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-   -   Reading Flow and Making Adjustments (http://www.sailnet.com/forums/cruising-articles/20183-reading-flow-making-adjustments.html)

Tom Wood 09-23-1999 08:00 PM

Reading Flow and Making Adjustments
 
<HTML><!-- eWebEditPro 1.8.0.2 --><P>How to Get the Most Out of Your Headsail</P><P>Optimal sailing performance requires adjusting headsail sheet angles so that the jib, genoa or staysail forms an effective airfoil. Even minor misadjustment of the lead block or tension on sheet, halyard, leech or foot cords can cause a dramatic reduction in performance. Yet it is common for us to sail on boats that have inadequate means of monitoring sheet lead settings or without means to make the proper headsail adjustments. </P><P><B>Reading the flow</B><BR>Before you can adjust the sheet lead, you must be able to read airflow over the sail. Every headsail should have pairs of telltales port and starboard just inboard about 8 to 12 inches off the luff. There should be one pair near the head, another up from the tack that is visible from the helm, and a third pair near the middle of the luff. Large sails may use a fourth or fifth pair. In addition it's helpful to have three or four single telltales located along the leech of the sail.</P><P>Telltales can be bought ready-made (Davis no. 950 Air-Flow Tells -- seven pairs for $5). But our favorites are quarter-inch-wide strips of black or bright-colored three-quarter-ounce spinnaker cloth attached with one-inch squares of sticky-back Dacron. We have found that black actually shows up best, even at night. Most sailmakers have scrap supplies. Your sailmaker might convince you to install telltale windows along the luff of your sail so you can read the airflow on the leeward side as well.<BR></P><TABLE cellSpacing=0 cellPadding=0 width="100%" border=0><TBODY><TR><TD height=15><B>Reading telltales</B></SPAN></TD></TR><TR><TD height=247><IMG height=6 src="http://www.sailnet.com/images/content/authors/wood/bullet.gif" width=6> When all telltales on the leech and luff stream aft smoothly in the wind the sail<BR>is an efficient airfoil. <BR><BR><IMG height=6 src="http://www.sailnet.com/images/content/authors/wood/bullet.gif" width=6> If the luff telltales all flow in the wind but the leech telltales flutter, the sheet lead is too far inboard or the leech cord is overly tight. Sometimes when reaching in heavy air, the leech telltales will flutter in the puffs if the main is eased. This usually indicates that it's time to reef the main.<BR><BR><IMG height=6 src="http://www.sailnet.com/images/content/authors/wood/bullet.gif" width=6> If the windward luff telltales all flutter while the leeward telltales flow, the lead is correct but the sheet is eased too far and the sail needs to come in more. <BR><BR><IMG height=6 src="http://www.sailnet.com/images/content/authors/wood/bullet.gif" width=6> If all the leeward luff telltales flutter but the windward telltales all flow smoothly, the sheet lead is probably correct but the sail is trimmed in too tightly. Slowly let out some sheet and re-check the telltales.<BR><BR><IMG height=6 src="http://www.sailnet.com/images/content/authors/wood/bullet.gif" width=6> If the luff telltales near the top of the sail flutter while the ones near the tack stream correctly, the lead is too far aft.<BR><BR><IMG height=6 src="http://www.sailnet.com/images/content/authors/wood/bullet.gif" width=6> If the luff telltales near the tack flutter before the ones near the head of the sail, the lead is too far forward. The leech telltales will generally curl inboard in this case. </TD></TR></TBODY></TABLE><P><B>Adjusting the lead</B> <BR><TABLE width=131 align=right border=0><TBODY><TR><TD><IMG src="http://www.sailnet.com/images/content/authors/wood/092399sl1.jpg" align=left></TD></TR><TR><TD height=19><SPAN class=storeyellow>Typical single and double sheet lead cars on spring stop track sliders.</SPAN></TD></TR><TR><TD height=118><IMG height=109 src="http://www.sailnet.com/images/content/authors/wood/092399sl3.jpg" width=150></TD></TR><TR><TD height=8><SPAN class=storeyellow>A towed-car system: The blue line is the sheet, while the yellow/green line hauls the car forward and the red line hauls it aft.</SPAN></TD></TR><TR><TD><IMG height=120 src="http://www.sailnet.com/images/content/authors/wood/092399sl2.jpg" width=150></TD></TR><TR><TD height=2><SPAN class=storeyellow>A running-sheet system: The red line is the sheet while the yellow/green runner adjusts the lead forward and aft.</SPAN></TD></TR></TBODY></TABLE>The most common method of adjusting sheet angle is with a lead block sliding on a track bolted to the deck. Some boats use trunnion snap shackle blocks on a perforated toerail. Many racing boats have two or more sets of tracks so the athwartships angle can be adjusted as well as the fore-and-aft sheet angle. Headsail sheets can produce very high loads making it difficult to slide a track block. For this reason sliders with tackles hauling the lead blocks fore and aft have become popular on larger boats. <BR><P>An older method of changing the sheet lead moves the sail rather than a lead block. A tackle between the headsail tack and the deck fitting allows raising the entire sail up the headstay. Raising a sail has the same effect as moving a sheet lead forward; lowering a sail moves the lead aft. This method is still viable for boats with hank-on sails.</P><P>Another method of obtaining a full range of sheet angle to the headsail is the running sheet. Here two blocks are fixed to the deck, one at the farthest forward sheet lead position and one at the farthest aft. The sheet itself passes through the aft deck block to the clew of the sail. A second adjustment line passes through the forward deck block and terminates on a block "running" on the sheet. The bitter ends of both sheet and runner are in the cockpit. When the runner is fully released, the sheet is led as far aft as possible. When the runner is two-blocked, the lead is as far forward as possible. <P></P><TABLE height=203 width=161 align=left><TBODY><TR><TD><IMG height=178 src="http://www.sailnet.com/images/content/authors/wood/092399sl4.jpg" width=175 align=right></TD></TR><TR><TD><SPAN class=storeyellow>This big cruiser has the ultimate in headsail sheet-lead systems, with inboard track, outboard track with 4 to 1 towed car and perforated toerail to attach a Barber hauler.</SPAN></TD></TR></TBODY></TABLE><P>For sailboats having only a single fixed point for the sheet lead block, or where a track is not long enough to fit all the sails, the solution is a Barber hauler (named after its inventor Merrit Barber). This is a short line with a hook or snatch block on one end to deflect the sheet, much like the running sheet system described above. We carry one with a single block having a cam cleat built in and a snap shackle head that can be attached onto a stout handrail, chainplate or stanchion base. By deflecting the sheet inward, outward or forward, we can achieve a perfect headsail shape.</P><BR><FONT size=2><B>Photos by Kathy Barron</B></FONT> <P></P></HTML>


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