Re: Shrouds & Bendy Rigs
Fri, 21 May 1999 00:48:34 EDT

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West Wight Potter Website at URL
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In a message dated 5/20/99 6:07:08 PM Pacific Daylight Time,

> OK, Judy, now that my opinions are out, it's time to learn something.
> I've never had the opportunity to experiment with a "bendy" rig, so
> I'm curious about the nature and magnitude of the effect. Your talk of
> 1"-2" changes implies a powerful cause/effect relationship.
> In the absence of an adjustable backstay, changes in sail flatness and
> draft position (to accommodate wind ferocity) are made by adjusting
> halyard, outhaul, and sheet tension -- and maybe the vang if one sails
> sans traveler. My question is: does creating a concavity in the luff
> by bending the masthead sternward offer an additional capability or is
> it just an alternate, easier way to adapt to increasing winds? More
> simply: are the results _significantly_ different/better or just more
> easily achieved?
> Regards,
> Bill Combs
> WWP 19 #439 (Aug 1987)
> "Ursa Minor"
> Fort Walton Beach FL

Hi Bill et al,

I'll take a shot at answering your question in more detail.

I'll try not to go into toooooo much detail. and bore you to tears.

Here's an advance tip before you wade into my sometimes dense writing. Pay
close attention to what part of the sail is being depowered by the trimming
technique. Different trimming techniques affect different parts of the sail,
but they all do the same thing, power up or power down one particular section
of the sail. The key is what part of the sail are you trimming.


PS. I tend to be completely blind to my typos and errors right after I
write something, so please forgive my obvious errors and confusing parts.
Cut and paste sometimes screw things up royally. It's the best I can do on
short notice.

Introduction and executive summary :)

Initially flattens *middle* third of sail and depowers the draft. Further
tightening loosens and depowers the *upper leach*.

Flattens and depowers the *foot* of the sail

Flattens the *middle* third of sail and depowers the draft. *Powers up the
upper third* at the leach by reducing twist off in gust, too much vang makes
boat very twitchy, causes increased heeling especially in gust.

Cunninghma (halyard, boom downhaul). Tightens luff of sail and moves draft
forward. Offsets tendency of draft to move aft in strong wind. Keeps COE
forward and *reduces weather helm*. Plays a very minor role in flattening
the draft

This is geared towards the P19 mostly.

If your shrouds are properly tuned, tightening the backstay doesn't bend the
tip of your mast aft half -- it primarily bends the middle of the mast
forward. Tightening it doesn't create a concavity along the leach of the
sail-- it primarily pulls the middle of the sail (where the draft is deepest)
forward and flattens the draft.

The outhaul flattens only the bottom 20-30% of the sail. The foot of the
sail doesn't give you more than a small fraction of your power. You can
significantly depower the sail by tightening your outhaul, but not alot
compared to flattening the draft. The draft is where the sail generates most
of its power.

In order to flatten the middle third of the sail where the draft is, you have
to bend the mast in the middle forward somehow. It's quite startling how
much flatter the draft gets when you bend the middle of the mast forward an
inch or two. It reduces the depth of the draft on my P19 sail by almost 50%
and depowers it significantly. By bending the mast, you depower the most
powerful part of your sail, right at the draft.

When you first start to tighten the backstay, you primarily flatten the draft
by moving the middle of the mast forward. If your shrouds are properly
tuned, the lower shroud limits any more forward bending of the middle of the
mast after about two inches.
If you continue to tighten the backstay just a little more, then the mast
tip gets pulled down and back a little. Just an inch inch of backward
movement of the tip loosens the tension on the very top part of the leach,
and it gets a little floppy. When a gust hits, it twists off and spills
wind. Your boat is more stable in the gusts. You have enormously increased
the comfortable and safe wind range of your sail, because now you've
depowered the very top of it, the part that makes you heel so much.

In the absence of a back stay, the only way to bend the middle of the mast
forward while our sailing and thereby flatten the draft is by using the vang.
Tightening the vang pulls down on the leach, and the tight leech bends the
mast. It pushes the middle of the mast forward, flattening the draft in the
mainsail. The vang's good for flattening the draft and depowering the middle
third of the sail, but there's a trade off. You've actually made the sail
less forgiving in the gusts by tightening the leach.

With a vang, the top of the leach is even tighter than before, and it can't
twist off and spill wind in a gust. The center of effort in the main is
effectively higher than before. The wind powers up the tip of the sail with
every gust, and that's a long lever arm with which push the boat into a hard
heel. Your boat is a lot twitchier in the gusts, and less forgiving of
helmsmen's error. Heeling over like that digs the chines in and increases
the weather helm, especially on a hard chined boat like a Potter or a Force 5
(my dinghy). Now you're fighting to keep her nose from rounding up into the
wind with every strong gust.

On a lot of boats with a vang and no backstay (like many catamarans, the
Laser and the Force 5, and windsurfing sails too) the top section of the mast
is tapered and much more flexible than the rest of the mast. That's to
restore the ability of the top of the sail to twist off laterally and spill
wind when a gust hits it. Even still, there's definitely a point of
diminishing returns with a vang unless you can hike way out.

Tightening the main halyard, as you suggest, moves the draft forward. So
does moving the boom down with the downhaul. That's a good thing thing to do
when the wind velocity increases. There's a tendency for the draft in the
sail to move aft as the wind builds, which gives you increased weather helm.
When it's really windy, you want to be sure your halyard is nice and tight so
your draft doesn't move aft.

But it's hard to adjust the halyard (or downhaul) once you're already rigged
and the wind starts to build. That's why most boats with a fixed gooseneck
have a cunnigham. The cunningham is a line that goes through a cringle in
the sail a few inches above the gooseneck. When you tighten the cunningham,
it has the same effect as tightening the halyard or the downhaul for the
boom. When it's really blowing, you want to tighten your cunningham.
it does flatten the draft just a tiny bit when you tighten the boom, main
halyard or cunningham.

The sheets don't do much to the sailshape. Mostly they just change the angle
of attack. The one exception is when you lock the traveller car in place on
the track (if there is a track) and two-block the main sheet (tighten it
maximally so the blocks on the boom are kissing the blocks on the boat).
Then you're pulling down on the boom, much like a vang does. This bends the
mast forward in the middle, depowering the draft, but tightening up the top
of the sail so it can't twist off in the gusts. Sometimes it's advantageous
to do that, but usually it's better to use a backstay or a vang if you have


Everything I wrote above applies equally to apparent wind, not just absolute
wind. After all, we all sail on apparent wind not absolute wind. High wind
trimming works well for pointing in moderate winds, low wind trimming works
well for downwind reaches in moderate winds.


Whew! That's a lot to try to explain. I hope I did a decent job. You can
read a better, more detailed explanation in any good sailing text. I was
mainly trying to sort out the different used for most common sail trim
techniques, what they do and why you might use them.