Standing rigging tension

[roberth]

I had all the standing rigging on my 29ft Seamaster 925 renewed in June. I suspect it has stretched a tad in the 1000 miles sailed since then - there certainly seems to be more play in the backstay, and the forestay/furler has a slight curve to leeward, especially in a high F4 upwards, which seems to be affecting pointing ability. Shrouds also seem not quite so tight.

My question to you experts out there - is there a rule of thumb on rig tension, and a quick way of checking it?

I have also noticed a slight degree of rusting within the strands of the wire, and rust stains on the deck by the fittings. Is a certain amount of this inevitable with stainless rigging? Or, as I suspect, does this sound like cheap Chinese stock has been used, despite assurances to the contrary and a bill of £750?

Thanks in anticipation.

Robert

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[Roberto]

When using a new wire rope, you should expect roughly 0.05-0.1% constructional permanent stretch (as the single strands bed in against each other), that means <1cm every 10m of wire; that will modify the length of the rope permanently, whereas after that as you will sail the rope will stretch and recover back its length (provided the elastic limits are not exceeded)

If you notice much more permanent stretch than that it may possibly be due to other causes.

If otoh you just want to adjust the tension on your rigging, then a quick method is shown on Selden website, download "Hints and advice"




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[bruce]

standard appears to be 10/15% of working load of stay material...

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[Colvic Watson]

Is there a handy way of measuring how tight they are vs how tight they should be?

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[roberth]

Precisely

Robert

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[bruce]

over here there is a guage make by 'loos' about $40.

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[ashanta]

this months PBO refers to tension as a % against breaking strain (20 something %)
but you will need a tension guage and you will need to find out what the breaking strain is of the wire you are using. You should tension backstay for going to windward and slacken off the backstay when running or when you are not using the boat. Do you have a backstay tensioner? Your rig is very similar to mine.

Regards.

Peter.

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[roberth]

No, as with many cruisers, bottle screw only so effectively fixed. Have also heard that that cap shrouds need to be double the tension of others?

Thanks for all your replies - certainly have given some leads to follow up.

Robert

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[Fin]

Sailing to windward the leeward cap shroud/inners shouldn't be loose. Try to pick a day giving about 12-15kts wind and flat water if you can find it. Sail closehauled and check to see if the leeward bits are loose. If so then tighten them up remembering the amount of turns you have used. Tack and then do the same amout of turns on the other side. Do this until there is no slack and check that the mast is still upright. That should be a good rough starting point. You will only really get an idea of your rig tension if you connect up a tension gauge. Off the top of my head you will stuggle to overtension wire above 5mm by using hand tools but some-one please correct me if I'm wrong!

Hope that helps.

Fin

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[john_morris_uk]

You can also put some tape on the wire at two metres up from the joint. You then tighten the wire to stretch this amount by so many mm for so much % of strain. Can't remember the figures but its a measurable amount. I was sceptical when I read about the trick, but it works and save the cost of a strain gauge. If you look on the mast manufacturers sites I have no doubt you will find the figures.

Just check that the whole thing is upright and evenly tensioned etc.

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[VicS]

1 mm of stetch in 2m represents 5% of the wires breaking load. Hence for a tension of 15% you need 3mm of stretch. Do not exceed 25% of the Breaking Load.
This rule of thumb applies regardless of the wire diameter.

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[Egbod]

The measured stretch figure is fairly accurate. I have a tension gauge which is very inaccurate but having measured the stretch I now know the correct calibration for my shrouds. I have tried three different gauges and do not trust their accuracy. One was out by about 80%.

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[oldharry]

Could be that the hull has moved a bit in response to the load, rather than the new wire. Elderly wooden boats near their sink by date its not uncommon to see the hull being pulled out of shape by an over enthusiastic rigger. Any hull is bound to move a little when loaded, and the shock loads imposed by beating to windward will soon cause the hull to take up if its going to.

