I regularly read that bending the mast back will flatten the main sail but I have trouble visualising this. Seems to me that bending the mast back would give more body to the sail, allowing it to fill out more. Can somebody enlighten me on this. Thanks.
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Flattening the mainsail
When the mast is bent backwards the middle of the mast is pushed forward. When this happens the sail attached to the middle of the mast is drawn forward with the mast. This removes the draft(camber) in the sail thereby flattening it.
When the mast is bent backwards the middle of the mast is pushed forward. When this happens the sail attached to the middle of the mast is drawn forward with the mast. This removes the draft(camber) in the sail thereby flattening it.
There is fullness cut into the forward
edge or luff of the sail. When that is pulled tight against a straight mast the sail won't be flat. Bending the mast to the same curve as the cut of the sail will flaten the sail. Same thing Alan said just different words.
edge or luff of the sail. When that is pulled tight against a straight mast the sail won't be flat. Bending the mast to the same curve as the cut of the sail will flaten the sail. Same thing Alan said just different words.
alan and ross are both correct.
to flatten a main on a frac rig by tightening the back stay it moves the middle of the mast forward thereby flattening it. on a masthead rig if you have an adj. back stay it only will change the head stay tension. to flatten a main on these there are two things that can be done. one is to have a cunningham on the main an increase the halyard tension which pulls the draft forward and thereby flattening it the other is to have what is called a flattening reef point put in the main. this reef point say on a 30' boat is usually about 1 ft to 18" above the clew on the main. this usually is set at the same point as the cunningham eye. to flatten you pull in both tight. At the same time you also tighten your outhaul. hope this helps. s/v Que Pasa?
to flatten a main on a frac rig by tightening the back stay it moves the middle of the mast forward thereby flattening it. on a masthead rig if you have an adj. back stay it only will change the head stay tension. to flatten a main on these there are two things that can be done. one is to have a cunningham on the main an increase the halyard tension which pulls the draft forward and thereby flattening it the other is to have what is called a flattening reef point put in the main. this reef point say on a 30' boat is usually about 1 ft to 18" above the clew on the main. this usually is set at the same point as the cunningham eye. to flatten you pull in both tight. At the same time you also tighten your outhaul. hope this helps. s/v Que Pasa?
Don't try mast tuning with masthead rig
I fault a lot of the sources I've seen on sail tuning for not going into the differences between rigs before advising people on how to tune them. Fractional rigs are typically designed for racing and are meant to be tuned. Masthead rigs are typically designed for cruising and are not suited for tuning by the stays. You have a fractional rig if your forestay is attached on the front of the mast somewhere below the mast truck, say 80% of the way up the mast (hence the "fractional" designation); or if you have a babystay down lower in addition to your forestay. Masts on fractional rigs often have a fore-and-aft curve to give them strength in anticipation of being tuned; many have spreaders swept aft with no backstay. I think the special cut of the luff Ross was referring to would be found on sails made for fractional rigs or dinghys, but not on cruising sails for masthead rigs. Masthead rigs have forestays that attach at the top of the mast at the same height as the backstay. The mast is usually a straight-up stick and the spreaders are not swept back. Don't try any major fore-and-aft tuning on this type of rig; just keep the shrouds and stays at proper tension and concentrate on other sail trim techniques with vang, sheets, traveler, cunningham, etc.
I fault a lot of the sources I've seen on sail tuning for not going into the differences between rigs before advising people on how to tune them. Fractional rigs are typically designed for racing and are meant to be tuned. Masthead rigs are typically designed for cruising and are not suited for tuning by the stays. You have a fractional rig if your forestay is attached on the front of the mast somewhere below the mast truck, say 80% of the way up the mast (hence the "fractional" designation); or if you have a babystay down lower in addition to your forestay. Masts on fractional rigs often have a fore-and-aft curve to give them strength in anticipation of being tuned; many have spreaders swept aft with no backstay. I think the special cut of the luff Ross was referring to would be found on sails made for fractional rigs or dinghys, but not on cruising sails for masthead rigs. Masthead rigs have forestays that attach at the top of the mast at the same height as the backstay. The mast is usually a straight-up stick and the spreaders are not swept back. Don't try any major fore-and-aft tuning on this type of rig; just keep the shrouds and stays at proper tension and concentrate on other sail trim techniques with vang, sheets, traveler, cunningham, etc.
