Predicting changes in apparent wind will make you a safer, faster sailor, says Martin Watts
Newcomers to sailing soon learn to appreciate the differences in apparent wind on different points of sailing, writes Martin Watts.
We’ve all slogged to windward in a Force 5, clad head to toe in waterproofs as spray flies over the deck, only to see a boat sailing in the opposite direction, downwind, with the crew lounging around in shorts and T-shirts enjoying what looks more like a Force 3.
However, the apparent wind can also change whilst sailing on a leg of a route, and understanding these changes and how to respond to them can improve not only speed, but also the efficiency of how a yacht is handled, comfort on passage as well as safety by avoiding losing control in broaches and other incidents.
Changes in true wind direction are obvious, but there are other reasons for apparent wind changes.
I’ll start by looking at what causes these changes and then how we respond to them.
If the true wind speed suddenly increases in a gust, then not only is there an increase in apparent wind speed, but the apparent wind angle (AWA) changes in direction, moving further aft with respect to the yacht.
Conversely, if there is a decrease in the true wind speed, then the apparent wind speed decreases, but it now moves further forward in relation to your boat.
There are also changes in the apparent wind due to variations in the speed of the yacht.
If the yacht’s speed drops, then the apparent wind decreases and moves further aft.
If the yacht speed increases, then the apparent wind speed increases and the apparent wind angle decreases, moving further forward.
This effect can clearly be seen on the foiling catamarans who have their sails sheeted in all the time.
In practice, any change in apparent wind speed and angle are usually a combination of both of these mechanisms.
The common sight is a strong gust hitting a yacht.
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The extra power and the fact that apparent wind is now more beam-on to the yacht means the sails are over-sheeted and there are large lateral forces that make the yacht heel, which result in a less efficient hull shape requiring more rudder to be used to keep the yacht going straight.
This all slows the boat down, and so the apparent wind shifts even more beam-on. The rudder eventually loses grip, and we end up with a broach.
So how should you respond to these changes in the apparent wind speed and direction?
The actual response will vary on the yacht, the crew and the conditions, and on whether you lean more towards the ‘racer’ or ‘cruiser’ ends of the spectrum.
Apparent wind: dealing with gusts
If this is the case, the wind direction of the gust may be the same as the higher-level wind direction.
In the northern hemisphere, this will typically be veered compared to the sea-level wind.
The result is that if you are sailing on starboard tack, you may get a lift to windward, while on port tack you would be headed.
It’s slightly more complex than this, however, as downdraught gusts fan out as they hit the water and so it helps to identify where the yacht is in relation to the gust fan so that the change in wind direction can be predicted.
If you are sailing towards the centre of the gust, on either tack, you are likely to be headed, while sailing away from the centre, you are likely to get a lift.
If you are hit by a gust when sailing upwind, the increase in true wind speed will move the apparent wind aft, giving you a lift in which you can luff up slightly, or if you are on course and don’t need to make more progress to windward, you can ease the sails to gain more boat speed and avoid excess heel, or do a combination of both.
If you are less concerned about performance, it is easier to just luff up slightly, which makes use of the lift, and also spills some power from the sails.
When a gust hits, its leading edge will often bring the strongest wind, so in blustery conditions, it pays to be looking upwind spotting approaching gusts, even if you’re just cruising.
This gives you a chance to luff up significantly before the gust hits, so that the sails are lifting slightly and the yacht is not overpowered as the gust hits.
You can then slowly bear away again, keeping the boat under control and maintaining good boat speed.
As boat speed increases again and the apparent wind moves forward, both groups may end up sailing at a slightly lower heading than before.
In contrast, if you aren’t ready for the gust, you will heel and increase rudder angle, slowing the boat down, and if you then luff up, you will do so into the lighter wind behind the gust, further losing speed.
When sailing on a reach, easing the sails and bearing off slightly, so that the direction of effort is more in line with your heading, will give the biggest increase in speed.
Racers steer to keep the boat ‘under the sails’, reducing heel to keep the rudder gripping particularly when fully powered up.