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[William_H]

I agree with Oldharry you must consider the hull structure when trying to tighten stays to 15% of Max working load. I think it is far too much myself. Your problem could be compression post compressing if it is a deck stepped mast or hull distortion however it is probably just the wire stretching. By the way you should consider fitting a backstay tensioning device. Most backstays branch to two wires about 1.5 metres above the deck one goes ech side of the transom. You make up a pair of flat plates about 30 cms long 5 cms wide of Al or SS and you fit a sheave wheel at each end between the two plates. This is fitted on to the two backstays the wire going inside the sheaves such that if the plate is pulled downward it pulls the two backstays together and so increases the tension. The plate with pulleys is pulled down by a 4 purchase tackle with cleat. Some boats have a single backstay to the deck and fit a turnscrew with a big handle on it so you can easily increase tension. Of course y90u could go to hydraulics etc. Not many keel boats don't have backstay tension adjustrment,This will allow you to tension up the forestay by tensioning the backstay and should improve your windward sailing. The tighter backstay should also flatten the mainsail giving better handling ain stronger winds. Well worth the effort. regards will

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[Salty John]

Lots of good information already given. Here is some blurb regarding rig tension based on advice from the manufacturer of the most widely used tension gauge:


The following table gives recommended initial tension settings for 1 x 19 wire rope. It should be noted that optimal rig tension will be a function of the boat design, the rig and the cut of the sails.

Wire diameter, Breaking strain, Forestay tension, Shroud tension
2.5mm 550 kg 80 kg 55 kg
3.0mm 950 kg 145 kg 110 kg
4.0mm 1500 kg 230 kg 160 kg

5.0mm 2000kg 340 kg 230 kg
5.5mm 2800 kg 450 kg 320 kg
6.0mm 3700 kg 590 kg 390 kg
7.0mm 4500 kg 680 kg 450 kg

Leading sail-makers provide rig tension information for certain boats and sails and these should be followed where applicable. If no specific information is available use the above table and the following comments to establish a rational basis for tuning your rig.

Forestay tension – masthead rig: It is almost always advantageous to set the forestay tension as high as possible within the limits of structural strength. Generally, it is possible to use 15% of the breaking strain of the wire as the forestay tension. The backstay should be adjusted to maintain a straight mast with the desired forestay tension. The backstay tension will be less than the forestay tension because the backstay makes a greater angle to the mast than does the forestay.

Note that rollerfurling jib tension can only be set by adjusting backstay tension.

Forestay tension – fractional rig: Because the forestay tension cannot be directly balanced by the backstay tension some mast bend is accepted and the sails are cut to accommodate it. Forestay tension of at least 15% of the wire strength is desirable but, if this results in excessive mast bend, it may be necessary to back off the tension.

Upper and lower shroud tension – masthead rig: The initial rig tension should be high enough that the leeward shrouds do not go slack when sailing close-hauled in a brisk breeze. The proper tension for your boat can be found by a few test runs under sail and then the Tension Gauge can be used to record and maintain this value.
For many boats a shroud tension of 10% to 12% of the wire strength is adequate. In some rigs it may be advantageous to carry a bit more tension in the uppers than the lowers.

Upper and lower shroud tension – fractional rig: In most cases the same comments apply as for masthead rigs. However, there is one exception. Where the upper and lower shrouds on a fractional rig lead to chainplates located aft of the mast – swept back spreaders – most of the forestay tension is balanced by the upper shrouds. A shroud tension as high as 20% of the wire strength may be required to achieve the desired forestay tension. Never exceed 25% of the wire breaking strength.

The Loos Tension Gauge is designed for use on 302 and 304 grade stainless steel 1 x 19 wire rope. If used on other wire types the readings will be repeatable, but the actual tension values in pounds or kilograms will differ from those given on the table on the gauge.


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[john_morris_uk]

Of course you have to consider the hull structure when tightening your rigging, but its a pretty pisspoor design that can't take the rigging loads that are being suggested here. Hopefully the designer will have gone through all the dynamic and static loads and produced a hull with chainplates strong enough to allow the boat to sail safely.

I might suggest that too many boats have their rigs TOO SLACK for effective and efficient sailing.

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[roberth]

Thanks to all of you for your advice / ideas. Certainly have no fears about the hull being up to the rigging loads - heavyweight Holman & Pye suggests a pretty good pedigree to me.

Robert

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[macd]

Thanks. The Selden info (on www.seldenmast.com) is priceless.

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[mirabriani]

Be careful not to overload your backstay tensioner.
I was on board a Limbo when the helm yanked on the tensioner.
The forestay was pulled out of the foredeck and we lost the mast
overboard. 10 or 15 minutes of intense activity later we were motoring.
But not before the yard workboat came out to salvage us!

Regards Briani

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[fireball]

Extreemly handy - especially as I have to re-evaluate our rigging - slack forestay !!

Direct link to the advice for those (like me) had difficulty finding it!
<A target="_blank" HREF=http://www.seldenmast.com/download.cfm?download=7581&webnode_id=2099&filename=595-540-E.pdf>http://www.seldenmast.com/download.cfm?download=7581&webnode_id=2099&filename=595-540-E.pdf</A>

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