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Performance Racing Trim
by Bill Gladstone


Chapter 6 -
Mainsail Trim & Controls

6.1 Introduction
6.2 The Mainsail Trimmer
6.3 Mainsail Power
6.4 Controls
Sidebar - Vang Sheeting
6.5 Reaching & Running
6.6 Conclusion
Fully Battened Mains

6.1 Introduction

The jib leads the boat upwind; the mainsail provides balance and control. Proper main shape is a shape which complements the jib, and balances the helm, while pushing the boat to the proper mix of pointing and speed.


We have more control over mainsail shape than we do genoa shape; and we are required to do more with it. After all, we have all those genoas, but only one main. Having so many controls is a double edged sword; sometimes it is hard to know what to use when. Fear not. If things don't work out, blame the tactician. Fig. 1.

Fig 1. - A well trimmed main will provide speed, pointing, and balance to our upwind performance.

In this chapter we will look at the role of the mainsail trimmer in upwind performance, and the various sources of power for the main. Next, we will consider each mainsail control and how it impacts sail shape and power. We'll look at initial trim settings for upwind sailing, and refinements for varied conditions. Reaching and running trim will be covered in a separate section. Later, in Chapter 8 we will integrate main trim, jib trim and driving techniques in a variety of sailing conditions

6.2 The Mainsail Trimmer

The mainsail trimmer is responsible for monitoring the boat's upwind performance, trimming to keep the boat sailing fast, pointing high, and in balance. Fig. 2.

Fig. 2 - The mainsail trimmer works with the jib trimmer and driver to keep the boat sailing fast and pointing high.

Monitoring performance involves information from on and off the boat. Performance against adjacent boats is one key input. Boat speed and apparent wind angle provide additional information. Also important are the helm balance, heel, and path of the boat through water. For example, if the boat is pitching or the course unsteady the main trimmer can make adjustments to help.

The ability to make quick adjustments to the main in immediate response to changing conditions distinguishes it from the jib. Jib trim is trimmed to an average, with small, incremental adjustments to changing conditions. By virtue of the ease with which the main can be adjusted, and because of its effect on balance, the main is played much more aggressively. This is particularly true in puffy or wavy conditions.

The main trimmer has more control over shape and power than the headsail trimmer. In the next section we will take a look at each source of mainsail power. Following that we will look at each mainsail control.

6.3 Mainsail Power


There are three sources of mainsail power, as with any sail: Angle of attack, shape, and twist.

Power through angle of attack

The main derives power first through angle of attack. Trim the sail in, and you add power. Let the sail out and you reduce power. Heading up also reduces angle of attack and power.

Angle of attack is increased by trimming the sheet, raising the traveler, or falling off. Fig. 3.

Fig. 3 - Angle of attack is the first source of mainsail power.
Boat A - Power is reduced by easing the sail out or by heading up.
Boat B - Angle of attack is increased by trimming the sail in, or by falling off.

Power through shape

Deeper sails generate more power. Flat sail shapes generate less power (and less drag). Sail shape is adjusted through a variety of controls. Mainsail shape is controlled by mast bend and outhaul tension. Fig. 4.

Fig 4. - Another source of mainsail power is sail depth.
Boat C has a deep, powerful mainsail.
Boat D has a flat sail, which generates less power (and less drag.)

Power through twist


A closed leech generates more power. A twisted, or open leech, spills power. The mainsheet is the primary controller of main twist. Fig. 5.

Fig. 5 - Twist is the third control over sail power.
Boat E has an open, or twisted, mainsail, which spills power.
Boat F has a closed leech, with little twist, for maximum power.

Initially the sheet's primary impact is angle of attack, pulling the boom in. As the sail nears full trim the sheet increasingly pulls the boom down (not in). At this point the primary impact of trim is a change in twist.


The mainsail trimmer has an array of controls available to control each source of mainsail power. The next section will look at these controls one by one.


6.4 Mainsail Controls

In this section we will review each mainsail control, how it will be used to alter mainsail shape, and how it will impact the power of the sail.


The mainsheet is the primary mainsail control. The sheet controls twist and leech tension; which affect power and pointing. Trimming the main also changes angle of attack and overall sail depth. The mainsheet should be trimmed so the leech end of top batten is parallel to the boom. Fig. 6. When the boom is on center line the end of the batten should point straight aft. Fig. 7.

