How far can I bend my mast

[Polly1]

I have a folksong with a 3/4 fractional rig and a tapered mast. I can adjust the backstay and bend the mast. I read in a Selden rigging guide that 1.5 mast diameter worth of bend measured from the tip was the maximum. But that wasnt for my mast or rig.
I have a very baggy sail and want to flatten it as much as i can but dont want to break anything. More tension will also give me less forestay sag for the genoa when the wind picks up.
Any ideas on how far I should go?

 

[sarabande]

before bending the mast, have you tried to take out the fullness with the Cunningham, the boom outhaul, and vang ?

 

[Quandary]

On a Sigma 33 with a 3/4 rig we would induce up to about 400mm. of bend with the backstay, it looked like a lot more because of the addition of the rake, but if you bring the main halyard down tight to the gooseneck you can get a picture. This was as much as the backstay adjuster could manage and was only used to windward in a stiff breeze with full main, also when setting up the rig and adjusting the prebend. The mainsail was dacron in those days and as we bought only one new fore and aft sail each year it could be up to three years old, you would not need as much bend with modern sail cloth. The amount of prebend was less than half of this and was increased as the sail stretched. I am sure the mast could have withstood more induced bend but we did not have the tackle or strength to apply it. I would guess that proportionately you would be applying about 60% of above.
On my current 7/8th. rig with fairly fresh dacron cruising sails I have less than 150mm. of prebend but the mast is stiffer and being 7/8th. I can only apply about another 100-150mm. of induced bend with the backstay tackle. Like Sarabande advises, we make full use of the outhaul and cunningham as the wind increases.

Note - the above advice relates to rigs with swept spreaders!

 

[Seajet]

Top Tip;

a good new mainsail is not only cheaper than a new mast, it gives a lot of satisfaction and gets one to the desired destination quicker.

 

[30boat]

When you bend the mast you are also reducing the tension on the caps because the effective length of the mast is reduced.

 

[Lakesailor]

Any ideas on how far I should go?

Please report back when you find out how far was too far.

 

[Quandary]

When you bend the mast you are also reducing the tension on the caps because the effective length of the mast is reduced.

Prebend is introduced by hardening down the backstay to allow the cap shrouds to be tightened, it is then reduced a bit by hardening the lowers to a greater tension. In big events we did this tuning before every race. I have never heard of anyone breaking a rig by pulling on the backstay but again I stress that only a swept spreader fractional rig is designed for it. On a seriously raced Sigma 33 in the days of Dacron sails the rig tensions could be considerable, hence the puddle in the coachroof.

 

[boaterbaz]

I guess you can bend a mast until it breaks, bu that isnt very useful

 

[SHUG]

You can easily get into a horrible circle of adjustment which Sonata owners know only too well. Tighten the backstay to bend the mast and the middle part moves forward so that the forestay slackens . With baggy sails it can be "the law of diminishing returns".
Some of the older Sigma33s would end up with dished decks at the mast foot from efforts to get a good set with old sails by increased rig tension.
Get your sail fixed or get a new one!!!!

 

[Polly1]

Would the fact that I have straight spreaders with cap shrouds in line with the mast and only one lower each side attached to the rear of the mast foot affect any of the advice so far?

 

[saltylegs]

Would the fact that I have straight spreaders with cap shrouds in line with the mast and only one lower each side attached to the rear of the mast foot affect any of the advice so far?

yes. you won't be able to induce prebend with single inline spreaders and single lowers as they only hold the mast in column.
you will need a baby stay or similar to set up prebend.

ps lowers attached to mast foot???

 

[30boat]

yes. you won't be able to induce prebend with single inline spreaders and single lowers as they only hold the mast in column.
you will need a baby stay or similar to set up prebend.

ps lowers attached to mast foot???

To the rear of the mast foot!

 

[William_H]

Mast bend

I think you will find you can bend the mast to an alarming degree and as said I have never heard of anyone breaking a mast this way.
I once did a short sail witha friend who had got the sailmaker top come along because his new sail had a lot of creases. The sailmaker grabbed the back stay tensioner and yanked it on to a point where I was worried b7ut it pulled that sail flat. Obviously it was not his mast. Certainly on a fractional rig 3 or 4 times the fore and aft dimension of the mast section would be normal for a fractional bendy mast rig.
I think the number 1.5 might be for a non tapered mast head rig.
The bending of the mast not only flattens the sail but allows the top of the mainsail to fall away so reducing
heeling power. good luck olewill

 

[30boat]

The amount of bend achievable depends on the type of mast.My old Kemp mast was so flexible I needed to fit checkstays to control it but the much larger section Z Spars replacement is a lot stiffer.I could bend the old one with very little effort but I suspect that If I tried to get the same result on the new one I would break something.The rig is a 3/4 fractional.