How do you measure forestay tension with furler in situ

[skyflyer]

My forestay has a turnbuckle but insufficient length of bare wire to fit a tension gauge as the furler mechanism covers most of it (sliding up over the foil just enough to expose turnbuckle)
How do you assess tension - is it a direct relation to the backstay tension (allowing for different angles etc?) Its masthead rigged with straight spreaders so the cap shrouds do not exert any fore or aft component.
If so, presumably the tensions in both backstay and forestay need to be increased equally incrementally to avoid an upper mast bend? (there are intermediate and lower shrouds, but only the lowered have a for and aft element, the intermediates are straight in line)

 

[Bobc]

Backstay tension should be roughly equal.

 

[Praxinoscope]

According to the Loos gauge instructions, on a masthead rig the backstay will have a lower tension than the forestay, unfortunately I don’t think they give a guideline for tha ratio, but I have found that a backstay tension set to about 60% of the recommended forestay tension seems to tension the forestay to approximately what it should be, but as the OP states with a headsail foil there does not seem to be perfect way of confirming this, apart from checking mast bend and watching performance of the headsail.

 

[Bobc]

According to the Loos gauge instructions, on a masthead rig the backstay will have a lower tension than the forestay, unfortunately I don’t think they give a guideline for tha ratio, but I have found that a backstay tension set to about 60% of the recommended forestay tension seems to tension the forestay to approximately what it should be, but as the OP states with a headsail foil there does not seem to be perfect way of confirming this, apart from checking mast bend and watching performance of the headsail.

I would say that is right if you have swept-back spreaders, but if your spreaders are square the backstay should surely be the same as the forestay. If not, then where is the rest of the forestay tension going other than a bit of load from the weight of the boom?

 

[Praxinoscope]

I would say that is right if you have swept-back spreaders, but if your spreaders are square the backstay should surely be the same as the forestay. If not, then where is the rest of the forestay tension going other than a bit of load from the weight of the boom?

I’m not sure about the effect of the spreaders, you may well be correct, , but the Loos instructions give the desired backstay tension as lower than the forestay because of the greater angle of the stay to the mast rather than taking into account the spreaders.

 

[dunedin]

I would say that is right if you have swept-back spreaders, but if your spreaders are square the backstay should surely be the same as the forestay. If not, then where is the rest of the forestay tension going other than a bit of load from the weight of the boom?

Angles. The horizontal distance from mast to forestay fitting likely shorter than from mast to backstay tang, meaning backstay under less tension to give the same horizontal pull. Then adjust for any difference in height of forestay and backstay attachment on mast to calculate precise difference.
Or just say that‘ll do. (Of course if have adjustable backstay it will vary as crank on backstay tension)

 

[Bobc]

I’m not sure about the effect of the spreaders, you may well be correct, , but the Loos instructions give the desired backstay tension as lower than the forestay because of the greater angle of the stay to the mast rather than taking into account the spreaders.

There is some sense in that. I guess it depends how far back the mast is in the boat.

 

[johnsilver2]

Do you need to know forestay tension precisely?
Backstay tension is variable to control sail shape especially foresail luff sag to leeward and mainsail via mast bend. Just mark the backstay tensioner for max tension.
p.s. on all my boats the forestay has been larger than the backstay there max backstay tension is less than the max forestay tension by default.

 

[Praxinoscope]

Do you need to know forestay tension precisely?
Backstay tension is variable to control sail shape especially foresail luff sag to leeward and mainsail via mast bend. Just mark the backstay tensioner for max tension.
p.s. on all my boats the forestay has been larger than the backstay there max backstay tension is less than the max forestay tension by default.

Backstay tension adjustment is not always available for the control of sail shape, certainly it is usually there on fractional rigs, but I would suggest that this is not the case with masthead rig, which is what the OP has, and in this case it is preferable to have the forestay and backstay tensioned ‘correctly’ when setting the standing rigging.

 

[johnalison]

Backstay tension adjustment is not always available for the control of sail shape, certainly it is usually there on fractional rigs, but I would suggest that this is not the case with masthead rig, which is what the OP has, and in this case it is preferable to have the forestay and backstay tensioned ‘correctly’ when setting the standing rigging.

Two out three of my M/H-rigged boats had backstay tensioners, from 22-29', though I would set the tension tight before adjusting the other rigging.

 

[LadyInBed]

My forestay has a turnbuckle but insufficient length of bare wire to fit a tension gauge as the furler mechanism covers most of it (sliding up over the foil just enough to expose turnbuckle)

Mine is the same. I've not got a tension guage so I just adjusted the twin backstays until the foresail would unfurl without seeing and feeling any bend in the aluminium furler.

 

[captainboo]

According to Selden's rigging guide for a masthead rig if you tension the backstay to a maximum of 30% breaking load then the forestay will be at about 40% due to the different angles - but only for short periods. Normally backstay set to about 15%.

 

[Laminar Flow]

The main purpose for forestay tension is to keep the stay from sagging off to lee. To be sure, it is impossible to achieve a straight stay, that would require infinite tension. Overtensioning is a sure way of losing a mast as the load increases exponentially.
A forestay of 11m length should be permitted a sag/deflection of 25cm. You control the tension by observing the deflection in the forestay.

To calculate:

Z = 2/3F x l / 8 x f = (Newton)

Z = Tension in stay
F = Wind load in N
l = Length of stay in m
f = Sag of stay in m

The load on a rig is generally calculated at 1lbs/sqft or 48 N/sqm

 

[skyflyer]

Many thanks everyone. The boat can have a backstay adjustment/tensioner retrofitted but it isnt standard kit.

The angles are easy enough to measure which on the face of it gives 65 deg for backstay and 74 deg for forestay, so using cosine rule that would make the relative horizontal element of the tensions as .27 to .42 to equal out, or 1 to 1.56 - in other words the forestay tension will be 56% greater than the backstay tension.

BUT this assumes two things:

1. That the mast is pivoted at its base and has no inherent stiffness to resist the tensions from the stays, and
2. That the shrouds do not impart any horizontal load on the mast.

Clearly NEITHER of these assumptions are correct and thus the forestay tension isnt simply a multiple of backstay tension - hence my question!

 

[Aeolus]

Are there any similar boats where you're moored that don't have furlers? You could measure the relative tensions on them and get some data.

 

[Laminar Flow]

Many thanks everyone. The boat can have a backstay adjustment/tensioner retrofitted but it isnt standard kit.

The angles are easy enough to measure which on the face of it gives 65 deg for backstay and 74 deg for forestay, so using cosine rule that would make the relative horizontal element of the tensions as .27 to .42 to equal out, or 1 to 1.56 - in other words the forestay tension will be 56% greater than the backstay tension.

BUT this assumes two things:

1. That the mast is pivoted at its base and has no inherent stiffness to resist the tensions from the stays, and
2. That the shrouds do not impart any horizontal load on the mast.

Clearly NEITHER of these assumptions are correct and thus the forestay tension isnt simply a multiple of backstay tension - hence my question!

If the mast is deck stepped then it is, for calculation purposes, considered a pin/pin situation and indeed pivoted.

In a masthead rig, again for calculation purposes, the shrouds are not really considered for fore&aft stiffness, hence the greater longitudinal section of the mast.

It is by far the simplest approach to determine tension by the amount of forestay sag - after all, the whole purpose of this exercise is to keep the sag to a minimum. Don't even try to remove it entirely, you will bring down the rig.