Fig. 6 - Trim the mainsheet to put the upper leach parallel to the boom, and the telltale flying most of the time.

Fig. 7 - With the main at full trim the boom should be on the centerline.


Leech Telltales

From this initial setting the sheet can be fine tuned to keep the upper leech telltales flowing, with an occasional stall. Usually this telltale behavior achieves the best mix of speed and pointing.

There are conditions when performance will improve with the main slightly eased or trimmed compared to the settings described above. You never know until you try.

If the sheet is eased slightly, so the telltales never stall, then speed may increase without any loss of pointing ability. At other times, trimming to the point of stalling the telltales half the (or more) time may result in higher pointing, though the added drag will usually cause some sacrifice of speed.

Secondary Controls
As you make adjustments to the secondary controls described below you will need to check and recheck sheet trim.

Never relent. You can always go faster.

More on the Mainsheet

In light air over-trimming the sheet will stall the sail. We seek all the power the sail can generate. This means trimming just short of a stall. Fig. 8, Boat A.

In more moderate conditions, higher pointing is possible by trimming the main hard, to the point of a partial stall. There are of course limits on how hard you can trim without a sacrifice of speed. The sheet should be eased to twist out the upper leech of the main when the boat is slow. Fig. 8, Boat B.

In heavy air over trimming the mainsheet will create excess weather helm. Some backwinding in the luff of the main is to be expected. Don't let a little backwinding trouble you - it is fast. . Fig. 8, Boat C.

Fig. 8 - Twist and power change with wind speed.
Boat A, in light air has a deep shape for power, with enough twist to ease flow.
Boat B is trimmed for moderate winds, with a tight leech and moderate depth.
Boat C is set up for heavy air with a flat sail shape and twist to spill excess power.

Boom Vang

The boom vang is primarily an offwind control. Upwind a very tight vang can add extra bend in the lower section of the mast. Snugging the vang upwind can also help control twist.

In light air a tight vang will close the leech, stall flow, and wreak havoc on performance.

We'll see more on the vang in the Reaching and Running section, below. At the end of this section we will also look at an alternative trim technique known as Vang Sheeting.

Mast Bend

After the mainsheet, mast bend is the second most powerful controller of mainsail shape. Mast bend is used to change the shape of the middle and upper portions of the sail. Mast bend is controlled by the back stay and/or baby stay and running back stays.

Mast bend flattens the sail by increasing the distance from luff to leech. Use bend to reduce power as the breeze builds, and for reduced drag and extra speed in smooth water. Use less bend (a straighter mast) for extra power in chop, or when sailing downwind. Fig. 9.

As secondary affects, mast bend also impacts twist and draft position. Concurrent to a change in bend the mainsheet should be adjusted to retrim leech tension; and luff tension must be checked as well.

Fig. 9 - Mast bend controls mainsail depth, particularly in the middle and upper portions of the sail. Increasing bend flattens the sail.




The outhaul controls depth in the lower portion of the main. The more we pull on the outhaul the flatter the foot of the sail becomes. On mains rigged with a flattening reef, think of the flattener as an extension of the outhaul, which takes over the outhaul function when the outhaul is at its limit.

The outhaul should be on part way whenever you are sailing upwind. As the breeze builds from light to moderate air the outhaul should come on all the way. The foot of the main should be stretched flat as the boat is over powered. In chop and waves the sail should be fuller for more power; in smooth water it can be set flatter for closer pointing. Fig. 10.

Fig. 10 - Use the outhaul to control shape in the lower portion of the main.



The traveler positions the boom, controlling angle of attack. Keep the boom centered (traveler to windward) until overpowered. Fig. 11.

Fig. 11 - The Traveler controls the angle of the mainsail.
Boat A - In the moderate conditions position the boom along the center line of the boat.
Boat B - On heavier air lower the traveler to control heel and reduce weather helm.


Gradually lower the traveler to leeward to control heeling as the wind builds. The boom should start centered with the #1 genoa. As you reach the upper end of the #1 the traveler may need to go down a foot or so. As the wind builds the range of play in the traveler increases. With a #3 the boom main be centered, or it may be eased to the quarter.

