Mast Bend

[muskie]

I got difference answers about the reason to bend the mast from different people. Could somebody clarify this for me? i.e. while running close hauled with strong wind, mast need to bend, why?

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[charles_reed]

2 Reasons

1. By bending the mast away from the leech you reduce its concavity ie you flatten it and reduce heeling forces.
2. You increase tension on the forestay and straighten the genoa luff thereby allowing you to point higher.

There are disadvantages to having a bendy mast.

1. If you watch a fractional rig beating with considerable bend you'll notice the lee shrouds are quite slack. This means all the loads are hanging off has the weather shrouds and the compression loads on the mast are unfair. Most designers cross their finger and hope the mast manufacturer allowed sufficient safety factor in his calculations, because the true loads are almost impossible to calculate (except that they're "considerable")
2. A corollary of this is that the mast, to be bendy, has to have a reduced cross-sectional area or it won't bend without distorting the hull. This leads to a pumping action (even with the restraint of the mainsail) and frequent leads to fatigue mode failure .

For that reason most ocean-cruising boats have a short(ish) masthead rig with no bend. The Alubat Ovnis are a typical example of a high latitude cannon-proof rig.

Guess what, I've a 9.5m long boat with 30m2 main on a 14.5m high fractional rig, to shift a boat around which has a design weight of 3000kg so I have ample first-hand practical experience

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[muskie]

Re: 2 Reasons

Hi Charle,

For point 2, straighten the genoa luff could point higher. Is it because of the Centre of Effort changed or some other reasons?


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[bugs]

The initial pre-bend in the mast is dictated by the cut of the mailsail.
The bend in the mast is a balance between back winding of the main (too little bend) and starvation creases (too much bend).

Further bend is applied as the wind increases and you want a sail with less depth in the draft as you head upwind.
Very bendy masts often have additional runners and check stays to add more stability to the mast to try to over come the damaging forces described by Charles.

Tighter forestay moves the draft forward in the genoa (more obvious on Mastheads than Fractional). This makes it point higher but can be harder to steer to since the angle of entry is narrower. Flat water is easy but if there are waves yo will need some depth lower in the sail to keep up speed as you work through the sails.

Go up to the forestay while underway and have somebody tighten the backstay - you will see the effect. (or tighten the foretay bottle screw on a masthead).

Lie on the coach roof and look up at the main and have a look at the draft.
Then have somebody pull on backstay, cunnigham, out haul etc . The difference is very obvious. The faster boats will spend a lot of time tweaking all the variables until the achieve the optimum shape for each wind strength and sea condition.

There was a book I think by John Hayes called winning races. I found it very helpful for explaining the setup of different masts, sail trim, etc.



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[charles_reed]

no

flattens sail and takes the draft forward mainly, tho' having less sag in forestay also helps.

If I'm going to windward in >F3 for any period I use the solent which, being tall and thin, gives a far better slot than an overlapping genoa as well as less side pressure and therefore less likelihood of forestay sag.

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[Sunnyseeker]

Only reason to deliberately bend the mast is to flatten the main, to reduce its power. Fractional rigs do this by allowing the mast to pivot about the forestay attachment. Top comes back middle goes foreward, if you pull hard on the top bit the forestay gets tighter too.
Very bendy masts have check stay to control how far forward the middle goes, and runners to pull the top back.
To change the entry shape of the front sail use the halyard, this is a seperate issue from rig tension and mast bend.
If the tensions too slack and its lumpy because they can bend the will and they start pumping which isn't good so wind up some more back stay/runner.
Best thing is to play with it in moderate wind and look up th emainsail track to see what its doing sideways, it shouldn't be doing much.

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[Rick]

Not sure that I agree with Charles on the disadvantages (all rigs have slack lee shrouds, regardless of bend), but the reasons I agree with. Masts on both fractional and masthead rigs get bent to flatten the mainsail - if you laid your main out flat, you'd find the luff and foot are both curved, and when fastened to the mast and boom, cause the three dimensional shape that we derive power from - by bending the mast you pull the fabric in the sail towards the bow, flattening the sail and reducing power.

On a fractional rig, you generally provide the bend by either pulling on the backstay, or in some cases, just using more mainsheet tension. Either way, the top of the mast moves back, causing the secion below the hounds to go forward. On a masthead boat, it's usual to have a baby stay or inner forestay that pulls the middle section of the mast forward.

You'll find lots of information about this at www.uksailmakers.com/lofts/socal/encyclopedia/encyclopedia8.html in general, and particularly www.uksailmakers.com/tipsmainsail.html.

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