(Editor's Note: This article can be used with any sailboat.) Twist is one of the most important elements of sail trim on an EC12, but its hard to get it just right, in part because of the lack of easily observable clues to proper adjustment and also because changing twist can dramatically change the balance of the boat. In order to make my conclusions about the effectiveness of a change in twist you should restore the boat to its previous balance by moving the rig fore or aft or by adjusting the rake. (A two-part article on fine tuning balance including the use of a special center of effort scale will follow soon.)
Twist can be thought of as similar to washout on a wing, present to delay or prevent tip stall at high
angles of attack. Beyond this the analogy is limited. Another way to think of twist is to consider it
the spanwise distribution of angle of attack. In smooth water and moderate even winds without wind sheer,
there will be a minimum twist governed by:
1. The need to eliminate tip stall.
2. The apparent wind blowing more from abeam at the head of the sail.
3. Various concerns having to do with inducing elliptical spanwise load distribution.
4. The need to match the twist of both sails.
Steward Walker's MANUAL OF SAIL TRIM describes this minimum twist for the main as occurring when all horizontal
tangents to the leech are parallel with the leeway angle. On full sized boats this setting will give the
best top end speed in smooth conditions and clean air. For EC12s I think that the centerline of the boat
should be substituted for the leeway angle. Consider this as having the aft end of each main batten pointing
straight aft. Even in these no waves, moderate wind conditions you can get badly burned with this method
currently used by a few GSMYC skippers. You will need silk or inked clear Micafilm wind rags attached to
the ends of the top two mainsail battens. Besides their general usefulness that is described in most full
size sailing texts, we use them to help set the main twist.
Incidentally, you must adjust or at least confirm twist settings by sail testing. On the water there is a change in apparent wind angle with height while on shore there is no such change ( in the absence of wind sheer). To understand this, remember that there is a change in the wind velocity with height. When sailing, the wind at the foot of the sail comes from further ahead than at the masthead because of the wind generated by the boat itself. Imagine what would happen if there were no true wind at the foot (an exaggerated velocity gradient). In other words, if you set the vang just right on shore it may be too tight when sailing.
My (Larry Robinson's) current feeling is that the full on vang position is found by gradually tightening the vang until the top main telltale just stalls at the same time that the leeward jib telltales stall as you slowly bear away. This means that the jib is suffering from leading edge stall when the top of the main is completely stalled out. There are only a few special conditions when you would ever want to use less main twist like when you are trying to gas Kelly, and even then it probably won't work. After setting the vang, the jib twist can be set using the air particle of the leech method described in Mikeys sail trim articles in THE MANUAL FOR THE EC12M. You can also determine the jibs minimum twist setting by watching the wind rags. Many sailors luff slowly, adjusting the jib twist to get the top and bottom windward telltales to lift at the same time; a so-called even break. Often it is more accurate to slowly bear away while watching the leeward jib telltales. The minimum twist occurs when the top wind rag stalls close to, but no sooner than the bottom one.
So far we have talked about minimum twist settings, but how often can we set up this way? Not very often!
These minimum settings will work in a 4 to 6 mph Juanita thermal but at Surprise Lake and at Coulon Park you
may take a quick trip to the back of the fleet with this setup. Many of the conditions in which we sail require
higher twist. Some are:
1. Waves (heading varies so with higher twist at least some of the rig will be at the correct angle of attack)
2. Variable winds (at least some of the rig at proper angle of attack)
3. Need for acceleration (force vectors of twisted portion of sail directed more foreword)
4. Over sailed (a Fisherman's reef, but note the downwind rolling problem on an EC12)
5. Wind sheer (different twist requirements on each tack -- see if telltales behave differently on each tack to check for this)
Clearly for a lot of racing more than the minimum twist must be used, but where to start? A GSMYC spy claims to have overheard Kelly advising a newcomer to adjust his vang so that the aft end of the top batten makes about a 15-degree angle with respect to the boom. This setting will give good protection from wind variation, waves and will give good acceleration.
Nonetheless since the top of the main is in effect not fully sheeted you will be giving up a small amount of top end speed when conditions are appropriate. You may want to just live with this problem at times, knowing that you will have a speed margin when the wind gets fluky. On the other hand you simply cannot allow the whole fleet to sail over you coming off the starting line. Here is one solution for this problem. Assuming that all other adjustments on the boat are appropriate (!), begin sail testing with the main twist set to Kelly's recommendation or perhaps 5 degrees more if its really light and variable. If you are fast enough compared to boats that have been your equal then sail with the twist as set. If you are slow and want to risk getting stuck with too little twist if the conditions should change (very bad), slowly tighten the vang as you continue to sail test making sure you don't go under the minimum twist settings. Do this by watching the telltales for the clues described previously.
Of course the skipper you are tuning against may be doing the same thing, in which case you will both tune yourselves to the back of the fleet. Better to check your tune against several boats and note their twist settings. Depending upon who you tune against, you may find that your setting is quite close to the minimum even if you would like to have more twist at least that's what happens to me when I tune against Jerry B. Remember that as you decrease twist (vang on) weather helm will increase. To make a reasoned judgment about your speed you will have to move the rig center of effort (CE) forward to restore what was hopefully the correct amount of helm to begin with. It is helpful to know the rough magnitude of the required CE change; for US22 going from a high twist set up to a low one will require that the CE be moved forward about .2 to .3 inches. You can do this by moving the entire rig this amount of by shortening the jibstay by 81% of the amount. These small changes are quite easy with the RSMT masthead jibstay adjusters used by Jerry Brower and Bob Wells. A measured drawing of these fittings will be published in the GSMYC newsletter soon. In practice you can adjust the vang until you get it about right, then check and readjust the jib twist, while making sure you have what you think is the correct amount of helm. Adjust the helm by changing the CE if necessary.