Forestay tension

[chrisbitz]

On sunday, we had a brilliant sail in 15kn (ish) winds, and in the gusts, I could see the forestay curving leeward.

I had already noticed earlier that the mast had a (reasonably good?) curve in it, (it's a fractional rig) so I was a little nervous of tightening the backstay even more.

I'm assuming that the forestay tension is directly related to the backstay tension, or can you tighten the forestay independently? The shrouds are level with the mast, so I doubt they would have any effect?

Is it quite normal for the forestay to flex?

Could I damage the mast by having too much backstay tension and curve?



On a sort of related note, should I keep the backstay tensioned when I leave the boat moored for the week, or should I relax it?

 

[sailorman]

http://www.uk.northsails.com/TIPSADVICE/GenoaTrim/tabid/7042/language/en-US/Default.aspx

 

[chrisbitz]

http://www.uk.northsails.com/TIPSADVICE/GenoaTrim/tabid/7042/language/en-US/Default.aspx

Thanks very much for the link, although I'm not sure what I should be looking at - it seems to be a page about Genoas, and doesn't mention forestay tension at all...?

 

[sailorman]

Thanks very much for the link, although I'm not sure what I should be looking at - it seems to be a page about Genoas, and doesn't mention forestay tension at all...?

check through all the pages

 

[daveyw]

Here's a PDF file for your consideration



https://www.google.co.uk/url?sa=t&r...0nEItawmE93DXbwsA&sig2=_CdAVmzz4gZZB4wjh8KB0A

 

[Javelin]

In light to moderate wind and assuming you have a reasonably slippery hull shape, when sailing upwind you should be sheeting the main on or just off the centreline.
You want the top leech tell tail, usually on the top batten, fluttering around 50% of the time.
To get this you will find you need a fair amount of tension.
As the wind builds you lower the traveller down to leeward.

OK the reason for writing the above is that much of your forestay tension is effected by your mainsheet and subsequently the leech tension in your mainsail especially with a fractional rig.
Applying backstay tension will only tension the forestay so much and then all that will happen is you will bend the mast which will then actually reduce forestay tension, again especially if you don't have swept spreaders or aft lowers restricting the mast bend.
Applying more backstay and increasing fore and aft bend will flatten the main sail which will help as the wind builds but this applies only so far before the mast becomes unstable and unable to deal with the compression loads.
By this time you will see a diagonal crease in the main sail from clew to the spreader area.

I'd need to see a photo of your rig to really help so much of the above is guess work based on assumption, not a great basis to give solutions.

As far as keeping backstay tension on, I have a hydraulic backstay and I ALWAYS back it right off once sailing is done for the day.
Why submit your rig and boat to avoidable compressive loads if you have a choice ?

 

[chrisbitz]

OK the reason for writing the above is that much of your forestay tension is effected by your mainsheet and subsequently the leech tension in your mainsail especially with a fractional rig.

Of course! I hadn't thought of it that way, but now you mention it, it makes sense...

I'll take some pictures next time just in case it's useful...

thanks!

 

[johnalison]

I once saw a quote from an engineer along the lines of "there is no such thing as a stressed line that isn't curved". The point is, how much?

 

[30boat]

With fractional rigs, forestay tension can be achieved by the cap shrouds in case of backswept spreaders or by running backstays in case of straight spreaders.On a backswept fractional rig you must tension the caps to about 20% to 25% of the breaking load of the wire which is substantially more than on a masthead rig (about 15%).The reason for this is that the backstay is there mainly to control the shape of the mast, not support it.Tightening it will tension the forestay up to a point depending on the stiffness of the mast section but beyond that,as was said ,you'll just bend the mast shortening it's effectve length and thus decreasing the tension.
Running backstays can be fitted to a backswept fractional rig together with checkstays.This way you'll have a very well supported rig but the extra complication is usually not worth the benefits.
Having said all that it is normal for the forestay to sag to leeward especially on this type of rig.My rig for instance is a 3/4 and I can't completely avoid it but since the foresail is small in relation to the main upwind performance is still very good.

 

[William_H]

With shrouds abeam the mast it is only the backstay providing support in the aft direction. So yes you should ease the tension on the backstay but only a little . Too loose will allow the rig to flop forward and aft.
As said forestay tension is provided by the main sail leach and the backstay. Both will be pulling aft on the top of the mast. So forestay tension is a function of the height between the mast top and forestay top and the stiffness of the mast.
As said it is impossible to get a straight forestay. However you should try to measure the deflection. If it is too much even witha lot of mast bend then diamond spreaders might provide more stiffness to the top of the mast so providing more forestay tension with less mast bend. Diamonds are stays from mid mast to spreaders facing forward at the top of the forestay to the top of the mast . Usually 2 spreaders about 45 degrees apart so mostly facing forward. Or you could try running backstays to the top of the forestay giving that forestay tension. I did sail on a boat which had running backstays and a load cell and readout measuring forestay tension but that might be a bit OTT for you. He eventually threw away the running backstays and modified to aft swept spreaders 3 sets. Anyway good luck and I am glad you enjoyed the sail. olewill

 

[johnalison]

A good sailmaker will take sag into account and cut the luff to match. Traditional Dutch craft have very saggy forestays but their jibs set perfectly.