rig tuning

[sailorbaz]

About 2 years ago, I printed off a posting about rig tensioning which involved fixing a tape measure to the cap shrouds and tightening to a small percentage of the breaking strain. Unfortunately last time I did this it rained on my ink-jet print-out and it is no longer readable! Does anyone remember the posting with values please? I need to set up the rig this weekend.

 

[alan]

Details can be found in an excellent book by Brian Toss on the subject of rigging.
Unfortuately my copy is on my boat! However I am going down to the boat later today and will post the details this evening.

Alan.

Alan Cloke

 

[vyv_cox]

This is the posting you mean, I think. It was posted almost exactly one year ago but several times before that also. A few people tried it and didn't seem to get very good results.

Vyv

Rigging is a bit of a black art, and I’ve never been terribly satisfied by the assurances that “it’s impossible to overtension using hand tools”. There are lots of books which will tell you how to tension rigging and, more importantly, set up the mast correctly. In the meantime, here’s a quick and easy answer.

Optimum tension is generally considered to be about 15% of the wire’s breaking strain for the cap shrouds; less for the lowers; and about 20% of breaking strain for the backstay on a masthead sloop. Fractional rigs probably need less, depending on where the forestay attaches to the mast.

You can buy rig tension gauges. They’re quite costly, and usually only fit a small range of wire sizes. And you may need to know the breaking strain of the wire first.

There’s another method which costs nothing and is easy to do. It also works regardless of wire size, and you don’t need to know what the wire’s breaking strain is. It relies on the fact that a 2-metre length of wire stretches by 1mm when 5% of its breaking strain is applied (and 3mm for 15% and 4mm for 20%). It’s easier to explain with a diagram, but as I’m not clever enough to draw one on my computer, here goes...

Assuming your mast is up and more or less in the right position, slacken off the shroud rigging screws (by equal amounts on port and starboard) and the backstay rigging screw. Take an ordinary DIY steel metric tape measure. Pull out just over 2 metres of tape. Fix the end of the tape measure to one of the cap shrouds, above head height, using sticky tape, so that the 2 metre mark on the tape measure is hanging a little way above the top of the rigging screw. Put a bit of string loosely around the tape measure and the shroud at the lower end, to hold the tape measure against the wire. Wrap a piece of masking tape around the shroud, in line with the 2 metre mark on the tape, and put a pen mark on the masking tape exactly at the 2 metre mark. Now, as you tension the rigging, you can see at a glance how much the 2 metre length of wire has stretched.

Tighten the rigging screws (by equal amounts on port and starboard) until the pen mark on the masking tape shows a 3mm stretch (which equals 15% of the breaking strain of the wire). You only need to measure the stretch on one side of the boat, because the other side will be equally tensioned. Then tighten the lowers so they feel a bit looser than the cap shrouds, keeping an eye on the mast alignment. Then put the tape measure on the backstay and repeat the operation, using 4mm stretch to indicate 20% of the breaking strain.

That’s your starting point. Watch the lee shrouds when you’re sailing. If they go slack, you need more tension. Purists may scoff (and probably will) but at least this method is something you can do yourself, easily and without incurring expense.

Hope this helps.

 

[bedouin]

Re: On the subject of rig tuning

I can recommend Ivar Dedekam's "Sail and Rig Tuning" that gives the details of the method you mention.

A further question to the experts out there - In all the books or articles I've read no one has commented on the relationship between the fore and aft lower shrouds. In the past I've always tensioned them evenly - but is it possible/desireable to use them to control mast shape?

 

[vyv_cox]

Re: On the subject of rig tuning

Indeed it is. I don't have forward lowers, just a baby stay. I use this to put in a little pre-bend. Amount varies from about 1 inch up to about half mast thickness depending on design, sail cut, etc.

Use lowers to keep mast straight close-hauled in winds about F4, sight up the mainsail groove and adjust accordingly. Remember to do the same on both sides. If you have forward lowers I would imagine there is a little more adjustment possible but the aim is still the same - keep the mast straight laterally with a little pre-bend.

 

[oldsaltoz]

G’day Sailorbaz.
You might try the following link: -

http://204.220.138.252/newspics/charts/8712mastheadrig.pdf

Andavagoodweekend Old Salt Oz……

 

[alan]

I refer to two publications as reference:
The Rigging Handbook by Brion Toss (Adlard Coles Nautical)
Sail and Rig Tuning by Ivar Dedekam (Fernhurst)

Cap shrouds should be 15-20% of breaking strain 0r 20% with fractional rig and swept back spreaders.
Back Stay 30% Max
Intermediates 12 - 15%
Lowers 10 - 12%

Breaking load for 1 x 19 stainless steel wire is:

5mm 1400 Kg
6mm 3200 Kg
7mm 4400 Kg
8mm 5700 Kg

If you use a 1 metre long sample then the stretch is as follows (staring from handtight):

10% = 1mm
15% = 1.5 mm

On a 2m long sample:

10% = 2.0 mm
15% = 3.0 mm

If you need any more info quickly then please e-mail me.

I can recommend the book by Dedekam which has lots of piccies to make life easier!!

Good luck,

Alan.
Italy.



Alan Cloke

 

[pvb]

It does work...

The post you reproduced was posted by me, Vyv, and the method described does work. I confess to having copied the method from a Selden booklet which came with my boat, and I reckon Selden ought to know a bit about rigging! The method's not super-accurate, but as a basic way for the average sailor to easily set up the rigging it's very useful.

 

[vyv_cox]

Re: It does work...

Not intended as a criticism of you!

I prefer to use a method in which the results can be observed, rather than to specify numbers that people adhere to in gospel fashion. Using observations of the amount of slackness in lee shrouds in combination with sighting up the mast and forestay seems to me to be a far more reliable method.