Twist: Twist is one of the most important elements of sail trim on an EC-12. It's 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 any 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, including the use of Larry's computer generated center of effort scale, by adjusting the rake. (See Chapter 12 for information on fine tuning balance.)
Adjustments Not Radio Controlled: 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 span-wise distribution of angle of attack. In smooth water and
moderate even winds without wind shear, there will be a minimum twist governed by:
Stuart 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 EC-12's we 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 setup. The top of the main may be completely stalled out. Here is an alternate method currently used by a few SMYC 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 you will have to pay an extra $5.00 with 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 shear). 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.
Our 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 when 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 many times national champion Kelly Martin, 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 Mikey's sail trim articles in The Manual For The EC-12M. You can also determine the jib's minimum twist setting by watching the wind rags. Many sailors luff slowly, adjusting the jib twist with the jib topping lift, (JTL) to get the top and bottom windward telltales to lift at the same time, a so called even break. It's often more accurate to slowly bear away while watching the leeward jib telltales. The minimum twist occurs when the top leeward wind rag stalls close to, but no sooner than the bottom one.
Last, in the upcoming weeks, Mesa MYC will be adding a couple more "points-of-interest" to it's Web page. We feel these added points will be on great additions of information for model yachters. Right now, they are on the drawing board and work will soon be started. We are looking forward to these additions and we hope you will too. Our goal to add more service to fellow model boaters, so check back is a week or two. We hope everything comes together quickly to deliver more model yachting information!
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 four to six mph Juanita Beach thermal but at the more
typical model yacht sailing sites with light and variable winds, 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:
Clearly, for a lot of our racing more than the minimum twist must be used, but where to start? Kelly Martin advises newcomers to adjust their 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 and 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 it's 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. Make sure you don't go under the minimum twist settings. You 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. It's better to check your tune against several boats and note their twist settings. Depending upon whom 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 Larry when he tunes against Jerry Brower. 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's center of effort (CE) forward to restore the correct amount of helm. It's helpful to know the rough magnitude of the required CE change. For US22 going from a high twist setup to a low one will require that the CE be moved forward about 0.2 to 0.3 inches. You can do this by moving the entire rig this amount or by shortening the jibstay by 81% of the amount. These small changes are quite easy with the custom masthead jibstay adjusters used by Jerry Brower and Bob Wells. (This fitting is shown in Figure 7.2 Chapter 8.) A newer option is the custom jib swivel adjuster now being evaluated by Kelly Martin. A diagram of this fitting is also included in Chapter 8. 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.