Slack forestay?

[mick]

As my current boat (HR 29) is the first I've had with a fractional rig, I'm unsure about how tight the forestay should be. It seems to be the case that a masthead rig can give you a tighter forestay, but even so my stay wobbles a bit more than I would like. The shrouds are good and tight. I thought tightening the backstay would help but my little book on rigging tells me that makes the forestay slacker(?). All comments welcome.

 

[Lakesailor]

from the excellent Kemp Sails technical data

Since the cap shrouds are responsible for countering forestay sag, they
need to be substantially tighter than those on a masthead rig. How
much help they get from the backstay depends on the position of the
hounds. With a 3/4 rig, for example, the backstay will do little more
than bend the topmast, whereas it exerts more influence over a forestay
that’s only a foot or two below the masthead.

 

[pvb]

Download Selden's tuning handbook, it has all you need to know.

 

[prv]

I thought tightening the backstay would help but my little book on rigging tells me that makes the forestay slacker(?).

Yep - pulls the top of the mast back, bending it and shoving the middle part where the forestay is attached, forward.

Pete

 

[lw395]

It depends on how stiff the mast is, and whether the shrouds are behind the mast or in line.
A bendy fractional rig with angled back shrouds relies on the shrouds to tighten the forestay. It is almost like a dinghy rig with a backstay added to bend the mast.
There will usually be running backstays to add forestay tension.
A stiff fractional rig with inline spreaders allows the backstay tension to bend the mast and add forestay tension. There may be check stays well below the hounds, tightening these reduces bend. The mast may be stiff through either being a large section, not having much top mast (eg 15/16th rig not 3/4) or having jumper stays to hold the topmast straighter.
Some rigs are a blend.
The lowers also have an effect by influencing bend, as does deck chocking.
When sailing the tension is quite different from the preload as the windward shroud tightens and the leeward one goes slacker.
Mainsheet tension also adds to forestay tension.

It's best to actually check what's going on. See how far you can deflect the forestay by hand with varying settings. Keep an eye on mast bend using a line from gooseneck to truck.

 

[johnalison]

A little forestay sag doesn't matter as long as the sail is cut to compensate. Traditional Dutch craft have very saggy jibs but can sail close because their sailmakers cut the sails accordingly, with some cloth taken out of the luff. All forestays sag a bit of course.

 

[Salty John]

Some information here - scroll down to the article on tuning with a Loos gauge.

http://www.saltyjohn.co.uk/cruisingresources.htm

 

[mick]

Thanks, all.

 

[dt4134]

Yep - pulls the top of the mast back, bending it and shoving the middle part where the forestay is attached, forward.

Pete

It doesn't work like that on any boat I've sailed on. You would need the caps well above the forestay for that to happen and that would be an unusual arrangement.

Normally you'd use the back stay in heavier weather to tighten up the forestay and allow the headsail to be depowered. It has the extra effect of tightening the headsail luff and drawing the draught forward. Not to mention flattening the main and adding twist to it. On one boat I raced on I found it almost more productive to play the backstay in gusts rather than the traveller, but it was hard work.