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Understanding and Controlling Upwind Sail Power

Upwind sail power comes from 3 sources:

  • The angle of attack.
  • Depth in the sail (draft)
  • Twist.

When trimming our sails we want to get our boat to full power and we must adjust for the sailing conditions by altering the three power sources listed above.

Sails are built so that they can perform in a variety of conditions but must be fine-tuned by the sail trimmer to achieve the designed shape of the sail.

The first source of power is the angle of attack.

At zero angle of attack, the sail is luffing. If the sail is luffing you need to trim in to increase power or the helmsman needs to bear off to increase power if the sail is already trimmed in as far as it can go. 

Power increases as the angle of attack increases up to the point of a stall. When the angle of attack is too great, flow stalls and power drops quickly.

The second source of power is sail depth.

Sail depth controls the power, acceleration, and drag of the sail. More depth creates more power and better acceleration while a flatter sail has less power and less drag.

As with angle of attack, power increases with depth up to the point where flow stalls and maximum power is achieved just short of a stall.

A flat sail is best when overpowered in heavy air and a flat shape is also fast in smooth water, as it creates less drag.

A deep sail is best to punch through waves and chop or to accelerate after tacking.

The twist is the third source of sail power.

Twist describes the relative trim of the sail high and low and a sail has lots of twist when the upper part of the sail is open.

Increasing twist reduces power and decreasing twist adds power.

Another reason that twist needs to be considered is that due to less surface friction, the wind is stronger at the top of the sail than at the surface and this is known as wind gradient.

The true wind and boat speed together create the apparent wind and a stronger true wind up high creates a wider apparent wind angle and stronger apparent wind the higher up the sail you go.

Sail twist is fine-tuned to match the sail shape to the prevailing wind gradient and we further fine-tune twist to wind and sea conditions.

The fine-tuning of twist is one of the most important and powerful trim adjustments we can make.

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HOW TO BE EFFECTIVE – STEERING OFF THE WIND

On reaches and runs it is essential that the helmsman and trimmer communicate and co-ordinate their actions.

As the helmsman, you must also respond to input from the tactician and changing sailing conditions plus the trimmer’s input based on sheet load and boat speed. The problem here is that there may be conflicting voices advising the steerer thus unsettling his concentration.

Since your tactics won’t succeed without good trim, it makes sense that the tactician talks to the trimmer who then gives feedback to the helmsman so the trimmer is the only one who communicates with the helmsman.

Steering on Reaches –

  1. Light to Moderate Air

On a reach, the fastest way between two points is a straight line and you should plan the reaches with that in mind only varying this based on changes in the sailing conditions or tactics.

In fluctuating wind conditions, work up in the lulls and down in the puffs as necessary to maintain speed, while holding a good average course. The trimmer will indicate when the sheet load is light head up, and when the spinnaker sheet is fully loaded bear off. The amount of course change required depends on wind speed.

When you must head up to pass another boat or defend your position let the trimmer know before making an abrupt change of course, to ensure that the manoeuvre is successful.

2. Heavy air

In heavy air, the helmsman is at the mercy of the trimmers.

The vang, main sheet, and spinnaker sheet must be eased when the boat is overpowered or it will round up and broach but it is fast to carry as much power as you can as long as you can control it.

Carrying some weather helm is OK as long as the rudder doesn’t stall, leading to a round up, this is once again a time when communication between the helmsman and trimmer is essential.

Steering on Runs –

  1. Light air (3 to 9 knots)

In light winds, the best sailing angle is about 140° true wind angle (40° above dead downwind). The angle changes very little as the wind speed fluctuates, so don’t head up in the lulls and off in the puffs except for tactical reasons. The fastest way to the next mark is to tack downwind and keeping the apparent wind forward is fast. A word of caution here though, this is boat dependant so it pays to practice to find out what is true for your particular class or boat.

2. Moderate air (10 to 15 knots)

The optimum speed and sailing angle change dramatically with every change in wind speed. For every knot of wind the optimum course shifts five degrees. In ten knots of wind, the optimum angle is 140° true wind angle and fifteen knots a 165° true wind angle is fastest. Do your best to respond to every change in wind speed, driving off with the puffs and heading up in the lulls.

3. Heavy Air (over 15 knots)

Aim for the mark, sail fast and keep control using the waves to surf wherever possible. Use crew weight to balance the helm, avoid sailing dead downwind and trim the spinnaker directly in front of the boat. Crew weight should also be moved aft to promote planing and to avoid the bow burying.

The helmsman should be forceful to keep control but also be mindful that smooth is fast, jerking the helm creates drag and slows you down.

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Sailing Wind Velocity and it’s Importance

 

In all sailboat races, you must sail toward better pressure as more wind velocity almost always means more speed. 

On the course look for darker water as changes in wind velocity are a lot easier to see than changes in direction.

More wind creates more ripples on the water, and these appear darker because of how they reflect light.

Be careful though to consider variations in sunlight and clouds when assessing heading over to darker looking patches on the course.

Other boats around you are also a great source of information about velocity across the course and be sure to not only take into account their angle of heel but also their heading.

Changes in heading maybe a puff, lift or knock so continual observation should give you the answers you are looking for.

Generally, increases in wind velocity make more difference when the wind is light. An increase of a few knots in the wind when it is light may increase your boat speed by a knot or more whereas an increase of a couple of knots of windspeed in the higher wind ranges may see no increase in boat speed at all.

