written by Stuart Walker on September 23 of 2011 and read by 1318
In several races recently I've suddenly recognized that my speed or height was deficient and been initially unsure what to do and in what sequence to do it. I decided to create a plan of action that would insure a more rapid response in the future and thought my scheme might be of general interest.
I. Basic Concepts -
A. Whenever you find that you are not going as fast or as high as your neighbor , you should assume that the problem is with you, that you are slow or low, because the only things you can control are in your boat. (And the problem usually is with you.).
B. The explanation for the deficiency is (almost) always defective sail trim - which, as Marchaj points out, is almost never correct.
C. Because only five elements of trim account for 95% of all slowness and/or lack of height, a quick response requires the consideration of only five elements (listed in the order of their importance):
<ol><li> Jib Sheet Tension (jib leech tension) </li><li>Jibstay Tension (jibstay sag) </li><li>Mainsheet Tension (main leech tension) </li><li> Backstay Tension (fore/aft mast bend)</li><li> Lower Shroud Tension (lateral mast sag)</li></ol>
(Unless severely maladjusted - and usually obviously so - all other controls (upper shroud tension, traveler position, outhaul tension, luff tension, even rake) have but a slight effect on speed or height, although after the major problem has been recognized, they may need additional adjustment.).
II. Correction of Problems
A. In light to moderate air, you are losing height to (not holding as high as) other boats on the same tack.
Response: Tighten leeches, decrease twist and make the sails fuller. Remember that fuller sails are higher, but slower. (In light to moderate air speed and height are independent and in opposition. Tensioning each of the five major controls to enhance height diminishes speed (and vice versa). Therefore, tensioning or easing beyond the optimal is counter-productive . Height is usually more important, but a compromise - an optimal tension between too much, which improves height, and too little, which improves speed - must be sought.)Modify each of the five elements separately and evaluate the effect. If there is no beneficial effect, discontinue the modification. If there is benefit, continue the modification until you are higher or faster or both.
<ol><li>Tension the jib sheet, decrease the twist and close the jib leech. </li><li>Ease the jibstay to make the jib fuller and move the center of maximum draft forward. </li><li>Tension the main sheet, decrease the twist and close the main leech. </li><li>Ease the backstay to make the main fuller. </li><li>Ease the lower shrouds to make the main fuller and move the center of maximum draft forward. </li></ol>
B. In light to moderate air, you are slower than other boats on the same tack.
Response: Open leeches, increase twist, make the sails flatter. Remember that fuller sails are higher, but slower. Modify each of the five elements separately and evaluate the effect. If there is no beneficial effect, discontinue the modification. If there is benefit, continue the modification until you are higher or faster or both.
<ol><li>Ease the jibsheet, increase the twist and open the leech. </li><li>Tension the jibstay (or, if necessary, the backstay) to make the jib flatter and and move its center of maximum draft aft. </li><li>Ease the mainsheet, increase the twist and open the main leech. </li><li>Tension the lower shrouds to make the main flatter and move its center of maximum draft aft. </li></ol>
C. In heavy air or gusts, you are slow, making excessive leeway, losing height relative to other boats on the same tack and heeling excessively. (It is not possible to distinguish deficiencies of speed and of height in heavy air - both have the same origin and both require almost the same treatment, speed. An increase in speed reduces leeway and corrects a deficiency in height.)
Open leeches, increase twist and make the helm so balanced that in gusts the boat drives forward rather than heels, and ceases to yaw to windward. Remember: Flatter sails are faster and cause less side-force, leeway, heeling, yawing and stalling. Modify each of the five elements separately and evaluate the effect. If there is no beneficial effect, discontinue the modification. If there is benefit, continue the modification until you are higher or faster or both.
<ol><li>Ease the mainsheet, increase the twist and open the main leech. </li><li>Tension the jibstay (or, if necessary, the backstay) to make the jib flatter. Tension the jib luff so as to overcome the flattening of the forward sections by the tighter jibstay and move the center of maximum draft forward. </li><li>Ease the jib sheet, increase the twist and open the jib leech. Let the jib traveler down. </li><li>Tension the lower shrouds so that the mast tip falls slightly to leeward in gusts and the main becomes flatter. </li><li>Tension the luffs of both main and jib as you tension the jibstay and/or the backstay so as to move the center of maximum draft of both sails forward. The farther forward is the draft, the less is the leeway and the greater the height, whereas the farther aft is the draft, the greater is the leeway and (in the absence of heeling), the greater is the speed. Adjust these controls no more than 'necessary' and be prepared to undue them as soon as the heavy air or gust has diminished. Ease sheets, open leeches and move the center of maximum draft forward in gusts and tighten sheets, close leeches and move the center of maximum draft aft in lulls. </li></ol>
Appendix I. - Basic Conditions of Sail Trim
Obviously, if you have your controls set up properly in the first place (as indicated by recorded experience), you are much less likely to need emergency adjustments.