They may even bear off more to stay in the gust, particularly in light winds, knowing that they will be coming back up to the original course line with a better apparent wind direction.
For a cruising sailor, the response to a gust when reaching can be rather different to this and is more similar to how they responded when sailing upwind.
Luff up slightly, or a lot before the gust hits if the gust is large to spill power from the sails, and then bear away as the gust eases.
Then as boat speed increases and apparent wind comes forward again, racers will sheet in, while cruisers may be content to bear away slightly to keep the sails full.
Things change again when running deep downwind.
With the wind astern and the sails sheeted right out, they are being pushed along, but there is no aerodynamic flow along the sails and they are not generating any lift.
The force on the sails is in the same direction as the boat’s heading, so there is no heel.
Rounding up towards the wind, far from depowering the sails, will do several things at once: it allows an airflow to develop and with it a significant increase in power; the apparent wind will increase as you are no longer running before the wind; and a large heeling moment, which could easily lead to a broach.
This is all exacerbated by the fact that there is a temptation to set more sail when downwind when things feel calm, only to find that you are hugely over canvassed as soon as you turn towards the wind.
So generally, the response is to stay heading in the same direction, with a key aim now being to stop the yacht burying its nose in the back of wave.
If you need to reduce power, sheeting in is a more responsive technique, or alternatively you could try reefing down to match your sailplan to the gusts.
When the true wind speed drops, when going upwind, the apparent wind moves forward and so the initial impression is that you have been headed.
The automatic reaction is to bear off, but you must do this gradually so that the yacht does not lose momentum as a result of large rudder movements and disrupting the airflow over the sails.
When the true wind speed drops on a reach, the apparent wind also comes forward, so you’ll need to sheet in or bear away to keep the sails pulling.
One of the common reasons boat speed suddenly drops is because of wake from powered vessels.
The first response is to try and minimise the effect of the wave by steering accordingly.
As the yacht’s speed drops and apparent wind moves aft, ease the sails or luff up so that the apparent wind angle is the same as before the wake hits, so that the sails remain set correctly.
Then as the yacht’s speed increases, bear away back on to the original course.
In light winds, it may pay to keep sailing a bit higher, with more apparent windflow over the sails, to build up boatspeed more quickly before bearing away.
Waves and tide
The effect of yacht speed on the apparent wind direction is clear when sailing upwind with the waves or swell coming downwind.
As the yacht climbs the front of the wave, it slows and so the apparent wind moves aft, so the helm can luff up slightly to keep the sail trim right and this can help the boat punch through the top of the wave.
As the yacht goes down the other side of the wave, it accelerates and so the apparent wind moves forward and so the helm must bear away to keep the sails drawing properly.
Apparent wind can also be dramatically affected by changes in the tidal stream. Sailing with the tide will increase speed over the ground and with it, apparent wind.
In terms of direction, the wind angle will move forwards, so sheeting in or bearing away will be necessary.
The more marked impact is that sailing upwind with the tide, the wind speed can increase dramatically, while on a run the apparent wind may drop a Force or two by comparison.
Things get more interesting when sailing across the stream.
With the tide on your leeward side, you would be favourably pushed towards the wind, ‘lee-bow effect’, while with tide on your windward side both apparent
wind speed and progress to windward would be reduced.
Traditionally, the mark of a good helmsperson was that they could sail a course with a steady compass bearing, so that the navigator could make good dead reckoning calculations.
Now with GPS and chartplotters, the yacht’s position is known, and the helm and crew have a greater flexibility to respond to apparent wind changes to maximise boat speed.
So, the yacht can be pointed on its desired course, the sails set accordingly and then the helm left to look around to predict the wind and wave conditions and steer to what feels right for the yacht and the conditions.
The aim is to be able to predict and react to any apparent wind changes, rather than just respond after the event.
Then, after a period of time, compare the actual and desired position or look at the cross track error (XTE) and alter the course heading and adjust the sails to suit.
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