Masthead rig bend
Rich, Our IOR racer not only has a masthead rig, but also a baby stay and hydraulic backstay. We do set the baby stay and apply the pressure on the backstay to create a bend in the mast and flatten the main. Why don't I see baby stays on newer masthead rigs?
Rich, Our IOR racer not only has a masthead rig, but also a baby stay and hydraulic backstay. We do set the baby stay and apply the pressure on the backstay to create a bend in the mast and flatten the main. Why don't I see baby stays on newer masthead rigs?
May have been an IOR thing?
Rick, certainly the important thing is the adjuster and not the position of the forestay--if you have a factory-installed adjuster they clearly intended you to be able to tune the mast (and maybe the babystay is there to hold the proper tension at the base?) I haven't seen babystays on any boats beyond the late '80's, but it's hard to say whether that's a matter of fashion (maybe they were used to enable tuning on simpler aluminum spars and manufacturers didn't like that approach?!?) or whether that configuration doesn't work with the new rules systems (remember, IOR--International Ocean Race--rule has been obsolete for at least a dozen years, though I can't remember the acronyms of the 2 current competing systems).
Rick, certainly the important thing is the adjuster and not the position of the forestay--if you have a factory-installed adjuster they clearly intended you to be able to tune the mast (and maybe the babystay is there to hold the proper tension at the base?) I haven't seen babystays on any boats beyond the late '80's, but it's hard to say whether that's a matter of fashion (maybe they were used to enable tuning on simpler aluminum spars and manufacturers didn't like that approach?!?) or whether that configuration doesn't work with the new rules systems (remember, IOR--International Ocean Race--rule has been obsolete for at least a dozen years, though I can't remember the acronyms of the 2 current competing systems).
Masthead Rigs and bend
On masthead rigs there are several ways to induce mast bend. Double lowers, a babystay, pre-bend, aft-swept spreaders all all ways to bend the mast. If you do a Google search on "masthead rig mast bend" you will get several articles that explain about the need for mast bend on masthead rigs. It doesn't take much and the maximum bend on a simple (no running backs or check stays) masthead rig is nothing compared to a racing fractional rig, but the bend is there and it is needed. I get about 1 1/2" of bend out of the mast on my C30. Many keel stepped masts carry 2-3" of pre-bend and get another 2-3" of bend with full backstay. Since the masts in cruising rigs are not designed to bend very much, the amount of sail shape control is limited compared to a fractional rigged boat.
On masthead rigs there are several ways to induce mast bend. Double lowers, a babystay, pre-bend, aft-swept spreaders all all ways to bend the mast. If you do a Google search on "masthead rig mast bend" you will get several articles that explain about the need for mast bend on masthead rigs. It doesn't take much and the maximum bend on a simple (no running backs or check stays) masthead rig is nothing compared to a racing fractional rig, but the bend is there and it is needed. I get about 1 1/2" of bend out of the mast on my C30. Many keel stepped masts carry 2-3" of pre-bend and get another 2-3" of bend with full backstay. Since the masts in cruising rigs are not designed to bend very much, the amount of sail shape control is limited compared to a fractional rigged boat.
To continue what Moody said
...this is one of the failings with frac rigs, no backstay and roller furling masts. They loose many sail controls that help to shape the main properly for various wind conditions. As a result the performance of the boat is heavily compromised compared to a similar sized boat that has the ability to make all the need sail adjustments.
...this is one of the failings with frac rigs, no backstay and roller furling masts. They loose many sail controls that help to shape the main properly for various wind conditions. As a result the performance of the boat is heavily compromised compared to a similar sized boat that has the ability to make all the need sail adjustments.
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