RIG TENSION ?

[Guest]

We have just re-stepped the mast on our Moody 27. It's a new boat to us so I do not know how it was set up. My question is how should the shroud/backstay tension be set up ? The mast is straight without any rake and plumb in the athwartship direction. I have read for shrouds you should adjust it so the lee shrouds are JUST tight on about a F3 wind hard on a tack. I have also read you should tap or ring them. I am worried that over tensioning the rig could cauise damage. Can anybody help ?

 

[Guest]

Really the best bet is to get a reputable rigger to tension the rig first time. Then you will have a good idea of what it should be in the future. It should not cost too much as its only about an hours work. Well it is not too bad on the east coast at least.

 

[Guest]

£50 - £60 quid in the Solent

 

[Guest]

Easy DIY way...

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.

 

[Guest]

Re: Easy DIY way...

Thanks PVB , It looks logical. We will try it. I thought afterwards that the best way to set the backstay up is to get the boat sailing upwind and tension it until the luff of the genoa looks straight and is drawing along it's lenght.

John

 

[Guest]

Last time I had the yard rig the boat they made a complete Spherical objects of it, and used mild steel split pins!

I suggest setting cap shrouds so that the top of the mast is in the middle of the boat - check with main halliard to each side of the boat. Set the intermediates and lowers so that the mast is straight.

The cap shrouds should be tight enough to be difficult to move with one finger.

The intermediates should feel a little tighter and the lowers tighter still, shorter wires stretch less.

Set the forestay and back stay to the correct amount of rake for your boat. If you don't have a figure try about 7 or 8 degrees. Sorry but a bit of trigonometry may be required. Or perhaps an educated guess maybe 6 inches for your boat.

Sail the boat on a close reachin a moderate wind say 12 - 15 kts, certainly less than the windspeed at which you would put in a reef, and check the lee shrouds. If they are noticably slack tighten them until they are finger tight, count the turns applied to the the turnbuckles. Apply the same number of turns on the shrouds on the other side, check that the top of the mast is in the middle of the boat. Check that mast is straight by tightening the main haliard vertically down the mast and comparing it to the groove, adjusting lower and intermediate shrouds to straighten the mast by taking turns off one side and on the other.

Re-check rake and tension backstay and forestay to about the same tension as cap shrouds.

To check rake sail on a close reach in a light wind say 10 kts, if you have a small amount of weather helm its about right if lee helm rake the mast backwards, if heavy weather helm rake the mast forwards.

You can juggle this as much as you like, some people spend most of their time doing it, it comes down to personal preference at this stage.

Best of luck

 

[Guest]

Agree

This is an accurate but brief description of the process (no criticism intended). If you want a quite detailed description, worked up over a number of years of overcoming various problems, e-mail me.

 

[Guest]

One small tip here ...

It isn't easy to check alignment with the main halliard because the length depends on the tension you put on it, because of the catenery. To get round this, tie an angler's spring balance on the end, and pull it out to 20 lbs (or whatever, same either side) to measure exactly.

Bear in mind your yacht shape may not be quite true.

 

[Guest]

PVB ... a question ... or two ...

You've posted this advice several times in the last two years. It is most helpful, but when I tried it I felt that it added more tension than I really liked. Also new rigging seemed to stretch more than old. What is the origin of this advice? Does it apply equally to racing and cruising boats?

Incidentally, with a deck-stepped masthead rig I like to adjust the tension on lower stays so that the aluminium mast is absolutely straight, both when at rest and when reaching under full sail in a moderate breeze (though the mast may lean slightly in this case). But of course, fractional rigs are designed to bend. I'd be interested in comments on the desirability of induced bend, and how to set it up.

 

[Guest]

Re: PVB ... a question ... or two ...

I have seen very similar advice in a book "Sail and Rig Tuning" by Ivar Dedekaw (fernhurst books); which I found an excellent introduction to the subject.

It mentions exactly the techniques PVB suggests (although perhaps with slightly different figures). I am going to try this as I set up my rigging this season and see how well it works.

 

[Guest]

Re: One small tip here ...

Good idea Andrew, its better than my calibrated middle finger.

Perhaps in the absence of a spring balance a short length of strong bungee and a length of string could improvise the same thing.

 

[Guest]

PS to last

I am sure a Moody would never be out of true

 

[Guest]

Answers....

Andrew, the method I described was lifted from a little booklet published by Selden Mast, the world-class spar makers. The booklet is called “Hints and advice on rigging and tuning of your Selden mast”, and may still be available. I imagine the advice is suitable for both cruising and racing boats, and indeed the booklet includes a section on “Racing fractional masts”.

The beauty of the method is that it can easily be carried out by one person, without any special equipment, and that it works regardless of wire size. It also helps the amateur rigger like me to get something near the correct tension. Of course, it’s only a starting point, and there’s still a need to fine-tune under sail. Interestingly, Selden’s advice is that “the lee-side cap shroud should not be seen to slacken until the angle of heel reaches about 20 degrees”.

As far as inducing bend in fractional rigs is concerned, I’m afraid I wouldn’t have a clue (I sail a ploddy cruiser with in-mast furling!). If you can’t easily get hold of a copy of the Selden booklet, I’d be pleased to scan it and send it to you - just let me know by e-mail.