The traveler should be played constantly in puffy conditions to control heel and weather helm. Once the sail shape is set for the average conditions the traveler is used to make quick adjustments to overall power. Fig. 12.

There are times when it will be faster to leave the traveler set and play the mainsheet, adjusting twist, when overpowered. The preferred method depends on:

Sea State - In more waves playing twist is preferred, in puffy, smooth water conditions the traveler is preferred.
Boat Design - Heavier, smaller keeled, smaller rigged boats respond well to traveler play. Lighter, deep keeled, over canvassed boats respond best to twist adjustments.
Ease of Use - If one is easier to adjust, and the other is a pain in the I mean difficult, then you'll probably go faster using the control that works - until you can fix the one that doesn't!
Testing - Try both techniques. Which provides better performance for you, today, in these conditions?

Luff Tension

Luff tension adjusts draft position. Adding tension pulls the draft forward. The main halyard and cunningham control luff tension. Use the halyard until you reach its legal limits, then use the cunningham. Fig. 13.

Fig. 13 - the main halyard and cunningham control luff tension and draft postition. More luff tension pulls the draft forward, less tension lets the draft move aft.
Sail A shows the draft where we want it - just forward of the middle of the sail.
Sail B shows the draft too far forward. This happens when the wind drops, or you turn down to a reach. To correct this the cunningham or halyard should be released.

Draft position is not so much a power control as it is a drag control. If the draft moves too far aft it creates too much drag. As the draft is pulled forward drag is reduced, with some loss of power.

The draft should be just forward of the middle of the sail, at 40-45%, most of the time. When overpowered try to pull the draft further forward with extra luff tension. In light air chop a more draft aft shape can help add power.

Mast bend pushes the draft aft. As you add bend add luff tension to compensate. Also, don't forget to ease luff tension when you straighten the mast. Fig. 14.

Fig 14. - The draft moves aft as we bend the mast and as the wind increases.
Sail C is a draft sail.
Sail D shows a sail with added luff tension to compensate for the impact of mast bend.


Full length battens also impact draft position. For more on the relationship between draft position, luff tension, and battens, see the addendum to this chapter.


Mainsail Controls - Conclusion


Our goal in mainsail trim is the extract the appropriate power from the main - in balance with the jib. We also seek the optimum mix of power from the three sources of sail power - angle of attack, shape, and twist. With impact on all three facets, the mainsheet is the primary control of overall mainsail power. Fig 15.

Fig. 15 - Changing conditons call for changing mainsail shapes. The mainsheet remains our primary sail control.

Our secondary controls influence the depth, angle of attack, and overall power of the main. As adjustments are made to any one control they impact the settings of other controls.

In Chapter 8 we will look in detail at the balance between the sails and the balance among the various sources of sail power.

SIDEBAR - Vang Sheeting

Vang Sheeting is an alternative mainsail trimming method.

Conventional Trim

In conventional trim the mainsheet controls the angle of attack, and it controls twist. That is, the mainsheet pulls the boom in and down. As you near full trim the emphasis is on the down component - adjusting twist. The traveler takes over the in and out component - moving the boom to center line once the desired twist is achieved, or playing the main in and out in puffs. This technique does not rely on the boom vang as an upwind trim control.

Vang Sheeting

In Vang Sheeting the boom vang takes over the mainsheet's up / down control over the boom, and the mainsheet handles the in / out trim which is the traveler's domain in our conventional upwind arrangement. This technique does not require a traveler.

Vang Sheeting requires a very powerful boom vang, capable of handling the entire leech load. Vang Sheeting is particularly popular on two person dinghies and other boats without back stays. When there is no back stay the vang controls mast bend as well as twist. (The vang pulls the boom down and thrusts it forward. The forward force of the boom bends the mast.)

We all Vang Sheet

On reaches we all vang sheet. On boats using conventional trim methods upwind the vang takes over control of twist once the sheet is eased, and the sheet moves the boom in and out. Vang Sheeting simply uses this trim method for upwind trim as well.

6.5 Reaching and Running Trim


Upwind trim demands a balance of speed, power, and pointing. Reaching trim is simplified by eliminating concerns over pointing. Reaching trim calls for more power. Ease the outhaul and ease the back stay to add power. Don't get carried away easing the outhaul. Don't sacrifice area as you add shape.

