Flattening The Main Sail

Aug 4, 2009
204
Oday 25 Olympia
Just wondering about how to quickly and easily, really flatten the main on a non-bending mast to de-power. Would a wire sewn into the sail in an appropriate curve near the luff from headboard to cunningham cringle do the job? Has anybody seen or used such a contraption? I'm thinking of the safety factor aspects.
Gudenuph
 
Feb 17, 2006
5,130
Lancer 27PS MCB Camp Pendleton KF6BL
I moved this over to Sail Trim with Don Guillette. This subject I think would get wider response than just being in the Day Sailers forum. I think that Don could really give some very good advice on how to flatten your main.

However, there are other tools such as the boom vang that was mentioned, outhual, topping lift, cunningham that you mentioned, mail halyard, and don't forget the main sheet. Lots of ways to de-power the main sail.
 
Nov 8, 2010
11,385
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
A cunningham and a flattening reef point on your main will get you there.

The flattening reef (a cringle about a foot up and in from the clew) will really help flatten the body of sail.

The cunningham will pull the draft forward.

Flattening reef.
 
Dec 1, 1999
2,391
Hunter 28.5 Chesapeake Bay
While all the tools and techniques mentioned other posters are good advice, it can be just about impossible to flatten a mainsail that has been blown out of shape.
 
Mar 26, 2011
2,965
Corsair F-24 MK I Deale, MD
Interesting. I think I can add a few clarifications.

Vang. Not if the sheet is already tight. We are also assuming the mast will not bend. Thus, the vang only pulls the clew down, like the sheet. What a vang does do is reduce twist on boats with short travelers.

Flattening reef. Only effective if the sail was cut with broad seam near the foot that provides draft. Many sails are flat in the lower sections and out haul is all they need, or can use. Sail-specific. Also a racer thing, as often the sails cannot be stretched past the band.

Cunningham. Mostly for attached-foot sails; loose footed sails can often simply be down hauled with less distortion. Also a racer thing, if the sail is already max dimension. However, a cunningham can help if the track is tight and the halyard tension is not distributed all the way to the foot. Otherwise, just crank halyard tension.

----

Depowering is a whole nuther discussion.

We skipped rotating masts with prebend; as they rotate the draft in the middle is affected. Many catamarans. Reducing rotation flattens the sail, over rotation makes a bag.

Batten tension. If full length, ease the tension.

And as was stated, sometimes sails are blown out. Recutting can be fun.
 
Nov 8, 2010
11,385
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
Interesting. I think I can add a few clarifications.

Vang. Not if the sheet is already tight. We are also assuming the mast will not bend. Thus, the vang only pulls the clew down, like the sheet. What a vang does do is reduce twist on boats with short travelers.

Flattening reef. Only effective if the sail was cut with broad seam near the foot that provides draft. Many sails are flat in the lower sections and out haul is all they need, or can use. Sail-specific. Also a racer thing, as often the sails cannot be stretched past the band.

Cunningham. Mostly for attached-foot sails; loose footed sails can often simply be down hauled with less distortion. Also a racer thing, if the sail is already max dimension. However, a cunningham can help if the track is tight and the halyard tension is not distributed all the way to the foot. Otherwise, just crank halyard tension.

----

Depowering is a whole nuther discussion.

We skipped rotating masts with prebend; as they rotate the draft in the middle is affected. Many catamarans. Reducing rotation flattens the sail, over rotation makes a bag.

Batten tension. If full length, ease the tension.

And as was stated, sometimes sails are blown out. Recutting can be fun.
Rotating masts?? Pre-bend?? Batten tension?? The guys got an o'day 25!! ;^)

If his sail has a flattening reef, he should try and use it. And, most people find a hand-load cunningham MUCH easier to adjust while sailing then playing with their main halyard on a winch.
 

Joe

.
Jun 1, 2004
7,499
Catalina 27 Mission Bay, San Diego
On my loose foot main.. the out haul is main control for reducing draft depth. The vang... not so much. A Cunningham will re position the draft location more forward... thus creating a flatter surface in the rear half of the sail....which is the part of the main that generates most power.

However, as Phil noted, no amount of trim adjustment is going to work with a blown out sail. A recut may be in order if you can't get the outhaul and Cunningham to do their jobs.

Rotating masts and batten end adjusters are common on multihulls....... never seen a rotator on a boat with a backstay....
 

Joe

.
Jun 1, 2004
7,499
Catalina 27 Mission Bay, San Diego
Interesting. I think I can add a few clarifications.


Cunningham. Mostly for attached-foot sails; loose footed sails can often simply be down hauled with less distortion. Also a racer thing, if the sail is already max dimension. However, a cunningham can help if the track is tight and the halyard tension is not distributed all the way to the foot. Otherwise, just crank halyard tension.
Say what???? On my Nacra 5.2.... and all beach cats for that matter. There is no backstay... the primary method of bending the mast to flatten main is a powerful Cunningham. Most all beach cats under 22 ft.. will have a static halyard... the sail is hoisted and clipped... luff tension is controlled with a high purchase Cunningham. The old hobies were attached foot.... but none of the serious racing boats were rigged such.. they are all loose foot... because batten tension is more effective with a detached foot and batten tension is an important pre set trim tool, along with adjustable shroud tensioners, for these giant roach sails. Many designs don't even have a boom... and thus no outhaul. There is a "clew board" with multiple attachment points for mainsheet tackle... the angle of this attachment will affect foot tension in combination with the lowest batten.

A mast rotator does not affect draft depth or position...it improves sail's luff entry by aligning the elliptical mast with the angle of attack.
 
Last edited:
May 17, 2004
2,037
Other Catalina 30 Tucson, AZ
Joe from San Diego has pretty much explained the solution. The two main elements a mate has to contend with is draft depth and twist. Draft position is 3rd and the last element of the 4 elements is angle of attack, which is not a factor. The primary sail trim control for adjusting twist is the boom vang and the secondary is the mainsheet. You can't entirely eliminate twist because the sail maker built twist into the sail. The reason he built twist into the sail is that the wind blows 60% higher at the top of a 45' mast than at deck level and without built in twist you couldn't control the sail. The primary sail trim control for adjusting draft depth is the outhaul and secondarily is mast bend -- if you have a bendy mast. When you're messing with the boom vang, mainsheet, outhaul and mast bend you're also dealing with adjusting draft position.

As the wind pipes up it pushes the DP past 50% and powers up the sail so you want to bring the DP forward to around 45%. In addition to the boom vang, mainsheet, outhaul and mast bend (if you have a bendy mast) you also want to to use the cunningham or halyard.

So the bottom line is when flattening the mainsail you're using every mainsail sail trim control except the traveler.

When flattening the mainsail, don't forget the jib, especially if you have a masthead rig. With a masthead rig you can flatten the mainsail all you want but the powered up jib will keep you going at about 90MPH. You're fairleads are your primary sail trim control.

Brian: thank you for moving the topic. Finding interesting topics for the forum is not easy. I "trawl" the other forums looking for topics but sometimes I arrive too late and the topic has taken off and the answers/suggestions are sometimes completely off the wall. I think the main contributors on the sail trim forum are the most knowledgeable sailors I've ever run into and they are happy to share their knowledge. I know of no other forum like the sail trim forum and we all should thank sailboatowners.com (Phil/Bly) for providing it.