A. A few basic requirements!
A reasonably stiff mast that bends smoothly Rake within the range of 29' to 31' (when the mast is set up at the rake angle, which causes the mast base (when cut at the proper angle) to sit flat on the deck (or on its shim), a condition that results in the divorce of the jibstay from the backstay and vice versa (at least to some degree). Reasonably good sails. Set up the backstay to provide a minimum of 1'- 2' of fore and aft mast bend (to accommodate the luff curve of the mainsail) in the lightest air and never use less backstay. Set up the jib using the clew hole and the jibstay tension that in moderate air permits the draft of the jib to be more or less evenly distributed vertically (slightly flatter at the bottom than the top) and 50% aft of the luff horizontally with the tack about 3' off the deck. Ease the jibstay, the sheet and the tack downhaul as the wind diminishes and tension them as the wind increases. A hull - its age, stiffness, weight, weight distribution, keel shape, fairness, smoothness make little difference to performance (except in heavy weather when light weight and stiffness are beneficial). A properly trimmed old boat will beat an improperly trimmed new one, every time.
B. The Function of the Five Major Controls
<ol><li>The Jib Sheet - controls the jib leech - and therefore both the shape of the leeward (functioning) surface of the jib and the manner in which the flow off its leeward surface flows onto the leeward (bulging) surface of the mainsail - the most important determinant of aerodynamic force. Critically important . Must be accurately adjusted to the particular jib and the particular mainsail and altered for variations in wind velocity, gusts, waves and steering. </li><li>The Jibstay - determines the sag in the jibstay and therefore the shape of the leading edge of the jib, the fullness of the jib and the distribution of its draft. It initiates attached flow onto the jib's leeward surface and onto the main. Critically important. Must be accurately adjusted for variations in wind velocity and waves. </li><li>The Mainsheet - controls the main leech - and therefore the twist, fullness and distribution of the draft of the main. It is the chief determinant of the shape of its leeward surface (on which the jib is directing its flow) and of the side force created by the air flow (heeling, leeway, and yawing moment - balance). </li><li>The Backstay - determines the fore and aft bend of the mast and thus the flatness of the mainsail and the distribution of its draft, the twist of the mainsail and the tension in the jibstay (despite the resistance of the mast base pressing down on the deck). </li><li>The Lower Shroud Tension - determines the lateral sag, straightness or tip fall-off of the mast and the sag of the leading edge of the mainsail (therefore comparable to the jibstay and its control of the sag of the leading edge of the jib), the fullness of the forward portion of the mainsail and the distribution of its draft. </li></ol>
Appendix II - Preventing Problems
A. Establish a means of measuring trim changes so that fast settings can be recognized and reproduced. Keep a trim chart indicating optimal settings found for the major trim variables in varying wind strengths.
B. Estimate the wind velocity by crew position and record crew position along with wind velocity on your trim chart. (It doesn't matter whether you are correct in assuming that 'two crew sitting in the windward side of the cockpit' equates with 6 knots; what matters is that you know what the optimal trim for 'two crew sitting on the windward side of the cockpit' is, and can reproduce it.)
C. When attempting to improve trim, change one variable at a time and make the changes in small increments so that you can detect what change made the difference and what amount of change was required.
D. Seek an optimal balance between too little and too much; defective trim develops in both directions. Tightening the leech may improve height, but excessive tightening will cause slowing.
E. Before a start set the boat up in accordance with your trim chart - but on the 'light side' of the wind velocity you estimate to be present:
<ol><li>We almost always overestimate the wind velocity because we base our perception on the gusts.</li><li>And, when a performance deficiency is detected, it is essential to know in which direction the trim should be modified - more tension or less?. If you know the settings err on the light side, you can modify for heavier immediately - without trial and error. </li></ol>
F. Each crew member should be responsible to 'play' one important trim element - to adjust for minor variations in wind velocity, waves, temporary need for height, etc. - and leave everything else alone until a need for a major change becomes evident and is ordered. The crew member should recognize the need to make only small changes and to undo whatever has been done as soon as the need for the change disappears.
One of the most common sins is tensioning a sheet - or the jibstay or backstay - because of an increase in wind velocity - and then forgetting that it has been tensioned (leaving it excessively tight) when the wind diminishes.
On my boat I play the jibstay (at 24:1), my middle man plays the mainsheet and the foredeck man plays the jibsheet.