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Downwind Sail Trim 101 PDF Print E-mail
Written by Mike Johnson   
Friday, 01 June 2007 00:00

Note: This article appears in the Summer 2007 edition of The Laser Sailor

It is my opinion that sailing downwind fast is mostly a function of finesse (anticipation, body mechanics, steering, wave placement, S turning, …) combined with the proper setup of the rig and sail trim to provide maximum power. This essay will attempt to lay some groundwork for proper rig setup and sail trim for maximum power downwind. It is by no means authoritative and I believe there are many debatable topics in this area.

This issue will focus only on conditions where surfing or planning is possible (>8 knots of wind and >1 ft sea). Many of the concepts discussed also apply in light winds, however going through all the eceptions would dilute the central theme. Please keep this caveat in mind.

Twist is the curve the leech takes when the sail is under load. Twist is sometimes measured by a comparison in the angle of the top batten relative to the boom. For example, zero twist is when the top batten is parallel to the boom. Twist can also be measured by comparing the relative angles of each batten to one another. When going downwind the twist is mostly controlled with the boom vang tension.

Active Leech
As the sail loads up due to a puff or added resistance of stuffing the bow into a wave, the upper roach will fall off to leeward creating twist. When the sail unloads after the puff, the roach returns to the neutral position. When the leech is said to be “active” it will easily oscillate about the neutral position without any  help from body kinetics. Michael Blackburn uses these words to describe the concept: “The leech should move 2 to 3 foot of its own accord”. A good comparison is a spring that moves back and forth after a force is applied. Andy Vance says the leech should be “springy”. This natural action can be easily mistaken by a judge as “leech flick” if the motion appears to be rhythmically timed with body movement or mainsheet pumps. Watch this video clip to see Robert Sheidt demonstrating this concept (fast fwd to 2:15).

Velocity Made Good (VMG)
When sailing a course other than straight downwind your boat speed vector can be broken into two components: 1) dead downwind; and 2) perpendicular to dead downwind. VMG is the magnitude of component
(1). If you luff to a beam reach, your VMG drops to zero. This concept is important when considering
whether to momentarily sacrifice VMG to go after a wave that may result in a ride that provides double
the VMG if you just remained on the downwind course.

Sail Trim
Remember the caveat that this guide only applies when surfing conditions exist. At the windward mark
it is most common to set the cunningham, outhaul, and vang for the conditions. Once on the run it is
uncommon to adjust the trim of any control other than the mainsheet and the tiller. The only exception to
this is after a significant change to the wind (velocity or direction). Given this assumption, it is critical to set the cunningham, outhaul, and vang properly to get the maximum thrust from the sail. Here is a discussion of each setting along with my rules of thumb.

Cunningham Pretty simple. All the way off in all but very high wind. End of story.

Outhaul The outhaul and vang settings share a symbiotic relationship. If the vang setting is left alone
as you ease the outhaul, the foot moves toward the mast and the leech tension will decrease and create
twist. To achieve an active leech, it is critical to match the outhaul setting with a correct vang setting.

At the 2000 Cancun Master Worlds I arrived two days early and watched the finals of the Senior Worlds.
Scheidt, Ainsle, Blackburn, Suneson, and many other  downwind maestros were 4 months away from the
Sydney Olympics and we had 12 to 18 knots with 2 to 3 ft seas in water that looked like a very clean swimming pool. It was downwind heaven. I walked around the beach one morning and noticed that almost every boat had a similar outhaul rig. This was in the days when the outhaul was cleated on the boom and the purchase was achieved by wrapping the tail around the mast and back to several thimbles. I noticed that when  every outhaul was eased all the way to the knot (handle) the clew grommet was always about 9 to 10 inches from the fairlead on the boom. Watching the Gold fleet round the weather mark, every boat eased the outhaul so the knot went all the way to the thimble and thus the 9 to 10” setting. After seeing this I conclude the optimum setting for almost all times when surfing and planning conditions exist is having the clew grommet 9 to 10” from the fairlead. When in Rome...

Boom Vang
Similar to the symbiotic relationship described above, the vang and the mainsheet share a symbiotic relationship. Given a standard outhaul setting as stated above, the optimum vang setting to achieve the active leech concept will vary slightly depending on the wind and sea state. It has been said that the top and middle battens should be kept parallel to one another and also kept 90 degrees to the boats centerline. Not enough vang tension will create a situation where the top batten is slightly more twisted than the middle batten. Too much vang tension will create a situation where the leech will not react to a load / unload and therefore will be “inactive”. The optimum vang setting to achieve an active leech is somewhere between these extremes  and necessitates finding the optimum amount of twist.

My theory is that between 8 to 20 knots of breeze, one vang setting can do the job. This “one size fits all” setting is probably not what the pro sailors would subscribe to, however I have found it to be just fine for providing an active leech through this wind range. This theory assumes that the main sheet performs the fine tune and the vang the coarse tune, thus the symbiosis between these two settings. For those that do not subscribe to this theory, I’d like to hear yours.

I recommend placing a stopper knot in the vang tail so at the windward mark all that is required is to un-cleat it and let it run to the stop knot. In the upper wind ranges it will be necessary to pull the line in a few inches to limit twist. To determine where to put the stopper knot here is a procedure I use.
--Attach the vang to the mast and boom. Put the boom in the gooseneck and lift the end of the boom until the vang purchases goes tight.
--Put the sail on the mast.
--Grab the clew in one hand and the end of the boom in the other.
--Pull down and aft on the sails leech (without bending the mast) and lift up on the boom (without bending the boom).
--The vang stopper knot should be adjusted so the clew grommet is about 4-5 inches above the top of the boom.
--When you get a new sail (not yet stretched) it may be necessary to adjust the stopper knot if you have previously been using a relatively blown out sail. Steps 1 to 6 should get you in the ball park and may require some fine tuning to achieve the ultimate “active leech”.

This procedure sets the position of the stopper knot so that when fully eased the vang is in its loosest setting.

As stated earlier, the main sheet trim shares a symbiotic relationship with the vang setting. While the vang is set to achieve an active leech, the main sheet is used to keep the top two battens perpendicular to the boat centerline. As wind speed increases the average load on the sail increases, and therefore the twist increases. This requires that the boom always be trimmed at an angle less than 90 degrees to the centerline.

To keep things simple and allow the majority of focus to be on the finesse aspects of downwind sailing, I placed a mark on my main sheet that is 25 ft-6 in. from the end that ties to the becket. When this mark aligns with the forward boom block I am at the optimum trim for sailing dead downwind in the 8 to 20 knot wind range. This is only true given the previous discussion of the outhaul and vang settings and the symbiotic relationships. While this is another “one size fits all” theory, it is not something that I think should be rigidly adhered to. Since the main sheet is in your hand, the mark is helpful to keep the sail at near its optimum trim and not be grossly out of trim. When sailing by the lee the mark is eased 8 to 12 inches and similarly when heating up it is tighter by 8 to 12 inches. The mark may need to be moved slightly to match your style; however I suggest the 25’6” as a starting point.

My hope is that this guide will stimulate some experimentation and fine tuning of the theories made. There are many theories on what is fast and what is not. One irrefutable fact is that time in the boat (TIB) will do the most to improve your downwind performance. Getting the rig tune and sail trim create the framework that allow the TIB to be most effective. Good sailing.

Last Updated on Friday, 16 July 2010 14:51
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