Once you have found yourself in better wind velocity, do your best to stay there and it may serve you better to stay in a puff longer by pinching up a little, footing off into it or tacking/gybing to stay in the puff longer.

Beware of velocity headers and when velocity changes it affects the wind you see and as an example, when you sail into a lull your apparent wind goes forward which feels as though you have been headed even though the wind direction stays the same.

The mistake a lot of sailors make is to tack on a velocity header and tack is not only slow in light air but you could well be sailing on a knock on the other tack.

When you experience a velocity header, change gears to keep your boat speed up and continually be on the lookout for the next puff or shift.

The amount of wind pressure also affects your ability to survive in another boat’s bad air. In light air, wind shadows are bigger and much more hurtful.

In heavy air, you can sail fairly close to leeward of another boat and go pretty much the same speed.

Wind pressure impacts what you do in different positions on the first beat.
Having more velocity means you will sail faster with narrower tacking angles, so you’ll get to a lay line sooner. In light air both tacks take more time, so you can afford to spend more time on the shorter tack.

A factor that can have a big effect on wind pressure is current. When you’re racing upwind, the choice is easy – head for the part of the course where there is stronger current flowing toward the wind or less current going with the wind.

This will not only help you make better progress over the bottom, but it will give you better wind pressure as well.  

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Common Myths of Competitive Sailing

 

MYTHS REGARDING CURRENT ACROSS THE COURSE

MYTH – Your apparent wind will be different on each tack. Since you sail faster over the bottom on the down current tack, you’ll feel more wind on that tack and this will affect sail trim and speed.

TRUTH – The apparent wind on both tacks is affected equally by the current, so you will feel the same wind and therefore need the same sail trim on each tack.

MYTH – You can improve VMG by pinching to get the current on
the leeward side of your bow.

TRUTH – It doesn’t matter where you are heading since the current only
pushes your boat in the direction it is moving. So pinching will be slower.

MYTH – On a beat, it’s always better to sail the up current tack first.

TRUTH – Current affects all boats equally, so as long as you don’t overstand the mark it doesn’t matter where you are. But if the up-current tack is much longer, it may be better to sail that tack first.

MYTH – If the starting line is square to the wind, it’s better to start at
the up current end.

TRUTH – All boats are being pushed in the same direction by the current, so it doesn’t matter where you start on the line (as far as current is concerned). 

MYTHS REGARDING STARTING

MYTH – If you are in the middle of the starting line and you turn up so your bow is pointing straight into the wind, the end of the line that is closer to your bow is the end that is favoured.

TRUTH – Going head to wind in the middle of the line will help you determine which end of the starting line is farther upwind, but that end is not necessarily ‘favoured.’ The favoured end of the starting line is the one that will get you to the windward mark sooner, taking into account a number of strategic and tactical factors including which end is farther upwind (and by how much).

MYTH – You should be more careful at the start when the race committee has signalled with flags that a starting penalty is in effect. Because of this, you should be more conservative as you approach the line.

TRUTH – Even when there is no special starting penalty in effect, the consequences of being over the starting line prematurely can be severe. You should not change your approach to the start just because one of these flags is displayed the rest of the fleet will be hanging back so your chances of a great start are much better.

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Avoid Sailing Mistakes

An important ingredient to winning a yacht race is to make fewer mistakes than your fellow competitors.

Because of sailing’s complexities, even the best sailors will make mistakes but it is the avoidance of the major ones that are the most telling, minor mistakes will make a difference but should not be sweated over.

I have listed below things which will ensure that you avoid major mistakes. You need good planning, execution on the course and staying alert whilst racing to guarantee you avoid a disastrous result.

  • Read and absorb the sailing instructions before heading out and where possible carry a copy to refer to if time and circumstances allow.  If you are on a crewed boat have at least one other team member do the same. Write the most important or unique instructions on your boat with a Chinagraph pencil.
  • Constantly check the wind direction both before the race and during the event, this will help you to identify persistent or oscillating shifts and assist you to modify your strategy if necessary. Head out of the boat.
  • Constantly look around the course for differences in wind direction and strength. To head to the wrong side of the course in changing conditions because you were not constantly observing changes can be extremely costly.
  • Choose your lanes carefully to avoid sailing in dirty or disturbed air and tack or gybe away to stay clear.
  • Check current direction and strength and read tide tables to see if there is a likely change of direction and strength as the race wears on. What was correct on the first time round may, in fact, be very different the next time around.
  • Have a race plan before the start but be prepared to modify it if conditions or your position in the fleet changes, a constant re-evaluation may be necessary.
  • Sometimes even if you have rights in a mark rounding or crossing situation you may be better off not to force the issue. Avoid collisions, these could finish your day and by taking your right of way you could be pushed to the wrong side of the course. It is important to plan in each situation, this will avoid snap decisions which could end in disaster.
  • Don’t arrive at the course with minutes to spare, get out there early to settle the team and get their heads in the race. This also allows you to set the boat up for the prevailing conditions ensuring that you get off the line in as good a shape as possible. Having the setup wrong and the subsequent messing around to get it right will mean that you will probably not recover.
  • Don’t head out with an item of equipment that you haven’t used before. Try all new gear during training or two boat testing to evaluate its suitability or whether it is better than what you already have.
  • When rounding a mark, locate the next one as soon as you can. It makes little sense tactically, to blindly follow the fleet if you are behind and if you are in the lead, locating the next mark is fundamental in planning your strategy for the leg.

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