Reduce luff tension for the lighter apparent winds and straighter mast relative to upwind sailing. Set the boom vang to control the leech of the sail. Keep the top batten parallel to the boom and try to keep flow off the leech telltales.

Most importantly, ease the sheet. Ease until the sail luffs, then trim to stop the luff; ease and trim. An overtrimmed main is slow.

In heavy air reaching dump the vang to spill the leech when overpowered, and be prepared to dump the sheet to prevent a broach. If the boom is close to the water ease the vang and keep it from hitting water. When the boom hits the water it can't be eased properly.


On a run don't forget the main. While everyone fusses over the spinnaker the main is often neglected. Ease until the sail luffs or until it rests against the rig. There is no harm to the sail or rig - let it out. Since there is no flow on a run don't worry about the rig interfering with shape. Ease the back stay to straighten the mast for a powerful sail shape. Ease the outhaul to add shape without sacrificing area Set the vang to keep the top batten parallel to the boom. Ease the vang if the batten hooks in, tighten when the batten spills out. Fig. 16.

Fig. 16 - Reaching and Running
Don't neglect your main downwind. Let it out. Ease it 'til it luffs, and trim. On a run ease the sail out against the rig.
Outhaul: Eased for power.
Luff Tension: Eased - draft at 50%.
Traveler: Down.
Sheet Trim: Ease to luff, then trim.
Vang: Top batten parallel to boom, try to keep flow.


6.6 Conclusion

The mainsail is critical to the trim and balance of the boat. Use your mainsail to keep a balanced helm. When there is excess weather helm and heel de-power the main. When the helm is mushy and the boat lifeless power-up. The helmsman and mainsail trimmer must work together to optimize performance. The helmsman is at the mercy of the mainsail trimmer.

The next chapter covers helming. After that we'll look at trim solutions integrating the main, jib, and driver.

Addendum: Fully Battened Mains

Fully Battened Mains are Good

Full battens improve both the performance and the racing life of main sails. Full battens change some of our control and trim techniques, and they generally make mainsail trim easier.

Full battens eliminate some of our control over draft position. The battens' curve determines the draft position, and luff tension has little impact. At the same time, the battens prevent the draft from moving out of position, so our need for control is diminished. We can still pull the draft forward with extra luff tension, but the propensity for the draft to move aft with mast bend is reduced.

Tapered upper battens give us a sail with self correcting draft position. As the sail loads up and the draft starts to drift aft the battens soft forward section bends to hold the draft forward.

No More Poke!

The nagging problem in mainsail design and trim has been batten poke - the sharp crease and kink which develops at the inboard end of conventional battens - particularly the top batten. With full battens batten poke disappears. Kinked upper leeches are replaced by smooth even shapes. Fig. 17 - next page.

Full battens also provide a more stable platform for main sails and reduce wear from luffing and flogging. Since fully battened sails do not flog the life of a racing main will be extended enough to cover the cost of a proper set of tapered full length battens.

Fig. 17 - Full battened mains hold their own shape better than traditional mains, they are easier to trim, and they eliminate batten poke. Not only that, but they overcome the age old problem of batten poke, which gave mains a kink at the inboard end of the upper battens. And not only that, they last longer too!


One or Two?

Not all your battens need be full length. Depending on the size and roach of your main only the top one or two battens need be full length. While you're at it, make sure they are tapered. It is worth the additional cost to get tapered battens.

Not Quite Full Length Option

An alternative to full length battens is longer but not quite full length battens. These avoid the compression and loading along the luff which full length battens create. These are an alternative to consider, particularly on boats which luff slugs or slides, although they do not provide all the benefits of true full length battens.

Keep It Loose

Some sailors complain of difficulty flattening their fully battened mains. This is most often due to putting the batten in too tight. The full length batten should be inserted with no compression. Better the batten be too loose than too tight.

Another cause of trimming difficulty is soft battens. Get stiff, tapered battens.


If rule makers are preventing the use of fully battened mains in your area or fleet, join the lobby in favor of full battens. They are better. Catamarans, dinghies, and windsurfers have known it for years.


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