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-   -   Mainsail Twist for Waves (http://www.sailnet.com/forums/seamanship-articles/19284-mainsail-twist-waves.html)

Dobbs Davis 03-23-2001 07:00 PM

Mainsail Twist for Waves
 
<HTML><HTML><P><TABLE cellSpacing=0 cellPadding=0 align=right border=0><TBODY><TR><TD width=8></TD><TD vAlign=top align=left width=309><IMG height=247 src="http://www.sailnet.com/images/content/authors/davis/032401_dd_wcooper.jpg" width=309><BR><DIV class=captionheader align=left><FONT color=#000000><B>Moderate winds and uniform waves require&nbsp;matching sail trim.&nbsp;</B></FONT></DIV></TD></TR><TR><TD colSpan=2 height=8></TD></TR></TBODY></TABLE>The 60th edition of the Acura SORC in Miami Beach this winter produced a fantastic variety of sailing conditions to challenge the 138 entries in the event. Starting with postponements and light air on the first day, the southerly breeze built steadily throughout the event so that on the fourth day the wind speeds approached and exceeded 20 knots. This kind of conditions offers a great opportunity for learning lessons, and that’s just what happened here. <P>For everyone but the multihull competitors, the course areas were set east of Key Biscayne, just a couple miles offshore. For the duration of the event, the southerly breeze was slightly more to the west than the 'normal' southeasterlies, making for an interesting scenario in wind and wave sailing. Though the wind showed a tendency to bend around the southern end of Key Biscayne, the wave pattern was more uniformly from the southeast, and often at a significantly different angle than the wind. This made for a completely different kind of sailing on each tack upwind. On starboard, boats were taking the waves and chop head-on, while on port, the waves were hitting the boats on the beam for a slightly smoother ride. To optimize speed, balance, and pointing ability, this meant adjusting the sail trim in a different fashion on each tack—twisted but powerful on starboard, and a little flatter and less twisted on port.<BR><BR>For those of us sailing on the offshore one-design course (including Mumm 30s, 1D35s, and Farr 40s), the first challenge in these circumstances was to decide first how to set up the rig. Without the dynamic rig adjustment of running backstays or checkstays, the middle shape of the mainsail is controlled by mast bend, which in turn is controlled by adjustment on the diagonal shrouds. Since these could not be adjusted except between races, the question became whether to try and optimize the tuning for the deeper and more powerful shapes necessary on starboard tack, or the flatter shapes that were better for port.</P><P><TABLE cellSpacing=0 cellPadding=0 align=right border=0><TBODY><TR><TD width=8></TD><TD vAlign=top align=left width=309><IMG height=247 src="http://www.sailnet.com/images/content/authors/davis/032401_dd_wcooper2.jpg" width=309><BR><DIV class=captionheader align=left><FONT color=#000000><B>On starboard tack, the waves were pretty square, requiring more power in the sail trim.&nbsp;</B></FONT></DIV></TD></TR><TR><TD colSpan=2 height=8></TD></TR></TBODY></TABLE>On these two-spreader, runnerless rig designs, easing the diagonal shrouds (lowers are referred to as D1s and uppers as D2s) allows the mast to bend more and flatten the main. However, if made too loose, overbend in the mast section will compromise headstay tension, which is critical for keeping the jib shape from becoming too deep in breezy conditions. The challenge is always to decide how much bend might be too much, and only through trial and error will the exact adjustment be determined. The difference between a light air and heavy air settings might only be several turns on the diagonals' turnbuckles. Because of this, using the sailmakers' tuning guides in each of these classes is an excellent way to understand how and when to make these adjustments and match the mast bend and stiffness with the sails' designs.<BR><BR>On John Musa's 1D35 <I>Jacaibon, </I>we opted for having a little more bend and flatter shapes appropriate for port tack, reasoning that if necessary we could manipulate other controls such as the outhaul to deepen the main on starboard. Also, as the day wore on we saw a tendency for the wind to increase, and we wanted to make sure we would therefore be fast on the last beat of the race as well as the first. <P><TABLE cellSpacing=0 cellPadding=0 align=right border=0><TBODY><TR><TD width=8></TD><TD vAlign=top align=left width=289><IMG height=230 src="http://www.sailnet.com/images/content/authors/davis/032401_dd_rolex.jpg" width=289><BR><DIV class=captionheader align=left><FONT color=#000000></FONT><STRONG>Steep chop like that seen here can severely challenge a helmsperson, but trimmers can offer some assistance by powering up the sails.