Backstay use...

Monique

Monique

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September issue of PBO page 71...

Not sure I get the principle... With boat sailing to windward, statement is " I suggested he pull the backstay on to flatten the main sail".

In my boat, pulling the back stay does the reverse. Makes the main fuller...

Not doubting the author but it seems reversed to me; I pull the back stay when sailing downwind.

Explanation sought......:o
 
On a fractionally rigged boat (this will not work on a masthead rig) when tightening the backstay the top of the mast comes back while the middle of the mast moves forward, the amount of movement or bend is controlled by the lower shrouds (and the backstay). This bending of the mast tends to flatten the main and it certainly worked on my previous boat (fractionally rigged GK24)
 
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Tightening the backstay not only pulls the mast top backwards, but also pushes the mast top down. Tightening the back stay therefore creates a bend in the mast.

With the bottom of the mast remaining in its fixed postion, the middle of the mast is effectively moved forward as a result of this bending. Imagine pushing down on the top of a flexible broom handle while you hold the bottom fixed between your feet and you may get the picture.

At roughly the middlepoint of the sail (seen vertically, i.e. from top to bottom), the horizontal distance between the leech of the sail (stretched between the mast top and the end of the boom) and the luff of the sail (stretched along the length of the mast, which is now bent with the bend located roughly in the middel and pointing forwards) increases as a result of this bend. Therefore the sail has no choice but to flatten itself to accomodate this increased horizontal distance.

The only conceivable explanation for your sail becoming fuller instead of flattening is that the bend is not forward facing, but in fact faces aft. This could be caused by incorrectly tensioned rigging (shrouds), depending on your rig type. These determine the direction of the bend.

A picture paints a thousand words...
 
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Imagine a triangle with straight sides, pull one side out in a convex curve, the material to form that curve comes from the centre of the sail so tightens (flattens) it.
You will also notice the boom goes lower as you tighten the mainsheet to compensate for the change in position of the mast head.
As your sails get older and fuller you should be setting the rig up with more pre bend.

Apologies JW for repeating what you said, you type faster than me.
 
prv said:
I pull the backstay tight when going downwind, to hold the mast up. But my boat's unusual...
Click to expand...

Twister_Ken said:
And so, I suspect, is your mast!
Click to expand...

:-)

My only backstays are runners. So if I don't pull one tight it's only the slight backwards angle on my shrouds that stops the mast tilting forwards. Running downwind, all the pull on the mast is in this forward direction, so it seems prudent to have a backstay hard on except in light winds and smooth water. I don't need to consider the headsail shape as they won't set on a run; I furl them and goosewing the mizzen instead.

(I've actually heard from fairly reliable sources that the shrouds are sufficient to keep the mast up and the backstays are only to help with forestay tension - but the shroud angle really is not great and the engineering instincts in me are happier with something else providing backwards pull)

Pete
 
backstay

On my 7/8ths Fractional rig X-yacht the difference between no tention and full tention (combined with the cuningham) is like taking in the first reef. Makes depowering simple and very quick in that 16-20knt app wind range.
 
prv said:
:-)

My only backstays are runners. So if I don't pull one tight it's only the slight backwards angle on my shrouds that stops the mast tilting forwards. Running downwind, all the pull on the mast is in this forward direction, so it seems prudent to have a backstay hard on except in light winds and smooth water. I don't need to consider the headsail shape as they won't set on a run; I furl them and goosewing the mizzen instead.

(I've actually heard from fairly reliable sources that the shrouds are sufficient to keep the mast up and the backstays are only to help with forestay tension - but the shroud angle really is not great and the engineering instincts in me are happier with something else providing backwards pull)

Pete
Click to expand...

I would probably also have a backstay on, but not hard.

Friend of mine has a mini transat, where the only backstays are runners. Upwind the Backstay is wound on to tension the forestay and flatten the main as described. But downwind it's still on, but eased somewhat to allow the mast to straighten.
 
flaming said:
Friend of mine has a mini transat, where the only backstays are runners. Upwind the Backstay is wound on to tension the forestay and flatten the main as described. But downwind it's still on, but eased somewhat to allow the mast to straighten.
Click to expand...

That's a rather different boat to mine :). My mast is a solid wooden pole of moderate height and perfectly straight and vertical. No spreaders, and all stays and shrouds are attached at the hounds just below the masthead. If I saw it bending I think I'd be worried. Any bend (either natural or deliberate) will be between the mast and the gaff.

Pete
 
Just to add a question to a question...does one still tension the backstay / bend the mast to the same extent with in-mast furling? Or does this b**ger up the furling mechanism? I assume it'll have to be let go before furling in any case.
 
bluedragon said:
Just to add a question to a question...does one still tension the backstay / bend the mast to the same extent with in-mast furling? Or does this b**ger up the furling mechanism? I assume it'll have to be let go before furling in any case.
Click to expand...

We sail a Bav 390 with Selden Furlin rig (in mast furling). No problem to tension the back stay and flatten the sail. The degree to which you can bend the mast has to do with many other variables besides whether or not furling main sail (such as purchase on backstay, tension of shrouds, flexibility of mast etc.).

No harm is doen to the furling mechanism. It consists of an aluminium extrusion very similar to the extrusion on a jib furler, and essentially similar to the mast itself. In other words the furling extrusion will behave much like the mast does, and simply bend along with it.

Yes, I find that it is beneficial to straighten the mast before furling. Although some pre-bend always remains. The furling mechanism can deal with this.
 
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jwdorst said:
A picture paints a thousand words...
Click to expand...

mastbend_180.gif
 
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