Increasing backstay tension flattens main?

Aug 2, 2010
502
J-Boat J/88 Cobourg
I crewed on a boat last night with a rather baggy main and thought about this notion I have read that increasing backstay tension will flatten the main...which this sail needed! Thinking about the geometry, I cannot see how bending the mast will flatten the sail, my imagination says the opposite will happen. Can someone help me understand?
My boat has no easy adjustment for the split backstay so this is mostly theoretical, and very interesting for me!
Thanks,

Dan
 
Feb 20, 2011
7,990
Island Packet 35 Tucson, AZ/San Carlos, MX
I crewed on a boat last night with a rather baggy main and thought about this notion I have read that increasing backstay tension will flatten the main...which this sail needed! Thinking about the geometry, I cannot see how bending the mast will flatten the sail, my imagination says the opposite will happen. Can someone help me understand?
My boat has no easy adjustment for the split backstay so this is mostly theoretical, and very interesting for me!
Thanks,

Dan
Bending the mast takes some or all of the "belly" out of the mainsail, depending on how much belly it has.
Last night's mainsail could have been past its prime...
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
You need to think of the mainsail in three sections.

The outhaul can flatten the bottom 1/3, by pulling it tight.

On a fractional boat, putting on backstay can bend the mast, pulling the middle 1/3 of the sail flatter.

The top 1/3 will not change shape much, but that's normal and OK, it twists off.
 
Nov 6, 2006
9,884
Hunter 34 Mandeville Louisiana
but, like JackDaw says.. fractional rig is what that works on.
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
but, like JackDaw says.. fractional rig is what that works on.
Performance oriented masthead boats (like C&C) can also achieve this mast bend with a babystay. Lots of these boats have babystays with 8:1 purchase in place of turnbuckles to bend the mast.
 
Aug 2, 2010
502
J-Boat J/88 Cobourg
You need to think of the mainsail in three sections.

The outhaul can flatten the bottom 1/3, by pulling it tight.

On a fractional boat, putting on backstay can bend the mast, pulling the middle 1/3 of the sail flatter.

The top 1/3 will not change shape much, but that's normal and OK, it twists off.
OK, so the bend in the middle of the mast stretches the middle of the sail toward the bow and thus flattening the sail. Now my geometric mind sees it.
So this was a reasonable looking sail and I had the telltales flying in light breezes but I couldn't get rid of the luff in the first quarter of the main. Increasing back stay tension would have helped, though I do understand this older sail may have been past its prime too.

Thanks you Guys! I am going to start racing my own boat next week so please answer the coming questions as I work through sailing better!
Dan
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
OK, so the bend in the middle of the mast stretches the middle of the sail toward the bow and thus flattening the sail. Now my geometric mind sees it.
So this was a reasonable looking sail and I had the telltales flying in light breezes but I couldn't get rid of the luff in the first quarter of the main. Increasing back stay tension would have helped, though I do understand this older sail may have been past its prime too.
Yep.
2370e.jpg
 
Jul 7, 2004
8,402
Hunter 30T Cheney, KS
I think there's a chance of over tightening the backstay and causing problems too.

Some masts have prebend built in. Are sails made specifically for these? I don't see how this technique would work for them.
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
I think there's a chance of over tightening the backstay and causing problems too.

Some masts have prebend built in. Are sails made specifically for these? I don't see how this technique would work for them.
The sail should be designed for the static pre-bend, but adding more bend can still be done dynamically to flatten when conditions warrant.
 
May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
Dan, the leading edge of a mainsail is cut in an arc. then when the arc shape is attached to a straight mast your sail gets its fullness,

to flaten the fullness in higher winds you arc your mast to the shape of the cut in the leading edge to flatten your sail.

there are five ways to arc a mast: 1. pull down hard on the mainsheet, lasers and finn sailors do this
2. the use of a very powerfull vang
3. force the lower section of the mast forward

4. fractional rigs tighten the back stay against the tension of the forstay

5. use swept back spreaders and increase the tension there
 
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Jan 1, 2006
7,039
Slickcraft 26 Sailfish
Over tightening the backstay can bend the boat. Beyond the point where the boat bends there is no more positive effect. I used to have a Mark 25 which is a C&C 25 at heart with a tall spindly fractionally rigged mast with jumper struts. The power to add and subtract power so easily with a adjustment of the backstay was amazing. When I originally got that boat I worried about a rig failure. I learned that the rig was fine but the hull would bend from the rig. Overall, it was a good sailing boat if you didn't mind the oil canning.
 
