I'm having a S-bend in my mainsail when I pull the mainsheet hard enough, so there is no twist in the mainsail. the S-bend goes from the bottom of the sail up and over the second batten. I d' like to get rid of the S-bend as I 'm loosing allot of power this way.

would stiffer battens help? the battens I use are from fiberfoam (Scott Anderson)

I'm having a older sail with the same battens, which dosn't show this effect at all.
What could be the reasons for the S-bend?

thanks for any help and tips!

Andreas,
What kind of boat? I have seen this problem frequently on the Nacra 6.0 NA and it is usually mis-diagnosed as a jib slot problem. It actually had to do with (could be controlled by) adjustment of the main clew traveller and having stiff enough battens. It was actually caused by heavy leach pressure on the sail levering the middle of the sail to windward using as a fulcrum the line from the mainsheet/mainsail attachment point to the upper mast. The 3 main factors that contributed to this behavior were:

1. a boomless rig
2. a very roachy mainsail
3. a soft (very bendy) upper mast (above the hounds and diamonds)

Perhaps it has an outhaul that is not adjusted?

It's a Ventilo HT18 Uni rigged

The S-bend stays the same when I ajust the outhaul. When I release the mainsheet a bit, the S-bend disappears. But then I have to much twist.

Andreas,

I think that you are saying that the "S shaped" battens are "S shaped" when viewed from above, or below; from a "plan view."

I have diagnosed this problem, three different times. In all cases the cause was attributed to batten cap ties that were too tight. Often, you may think that there is no batten tension when you hold the batten, but the batten may be bent while you are holding it.

I suggest that you untie the battens and leave them untied while you perform a test hoist, and test trim.

If you are in a place where you can capsize your cat with the main trimmed to maximum tightness, you can then tie the battens snug while the sail is sheeted tight. This will ensure that you are not over tightening your batten tension settings.

Please let us know what finally solves your problem. Also, if your description of "S-bend" is different than mine, please tell me your description.

GARY

Andreas,

Would you perhaps have a picture you could share? It would help both to be able to see the whole sail plan in profile, and perhaps the S-Bend.

I will make a picture! the S-bend of the sail shape can be best seen from above or from down low. the leech bends to leeward while the front section of the sail looks normal.

Hi Gary

the "S shaped" battens are "S shaped" when viewed from above, or below.

I will try to untie the battens and perform a test hoist, when the cat is laying on the side.
I was also thinking about stiffer battens. But this would hurt the light wind performance.

Hello,
here is a theory for you: the load from the mainsheet is traveling in a direct line from the boom bail to the sail head. The roach of the sail is falling to leeward.
something is mismatched. mast stiffness or bend vs. sailcut is an obvious place to look. battens that are tubular in the back may help. fine tuning the various tensions involved may be all that is necessary.
have fun with it.

Hello Andreas,
It sounds like you have a sailmaking 101 problem. The luff curve made on your sail is more shallow than the luff curve your mast bends to in the upper part of your mast. When and where this happens,like at the first or second horrizontal batten down from the head for example, the sail goes flat here. The mast bend tries to pull the sail to a shape that is greater in width, luff to leech, than the sail is actually made to. Now the mainsheet load that was traveling up the leech of the sail migrates forward prematurely to the luff of the sail across this flat and tight sail area. This flat and tight area of the sail, one or two battens down from the top of the sail, takes the mainsheet load and delivers it to the mast one or two battens down from the top of the sail. Now the leech sail area above this point has very little tension in it and it falls off to leeward. If you take line of max leech tension from the clew to the second batten down from the top, for example, this line acts like a hinge. The sail area aft of this line falls off to leeward because the upper end of the leech at the square top is now falling off and it takes the leech with it. This will put an "S" shape into the sail. To correct this problem, your sail needs more luff round especially near the top. I would suggest you move the headboard aft about 2cm and refair the luff curve back into the original luff curve at the forward end of the gaff batten. This will put more luff round in the sail near the top of the luff curve and stop the sail from going flat prematurely at the second or third batten down from the top. Now the mainsheet/leech load will stay in the aft leech area of the sail until this load reaches the square top area of the sail where it will finally come forward and pull aft and down on the top of the mast. The leech will stand and the "S" shape in the sail will be gone. The hinge line has been eliminated.
Good luck,
Bill

Hello Bill

Thanks allot for responding! I understand your theory and it sure makes sense to me. But why is the S-Bend only at the 2 lowest battens? Should not all the battens bend, until to the top?

