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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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What Happens if The Windshifts in The Pre-Start?

In the last few minutes before the gun goes there is plenty happening. You must be observant of what is going on up the course as well as finding a spot on the line to accommodate you and your plan.

Keep an eye on boats that may have already started on your course, but if you are the only fleet out there, take note of changes in angle as you parallel the line or the different trim of sails that you need to make to maintain your course. 

As you are idling forward prior to cranking on to get up to speed watch how your sails are behaving, if nothing changes but the jib suddenly luffs heavily or fills, as long as it is not the effect of another boat nearby, you will detect a shift in the wind.

If you have been keenly observing what has been happening you should be ready to modify your starting plan and it might be time to reconsider where you want to start on the line.

If you see someone sailing upwind, and their angle is different than the angles you’ve been seeing, there’s a last-minute shift, and you may need to change your plan.

A word of caution though, make sure you consider the type of boat that you are watching and how its pointing characteristics compare to yours.

At the start, the shiftier the venue the more likely you’ll see a last-minute shift. This happens often on small lakes, or with venues with offshore winds when the course is located close to land.

In these situations, it can be safe to start near the middle of the line and with the mid-line start, you’re not fully out of the race if a shift happens in either direction. 

A fleet that starts before you are “tell tales” and their spread across the course gives you wind directions. If you see a boat that’s bow up on starboard, they’re likely in a right shift, if they are bow up on port, they’re probably in a left shift.

When looking at the boats in the fleet ahead and you see the leaders gybe set around the weather mark, you can be sure they’re in a right shift at the top of the course.

Watch what happens with that fleet as they continue downwind as this will give you some clues as to what has been occurring on that part of the course, just be aware that by the time you get there the wind may have switched back.

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Laylines & Getting Them Right.

Windward mark layline mistakes, unfortunately, are very common and can be extremely costly.

One of the most common things that sailors get wrong is getting to the layline too early. If you get to the layline too early, you can no longer play the shifts and you also lose tactical options.

More often than not it is really hard to judge laylines without a good visual reference and wind changes, dirty air, waves, or current are all outside factors that you need to take into account. 

Problems that can be created by getting to a layline too early:

  • A lift or increase in wind velocity causes you to overstand and sail extra distance.
  • Other boats that were below the layline may now be fetching the mark.
  • A header favours the boats inside the laylines, since they are closer to the shift.
  • In an approaching lull, you have fewer options to sail towards more pressure.
  • Boats not on the layline can tack on your air, leaving you with few options.

As you get about two-thirds of the way up each beat, work out your relative distance to the port and starboard laylines and consider your plan accordingly.

Are you a lot closer to one layline than the other? If so, make sure your strategy is sound. You must have a really good reason to keep going toward the closer layline so continue to evaluate all possibilities.

When considering tacking for the mark a simple test is that if you have to look back over your shoulder to see the mark, you’re probably on or past the layline.  

It is extremely important to know your boat’s tacking angle which is the difference in headings on each tack. 

Different conditions such as wind strength, sail trim, waves and dirty air will affect the tacking angle and in light air and the difference in light to heavy air could be as much as 30 degrees.

Learning your boats tacking angles comes from practising in various conditions and it does no harm to record these numbers on the boat for quick reference and to aid your memory in pressure situations.

Drawing tacking lines on the boat are one way of helping to call a tack.

Other boats are a great clue when judging laylines but just make sure that the boats you are referencing are trimmed on and sailing hard, they may have overlaid and are reaching down to the mark or they may have underlaid and are pinching to try to get up to it.

Even if you are very close to the layline there are a few reasons why you may delay your tack or you may even decide to tack early and they are:

  • There is a favourable shift coming.
  • There is more breeze coming as more breeze lifts you and decreases your tacking angle.
  • There are tactical reasons relating to other boats in your immediate vicinity.
  • There is an unfavourable shift coming.
  • You are heading into a lull.
  • There is no clear air on the layline.
  • If there is a big wave coming, perhaps delay until it passes as a tack right on it may stop the boat and cause you to underlay on the other tack.  

There are things you can do to practice judging laylines, but make sure you practice in varying conditions.

One drill I have found to be particularly useful is to use a fixed mark and tack at it from various distances with the aim of getting to it fully powered up, close-hauled and to pass within half a boat length.

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Headstay Sag and Why it Matters

Headstay sag affects everything from boat speed to pointing ability.

Sailboats with headstay sag often point higher and maintain boatspeed better in light-air conditions and one of the key methods to power up a sail is to induce sag in the forestay.

Dinghies obviously only have one jib and many sailboat classes limit the number of jibs that a boat may carry so the ability to be able to power up and down is very important in changing conditions.

When the headstay sags, it not only sags to leeward but also sags aft, which puts the luff closer to the leech, thereby adding depth to the jib. The key controls for manipulating headstay sag are shroud tension, mainsheet tension, and in some cases, headstay length.

In light air, the number one adjustment for headstay sag on boats with either deck-stepped and keel-stepped masts is varying the shroud tension. More tension effectively pulls the mast aft (assuming the chainplates are aft of the mast)

The other way to induce headstay sag is to minimize mainsheet tension. If the mast is stiff, trimming the mainsheet will quickly increase headstay tension and reduce sag. Ideally, in light and puffy conditions, when you ease the mainsheet in a lull, you’ll see the headstay sag to leeward, powering up the sail.

Easing off the rig isn’t the only way to increase headstay sag, chocking the mast at the partners, using a mast ram (if your class allows this) or moving the mast butt aft are other methods of achieving the desired sag.

When you sag the headstay, the maximum draft in your jib moves forward. To compensate and keep the draft aft, ease halyard tension, which also creates additional power. If you increase headstay sag but the halyard remains too tight, you’ll get a knuckle in the front of the sail and an entry that’s too deep, so you won’t gain the overall power you’re looking for.

A major effect of increased headstay sag is that it rotates the middle of the sail, changing the angle of attack causing the mid-luff of the sail to move to leeward as well as aft. At the same time, the mid-leech rotates slightly inboard, similar to the effect of weather-sheeting, which increases pointing ability.

With too much headstay sag the leech will rotate inboard too far, becoming extra sensitive to sheet tension and stalling too easily. Equally problematic, the entry angle becomes too extreme; when you bear off to rebuild speed, you have to sheet out too much to power up the sail.

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