May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
on my star and on my A scow the base of the mast at the bottom of the hull is movable to control rake / weather helm and some arc maybe.
 
May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
Dan, on a masthead rig, tightening the backstay is done to straighten the leading edge of the jib
 
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May 17, 2004
2,099
Other Catalina 30 Tucson, AZ
jackdaw's picture tells the story. Suppose you want to describe it to your dock neighbor or a person at the YC bar and you don't have jackdaw's picture with you. Here's how you do it -- use your index finger to show a straight mast. Now bend it and tell your friend to notice how the middle of your finger (knuckle area) pulls the fullness out of the middle of the sail. If your friend has a "telephone pole" (non bendy mast) tell him to forget the demo because the mast won't bend -- it will rake and the boat will think he picked up the mast and moved it aft.

Then your friend might ask to illustrate TWIST. Form your right hand as if you were going to salute. Next, twist your only fingers to the right and explain you're reducing pressure on the boom vang. That illustrates how air is spilled out of the top 1/3 of the sail. Spilling air reduces the power of the sail. Then explain your cranking on the boom vang as you turn your fingers into a salute to illustrate you're now inducing power back into the sail so that is drawing from top to bottom. .
 
Jan 2, 2015
35
Beneteau 340 Oceania ventura
the rigging on my Bene 34 was quite loose. if you shook shrouds the rig really bounced around forestay was quite loose too. We tightened the all shrouds and back stays. Now I'm thinking that mast is too straight as the in mast furling is jamming like ever before at the vert. batons. Should have consulted Bene before adjusting. But I think we took to much of the prebend out
 
Dec 29, 2008
805
Treworgy 65' LOA Custom Steel Pilothouse Staysail Ketch St. Croix, Virgin Islands
Now I'm thinking that mast is too straight as the in mast furling is jamming like ever before at the vert. batons.
There was another thread last week or so that gave me the opposite impression. The discussion was about in-mast furling jamming, and what I took away from it was that the mast might have been bent, causing the furling to bind. The impression I came away with was that bending the mast was, at least to some degree, incompatible with in mast furling. I have a friend whose Jeanneau 37 was having problems furling and unfurling is old and new sails in-mast, and I was planning to site up his mast to see if there was any bend in it. I'd welcome clarification from the experts on here.
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
There was another thread last week or so that gave me the opposite impression. The discussion was about in-mast furling jamming, and what I took away from it was that the mast might have been bent, causing the furling to bind. The impression I came away with was that bending the mast was, at least to some degree, incompatible with in mast furling. I have a friend whose Jeanneau 37 was having problems furling and unfurling is old and new sails in-mast, and I was planning to site up his mast to see if there was any bend in it. I'd welcome clarification from the experts on here.
The manual for our US Spar furling mast says "The mast must be as straight as possible (only 3" of pre-bend)". Sighting up ours I'd guess it's about 5" and we've never had any trouble.
 
Oct 29, 2016
1,915
Hunter 41 DS Port Huron
This is an excerpt from the Selden B&R rig tuning manual
The amount of a pre-bend depends on the cut of the
mainsail. A general rule is that the pre-bend should be
1% of the distance from the masthead to the gooseneck,
measured at the centre of this distance. The mast must
have no lateral bend. All pre-bend must be in the fore-
and-aft direction
 
Jul 12, 2011
1,165
Leopard 40 Jupiter, Florida
Off topic -- I love reading these sail trim questions with you guys! Thanks to each of the contributors for explaining this in simple, understandable terms. Even though I understand this, it is a pleasure to read each of your explanations.