Replacing a fixed length forestay.

[WesHeald]

Hi all,

I have a 1971 Ericson 39 with a Profurl R42 rolling furler. The forestay is a fixed length and was tensioned only by adjusting the backstay. There is no turnbuckle and there's no room for one inside my furler drum. the forestay collided with a bridge, and needs to be replaced. The whole unit is off, and I have the new cable, 3/8 1x19 and sta-loks. My intention was to match the cable length to the old cable, but I'm told that there is some permanent structural stretch in my old cable, that should be adjusted for in the new one. In other words, my new forestay should be a bit shorter than the old one because it will stretch. Does anyone know how much this stretch is so I can allow for it?

Also, I've only had the boat a year and the rig was never properly tensioned. Does anyone know the rig tuning values for this boat?

[model 10]

To bad it's not like most ProFurl setups that have room for a turnbuckle at the bottom.

[JPA Cate]

You need to know what kind of wire, 304 vs.316, to be able to tell how much the old one stretched, and how much the new one will stretch. The information is available on line.

I really hear Guy, wishing you had a turnbuckle in there. With the new wire being shorter than the wire you took off, it is going to be difficult to get the mast to bend forward enough for you to hook up the proper length stay. It can be a real struggle, at times, and you may to not only relieve backstay tension but other parts of the rig, too.

If you don't feel comfy with this, you could consider hiring a rigger, and asking questions along the way. You'll see him or her tighten the wire, then pull on it, repeatedly. When it's okay, he goes to another wire, and so forth. If you test it like he did, your muscles will inform you of the tension. And, you assume he got it right.

I'm sure there's rig tuning advice on the internet. Jim's always done ours. First you tune it in the berth, then take it out sailing. You want the mast to still be straight, not sagging off to leeward and no slack shrouds in about 20 kn apparent. He still checks it because the wire does stretch, and that is prudent, imo. sight up the sail track, and you can see if it is sagging off to one side or the other. Tighten till the sail track is straight.

It's not rocket science, but having a turnbuckle in there would make it a lot simpler, 'cause you've got no back up if it isn't right.

Ann

[UNCIVILIZED]

Any decent rigger will be able to give you the numbers on stretch. It's based on the known stretch properties of the wire, it's diameter, & length vs. the load it sees. The math is dirt simple. You can DIY it pretty easily, if you've a mind to.
And you might initially build the headstay a couple of toggle lengths too short, removing them as it settles in length wise. That, or possibly using link plates, with multiple holes for the pin.

Tuning the rig is more about dialing in the helm balance that you prefer, & in order to properly shape the sails, than anything else. Well, unless you're racing One Designs, then the tuning settings are already established & well published.
Pick up a copy of Ivar Dedekam's Sail & Rig Tuning, along with gleaning what info you can from the tuning guides on some of the more prominent sailmaker's websites.
https://www.amazon.com/Sail-Rig-Tuni...g+tuning+guide


I've been on plenty of boats where due to headstay length, we unhooked the backstay, & loosened the other rigging, while using a tackle connected to a jib halyard, in order to get the mast tilted far enough forward to hook up the headstay. If you've not done it, it sounds crazier than it is. Assuming the mast is keel stepped. And we did it this way on my deck stepped Ranger 33' too. Only with the backstay fully slackened instead of unhooked. You may even need to run the tail of the tackle back to a cockpit winch. But it's more than doable.

Knock on wood, you could probably even get it prestretched, as is the norm with Dyneema rigging. But the guys doing the stretching had better know what they're doing.
EDIT: You might also look into rod rigging style turnbuckles, as they're quite svelte as compared to those for wire. And adapting them for use with wire is pretty easy.

[Captsteve53]

Below is the magic formula if you are that worried about the stretch factor, but in reality if you cut your new wire say 1/2"- 1" shorter than your old wire(allowing for permanent stretch) and check where your adjustment was on your back stay (ie: was it in the middle or at the ends of adjustment?) Most all turnbuckles will have the adjustment to take for Permanent and elastic Stretch (especially at 3/8-10 mm dia) , with new fits allow the turn buckle to be near the very open end of the adjustment, so that when tension ed it should be near the middle of adjustment.

Set the mast up with your halyards to get initial correct RAKE (USUALLY AROUND-12"-18"), this is a good time to re check the fore stay length before cutting to final dimension.

Note:All 1 x 19 stainless wire stretches under load, but returns to its original length when the load is removed. 1 mm stretch per 2 m wire is equivalent to 5% of the breaking load, irrespective of the diameter of the wire.

Elastic stretch (mm)=
W x L
E x A
Where:

W=
Applied Load (kN)
L=
Cable length (mm)
E=
Strand modulus (kN/mm2)
A=
Area of cable= D2 x π
_____________- E x A
Where D=
Normal diameter of cable (mm)
Typical values for E are:
1 x 19
107.5 kN/mm2 (15.59 x 1066 P.S.I.)

Cheers Steve

[david samuelson]

Hi Wes,
There are 2 types of stretch here that concern you: elastic and unrecoverable stretch.
With the correct rigging for the boat, you will only experience elastic stretch: when you tighten the rig and when you are sailing. Today’s cables do not have structural stretch so there is no problem there. So if you change rigging after ‘normal’ use you can copy the length of the original cable.
Hitting bridges is not ‘normal’ sailing. This can put a load on the cable which exceeds the wires elasticity, so now we are into non-recoverable stretch, and alas, we do not know how much, and guessing the amount of stretch is a hit or miss solution.
My suggestion would be to buy a turnbuckle cylinder (Profurl part # P037070) which will allow you to add a turnbuckle – you will need to cut a bit off the top foil but it will not shorten the extrusion length so the same sail can be used.

Good luck. David

[bil56]

the good tension on the rigging cables is about 20% of the breaking load.
the best is to do this job with a tensiometer ...

elongation of 0.5 mm/m = 5% of breaking load.

[david samuelson]

Hi Wes,
Setting and/or knowing the tension on a forestay to which one doesn’t have access, because it has roller furler foils for example, it not easy. Some boats have 2 backstays, some have a split backstay and even if there is only one backstay the tension is seldom the same as the forestay.
In my opinion the 2 principal causes of foil failures, fasteners coming out, foils separating at the joins etc., are poor installation and not having the forestay tight.
When sailing, the forestay, and therefore the foils, bend from side to side. This bending tries to separate the foils from one another, thus working on the little set screws and the joiner.
Have the forestay tight, and yes, use the backstay tension as a guide, but with caution.
My 2 cents