Hi Andreas,
Draw a picture of your sail plan from the side view, luff,curved leech, square top head and foot. Now draw a dotted line form the forward end of say the second horrizontal batten down from the top, the very flat area of the sail, to the clew corner. This dotted line is the hinge axis. Aft of this line the sail falls off and in cross section the sail forms the "S" shape. The battens higher in the sail will have a greater percentage of their length aft of this dotted line, the hinge axis, and the fall off will be more pronounced here. The bottom batten or two down close to the foot of the sail will show very little fall off or "S" shape because such a small percentage of the batten length is aft of the hinge line. Does your sail have tapered battens in it? Are they very stiff in the aft 1/3rd of their length and soft in the front 1/3rd? Or are your battens of constant cross section and bendy everywhere?
Constant cross section battens, untapered pultruded fiberglass battens for example, are too bendy in the leech area of the sail and often proper control of the leech is lost. Sometimes the leech can fall off, other times it can hook back to windward.
Good luck,
Bill

Hi Bill

thanks again for taking your time and tips!
They way you describe it, would be logic, but it is just the opposite, because all the upper battens look fine. They have the right profile and only the lower two have this s-bend. It gets worse from above to downward.
I can pull the outhaul through and the s-bend persists.
the battens come from Scott Anderson ("Fibre Foam" Battens) in Austria and are tapered and straight. the stiffer end of the batten is at the leech. Maybe the battens are two soft? These are the same battens they use in tornado, so they should be ok!?

good sailing!

I assume you are the only one with that boat/sail there?

If you loosen the two lower battens (a lot), and then apply mainsheet+downhaul, what happens?

Sailing without any twist at all is usually slow in my experience. But your main should not make an S-twist at the lower two battens anyway. Is the sail specially designed to tweak the class rules, or is it a well known design in the class? What Bill Roberts already has written, is usually the reason for S-bend. Stiffer battens might help, but might ruin the shape otherwise.

Is your mast set up as per the class 'norm' with pre-bend (diamond tension) and spreader rake ?

It would be easier to come up with teories if you could supply pictures of the sail taken from the side and from below (while sailing or while sheeted in+downhaul as you do to recreate the problem)

Here is the picture of the sail with the s-bend.
Pic of sail with s-bend 300kB

first I have to say that the sail is specially designed for light winds. it has not allot of luff curve and allot of shape build in. It really seemed to me, that the luff curve didn't match the bending characteristics of the mast.

We experimented with the prebend of the mast. we started with 7 cm prebend after the recommandations of the sail maker. the more prebend, the more the mast will bend at the spreaders.

the more we tightened our diamonds the worse the s-bend.
we finally found the right setting, with only 3-4 cm prebend, witch is not much for a mast with 10m lenght.

the hinge, bill was talking about, was there, but not to the top of the sail, it wend from the clew to the spreaders.
that is wy the upper part of the sail looked good.

thank you all, for the usefull tip's! It really helped us thinking!

Pic of sail without s-bend 300kB

Hi Andreas,
Obvisously the sail luff curve is too shallow for the way your mast bends down low. The mast bend down low can be reduced by moving the mainsheet attachment point to the boom or clew of the sail forward, toward the mast. In doing this, a small component of mainsheet tension will travel down the boom and pull aft on the mast at the gooseneck and reduce the bend in the lower part of the mast. This will also reduce mast rotation and a mast rotation forcing system will probably be required.
The solution to your problem is a new luff curve made onto your sail. To guide your sailmaker, measure the bend characteristic of your mast. Tie the halyard to the end of the boom and sheet the mainsheet in firmly. Turn the boat over on land and support the mast at the hounds with a short ladder. Pull a tight string from the top of the mast to the gooseneck. Measure the distance from the string to the sailtrack every 50cm along the mast, top to gooseneck. This will define the correct natural bend characteristic of your mast. This curve may be too much or too little luff round for the desired overall sail camber according to your sailmaker. The sailmaker can adjust these offset measurements by multiplying them all by the same constant. For example: If the offset numbers are too large, too much mast bend, then multiply all of them by 0.8 or 0.9, whatever constant number is required to acheive the desired sail camber with this mast bend characteristic. The mast will always bend in a characteristic shape proportional to the data you took.
Bill

Don't do this.

It will result in one or more of the following

1, Minimum the halyard will just streach and not really bend the mast
2, The halyard will streach a little and then break, possibly causing your mainsheet blocks to impact each other and (poss) crack, and or boom shattering after the shock of the release.

Do this.

Shackle one trap wire to the mainsheet via the boom and sheet in having set the mast rotation at the required level.

Andreas: I expect your performance just improved a notch :-)

Dear Bill

Thanks a lot for studying my pictures!



You are right! I have to move the attachmentpoint of the mainsheet. I had a boombreakage this summer and the position of the boom attachmentpoint is not at the same place as before. I moved it a little back because, I was looking for more mast rotation, witch was successfully achieved.

the interactions between the sails and mast/rigging are complex!
thanks again for your help!

Hi scooby

thanks for that warning! I think your are right about my block on the top of the mast .

I think the best way to measure the mast bending, is using the sail without inserting it into the leech but attaching the head of the sail at the top of the mast as normal. then connect the cunningham and the mainsheet with the free hanging sail and pull them both tight.
To measure, use a small chord tied to the top and to the gooseneck, and go on as bill described before.

thanks all for sharing your knowledge!