forestay tension - design question

[snowleopard]

i need to know how i can work out how much tension is required in a forestay to achieve a given degree of sag in the forestay, given area, wind speed etc.

my problem is that with a flexible unstayed mast i have to load the jib luff dynamically so that it sets with the correct degree of sag so i'm trying to devise a spring mechanism but need to work out the loadings on it.

does anyone know where i can find a suitable formula? also, from a practical point of view, if you have an adjustable backstay, how much pressure do you put on it, say in 15 knots apparent and what are your i & j measurements?

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[Twister_Ken]

Not much help I know, but going upwind a saggy forestay is a BAD thing, whereas going downhill it is a GOOD thing. Therefore, do what you need to get a tight forestay upwind, and let off some backstay off the wind to put some fullness back.

Only numbers I can rememeber, on a Swan 411 we used to put 5000 lbs (or maybe kgs?) on the backstay using hydraulics, dropping it to nothing dead downwind. Masthead rig tho'.

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[Shanty]

This sounds like a fairly involved calculation - non-uniformly loaded catenary with an anchor point that deflects under load?

As for the loads imposed by the sail - have you thought about talking to a sailmaker? The design software they use may be able to calculate the loadings across the luff panel of the foresail - which is the loading on the forestay.


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[Salty John]

A very complex calculation, I suspect. On a masthead rig the forestay tension is about 15% of the breaking strain of the wire. For instance with a 7mm wire diameter (1x19 SS) the forestay tension would be about 680 kg. On a fractional rig, where the forestay tension is not directly opposed by the backstay tension it is usually lower, but 15% is desirable if the mast bend is not excessive. Some mast bend is inevitable and is compensated for by the cut of the sails. On your rig I assume the mast itself provides the resistance to the forestay. If 15% of the wire breaking strength is necessary to achieve a tight forstay with minimal sag on a conventional rig you could use this as your maximum setting. However, there could be significant bend in the mast by the time you achieve this, especially if your wire diameter is excessive.

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[Ships_Cat]

As far as I know all the codes either ignore the rig or give emperically based solutions for the breaking load of the stays/shrouds and that related to the righting moment (even for the stays) with rig tensioning loads not being an input, so they do not provide a solution for the actual forces. Guessing at the safety factor and using one of these may give you some order of magnitude, however, which you could apply to your case, but if I understand what you are trying to do it probably won't get you close.

Unless I had access to proven analysis (software which even if it exists for purchase is probably quite expensive) I think that I would be inclined to measure the actual loads with a load cell. Sounds like you need a friend in one of the big custom sparmakers /forums/images/icons/smile.gif.

As I say I am not sure exactly what you are doing, so the following may be all rubbish, but is there a problem using a spring in that your rig loads will probably increase in a non linear way (increasing more rapidly as wind strength increases combined with high stability of a cat) whereas a spring will obey Hookes Law which is linear?

John

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[snowleopard]

clarification...

i lost the forestay through fatigue resulting from snatch loads when the mast flexed. i'm now looking at using a spring/damper arrangement to keep the forestay tight while absorbing shock loads. i'm trying to work out the lever arms and spring loading.

i'm pretty sure i'll have to have the jib cut to allow for sag but firstly i need an idea of how much tension i need to keep the sag down to say 40cm over 13m luff length.

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[Jacket]

Re: clarification...

<i need an idea of how much tension i need to keep the sag down to say 40cm over 13m luff length. >

If you can find out the Youngs modulus of the wire, and let me know the diameter, thats a fairly easy calculation (I think- I'd better go and look it up again, incase my memory's playing tricks on me, but I'm fairly sure its just a development of the catenary equations, with stretch of the wire factored in).

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[snowleopard]

Re: clarification...

the big factor is the pull of the sail. i=12m, j=3m and max windspeed 15 knots. forestay is i think 7mm. no idea on the modulus. good luck!

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[bedouin]

Re: clarification...

Do you really mean 40cms? That sounds a bit excessive. I tend to think of the sag being of the order of a few cms - certainly less than 10cms

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[snowleopard]

Re: clarification...

bear in mind that we're not talking about a hydraulic backstay and tons of loaf- it's an unstayed rotating mast so the forestay loads have to be kept low. the jib is cut to allow for the large sag.

incidentally, unlike a conventional rig, we furl the jib as the first reef, then sail on main alone, rather like a laser!

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[Jacket]

Re: clarification...

Ah, forgot that you had an unstayed rig. That makes the calculation much harder. Without knowledge of the stiffnesses of the mast and boom (if you call it that) its not even possible to come up with a ballpark figure.

But the designer of the rig must be able to give you some guidance, surely? Else how did he specify the strength of the spars in the first place? Its not like a conventional rig, where it can be done using rules of thumb based on decades of experience.

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[snowleopard]

Re: clarification...

what i'm talking about is a spring which provides the tension in the forestay. as the mast flexes the spring will take up the slack. the question is how much spring loading is needed to achieve a reasonably tight luff?

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[Salty John]

Re: clarification...

I think I'd introduce a shear pin to protect the stay from an excessive shock load. The shear pin would break at the maximum safe working load of the shroud and allow the spring to take over. The spring would likely be a series of cup (belville) washers to give a compact tubular shock absorber. Having a spring permanently in use could get a little weird in a head sea - the forstay would be like a tight rope with an acrobat jumping up and down on it. Of course, get the sums wrong and you'd need a bucket full of shearpins in the same circumstances!
Good luck.

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