</STRONG></DIV></TD></TR><TR><TD colSpan=2 height=8></TD></TR></TBODY></TABLE>Fortunately, the other controls we had on sail trim were much more accessible than adjusting the diagonals. Deeper and more powerful shapes on starboard tack prompted us to use slightly less headstay on the 1D35, and even a little less outhaul on the main and less halyard tension on the jib. After tacking to port, however, we'd pull more outhaul on, pump on more headstay, grind on more halyard tension, and tighten the cunningham to help flatten the shapes for smoother water. These we'd keep fairly constant while on each tack, unless there was a significant change in wind speed.<BR><BR>The most dynamically adjusted aspect of sail shaping that we tried to manipulate was the twist in the leech. On starboard tack, the impact of taking the waves head-on made the boat pitch more readily than on port tack. This meant the angle of attack of the wind on the sails would vary widely, especially at the top of the spar. So, in order to reduce the risk of having the sails over-trimmed and therefore stalling the flow of air, we would put more twist in the main and maybe a little more as well on the jib. This meant easing the jibsheet about an inch or two from where it was on port tack, and a similar and sometimes greater amount of ease on the mainsheet. While a little pointing was sacrificed, the twisted shape made the boat perform consistently faster through the waves, with the balance on the helm and heel angle controlled through adjusting the traveler. <P><TABLE cellSpacing=0 cellPadding=0 align=right border=0><TBODY><TR><TD width=8></TD><TD vAlign=top align=left width=289><IMG height=185 src="http://www.sailnet.com/images/content/authors/davis/032401_dd_rolex2.jpg" width=289><BR><DIV class=captionheader align=left><FONT color=#000000><B>Adjusting sail trim to meet conditions can mean the difference between the unobstructed view up front&nbsp;and the view in the back of the pack.</B></FONT></DIV></TD></TR><TR><TD colSpan=2 height=8></TD></TR></TBODY></TABLE>On port tack, the flatter water meant we needed less powerful shapes in the sails to accelerate, and the mast was not pitching through huge angles of gyradius. More headstay, halyard, outhaul, and cunningham tensions were applied, and if we had a backstay, we'd have pulled that on harder as well. Here also the challenge was not to keep the side-on waves from pushing the bow too far off to leeward, so a little less twist in the main kept the boat pointed consistently higher. This meant carrying a few inches more mainsheet tension than on starboard tack, although never was the main trimmed so hard as to have the top batten pointing to weather of centerline. This trim setup allowed us to maintain height as well as speed, with the traveler eased in the puffs and trimmed in the lulls.<BR><BR>After all this work getting upwind, going downwind was a blast, especially on port gybe where at relatively close true-wind angles we were all easily surfing down the waves. Fortunately both the finishes and the harbor were downwind, so it made for great endings to some long days!<BR><BR>If you encounter similar conditions when you race, keep in mind this simple lesson from the 2001 Acura SORC—when sailing upwind, the bigger the waves (and the more head-on they are), the more the twist. <HR align=center width="75%"><P clear=all><P><STRONG>Suggested Reading: </STRONG><P><STRONG><STRONG><A class=articlelink href="http://www.sailnet.com/forums/showthread.php?threadid=20663">Shifting Gears Upwind</A></STRONG>&nbsp;</STRONG>by Rich Bowen <P><STRONG><STRONG><A class=articlelink href="http://www.sailnet.com/forums/showthread.php?threadid=20696">Basic Roll Tacking</A></STRONG>&nbsp;</STRONG>by PJ Schaffer <P><STRONG><STRONG><A class=articlelink href="http://www.sailnet.com/collections/Racing/index.cfm?articleID=ddcksn0325&amp;coll_cat=Teamwo rk&amp;Coll_name=Team%20Work%20and%20Technique"htt p://www.sailnet.com/forums/showthread.php?threadid=20702"http://www.sailnet.com/store/buying_guide.cfm?guide_id=1000">Headsail Sheet Lead Systems</A></STRONG> <P><P><TABLE cellSpacing=0 cellPadding=0 align=center border=0><TBODY><TR><TD height=8></TD></TR><TR><TD vAlign=center><A href="http://www.sailnet.com/store/item.cfm?pid=22508 "><IMG height=100 src="http://www.sailnet.com/images/content/authors/davis/032501_addd_line.gif" width=320 border=0></A></TD></TR></TBODY></TABLE><P></P></HTML></HTML>


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