrns have a long periodicity that you may not even detect after a practice beat and first leg of the race. At other times the wind will make a permanent shift so to speak, for the rest of the race.

Here again we want to use the weather signs – waves, clouds, smoke and flags on the shoreline etc to then compare to the weather forecast. Many are influenced by frontal activity, some are solar modified in gradient breeze while in sea breeze driven by the sun, the wind can follow the route of the sun or move towards a large and warmest land mass as with the SE of England where it goes right, or in contrast with the first example, left on the coasts of Norway following the sun.

Some are influenced very much by topography near or even quite far from the race course. Sun warmed land can drastically alter wind direction over time and train an isobaric wind into a new direction. Major valleys and mountain ranges can also train a wind, and some can force the wind around them until the wind picks up velocity and surges down or between them, turning up on the race course quite suddenly from an unexpected direction.

As in ‘being on the right side’ of a predicted osciallting shift, we want to get on the right side of the course for the ‘permanent shift’ ideally before it hits, or be on our way as soon as we confirm that it is underway. Knowing when for example a front is coming through or a sea breeze is fully estbalished is a real race winner as even if you need to sit in an apparent doldrums for some minutes on that new side, the gain can be enormous and very often you stand the windward mark within a single tack or so even if you are only a third of the way up the course!

So wind shifts are not a complete mystery , we have many clear weather signs above and around us to consider and we can use the compass more often, and even engage a stop watch to experiment and discover if a typical days sailing does involve a periodic and therefore predictable oscillating shift pattern. 

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