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Go Back   SailNet Community > Out There > Racing > backstay adjuster
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Topic Review (Newest First)
01-25-2003 01:08 PM
geohan
backstay adjuster

Jeff: Thank you very much for your comprehensive reply relative to mast bending for draft control. I see the bendy rig in a new light now and really appreciate your time and effort. Thanks again, George
01-25-2003 08:58 AM
Jeff_H
backstay adjuster

I am not sure that I understand where you are going with this. I have sailed on traditional boats that had the sails laced on and have had brails rigged to be able to reef the sail to the mast rather than the boom. The problem with both is that your are dealing with pulling in a lot of line in order to alter the shape of the sail. Even the equivilent of a flattening reef could easily be yards of line that needs to be overhauled between the tension that you would want on a higher wind beat vs the shape you would want cracked off in the same conditions. With the high frictions involved, hauling that much line in and distributing the loads sufficiently evenly between the area of the differing loads on each cringle that result from the different tributary areas of the sail, becomes a very difficult if not imposible problem.

Beyond this you would be gather a turbulence producing bundle of sail right at the leading edge where clean air flow is most critical.

While there is a tendancy to think of a mast as a simple compression member, the reality is that masts have almost always been bending members as well. The ability and propensity of a wooden mast to bend has been an safety component of traditional boat design for centuries. Mast flexibility as a safety valve was well understood on an intuitive basis as early as the mid-19th century and is reflected in sharpie an skiff rigs by the early 1800''s.

Herreshoff''s understanding of role of mast bend was so acute that S-Boat designed in the first decade of the 20th century had a mast that was actually precurved for that purpose.
The first academic study of the role of mast bend on sailing that I know of occurs in Manfred Curry''s treatice on the Aerodynamics of sailing in the 1930''s.

In other words, mast bend has played a key role in sailing rig design for a very long time. It is only in the late 1950''s, that you find rig design changing to masts so stiff that fore and aft flexing is reduced to an absolute minimum. This is partially in response to the artificial shift to masthead rigs that resulted from boats trying to beat the CCA racing rule. Masthead rigs require much higher rig loads and so also develop much higher compression loads on the mast. Buckling in aluminum spars became a very real concern.

With the change in the racing rules, there has been a shift back to the greater efficiency of a fractional rig and to ''bendy'' rigs that can allow rapid, subtly adjustable, on-the-fly shifts in sail power. There is now quite a long track record on these bendy rigs as they first began to become popular in the mid-1970''s. While the more extreme cases have not proven to be extremely long lived, the more conservative bendy rigs, such as the J36 which also incorporated raked back spreaders and lowers to minimize pumping,have stood up very well indeed. Using an example of a boat that I know particularly well, the Farr 11.6 (Farr 38) The Farr 11.6 was designed around 1981 and has a very early very bendy rig. It depends on swept back spreaders and lowers to prevent over bending and pumping. Farr 11.6''s began racing in the South Atlantic out of South Africa in the early 1980''s. To this day they still race as a semi-one design class in the Capetown/Rio (comes in somewhere else these days) and some of these boats have been doing this race almost every running. This is a race known for its heavy air conditions. In talking to a fellow who was active in this venue, some of these early boats are just now replacing thier spars after 20 years. Given the very hard useage, pretty primitive design and workmanship on some of these spars, and the move to penalty chutes that these spars were never intended to live with, that is not unreasonable. When we had my spar down last year, (Mine has had 20 years of hard use) we found no signs of fatigue and franlkly the spar looks very good. Which is a very long way of saying that I really don''t think that mast bend offers any real problem in terms of spar durability or safety as long as the spar design is not taken to an extreme.

In other words, you are not behind the time, but are operating on a theory that was only popular for a very short period in the history of sailing rigs.

Respectfully,
Jeff

01-25-2003 07:27 AM
geohan
backstay adjuster

Jeff: Please suffer one more tug on this thread. Was your experience with lace lines that of attatching the main to the mast or were the lines solely on the sail itself? Can a STIFF wooden mast be bent safely and sufficiently to really flatten and depower the main sail? What would you think of a second luff rope (wire) sewn to the sail on a curve matching the desired mast bend (like an adjustable leach line)? Maybe I''m just behind the times but I do like to keep compression members in-column if I can. Thanks for your patience, George
01-24-2003 06:51 PM
Jeff_H
backstay adjuster

I have used lace lines on small traditional boats. The problem, even on a small boat is that you really cannot precisely control shape as the stretch in the line, friction at the grommets and inability to evenly distribute the tension results in an irregular distribution of the flattening. There were some sails made with a zipper foot, or luff back in the 1970''s but this was not all that effective and was a bear to engage in the kinds of big gusts that flattening is best at dealing with. Mastbend with a backstay adjuster is such an easy and incrimental adjustment that controls both the mainsail and jib at the same time, and is reasonably inexpensive to build, that I have a hard time visualizing the advantage of going to a lace or zipper luff.

Respectfully,
Jeff
01-24-2003 03:55 PM
geohan
backstay adjuster

As an alternative to trying to bend a stiff mast for mainsail draft control, have any of you seen or used a system having nearly parallel lines of gromets just abaft the luff with a lacing line to gather in the excess draft much like a draft control zipper along the foot? I''ve read about it somewhere and have wondered how well it might work or if it is even racing legal. It seems like it could allow for a lot of draft control.
01-21-2003 07:03 PM
dchew
backstay adjuster

I second JeffH''s recommendation for a cascading backstay. It provides more power than the pinching system. The Merit 25s in the SF area came with the pinching system and we''ve all moved over to a 48:1 cascade.

Also, consider how much you need to bend your mast. I read in Conner''s book that the typical cruiser will bend their mast 1% of the total length. A racer will push it to 1.5%.

If you''re NOT bending your mast that much, you''re throwing away a lot of potential draft range.

Also, when it comes time to order a new main, find a sailmaker that will measure you mast bend at off, half, and full on backstay tension. It takes about 2 hours to do this, but your mainsail will play like a Steinway piano.

Douglas Chew
s/v Challenger
Berkeley, CA

09-12-2002 04:40 PM
geohan
backstay adjuster

I guess that I would have to admit that a lever type backstay adjuster ins''t for everyone. I wouldn''t want to pay shop rates to have one built. I do insist however that a handy do-it-yourselfer could fabricate such a device that is capable, convenient, aesthetic and economical. Capability is foremost and you can get that for $15. Convenience and aesthetics are extra, a lot extra.
Cheers, George
09-12-2002 08:44 AM
JeffH
backstay adjuster

The real problem with hydraulics on a small boat, besides maintenance, is that they are hard to put on and off quickly. With a block and tackle system it is very quick to make repetitive adjustements.

Jeff
09-11-2002 02:55 PM
chris1f
backstay adjuster

Our local rigger also recommended a hydralic system, but at $1000 bucks installed, it seemed too expensive. I''m sure you''re right about the money, but I was trying to do this with about $350 or less.

Thanks,

Chris Foley
Cal 28-2
Morning Star
09-11-2002 01:16 PM
smoke
backstay adjuster

Are you sure you don''t want to try one of these? I''ve tried all the other versions you''re considering. Believe me, the extra cost for a good Sailtec or Navtec is worth it. Looks neater, works better.

http://www.pyacht.net/cgi-local/SoftCart.exe/cgi-bin/pagegen.pl?U+scstore+xmts1056ff3ba63b+pr+sailt10SI .html
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