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Old 17-05-2017, 16:05   #1
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How much to tension the lower shrouds?

Sitting on the front deck whilst sailing, I noticed that the front of the mast pivot slightly (about 1/8 of an inch) when the boom swung over. Concerned about this, I wrote to Sparcraft, send them some sketches and images. They replied that the lower shrouds needs tensioning. I checked and did indeed found that the lower shrouds where somewhat loose and needed to be tensioned.

I replied to Sparcraft and asked how much to tension the lower shroud. The reply I received is "until they are tight". To me, this is completely unclear - how tight is tight? Can anybody with some experience shed some light?

Also see the image which illustrates the problem. The mast is around 18 meter and is for a 50 foot catamaran.
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Old 17-05-2017, 16:06   #2
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Re: How much to tension the lower shrouds?

Sorry, here is the image.
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Old 17-05-2017, 16:14   #3
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Re: How much to tension the lower shrouds?

I just replaced my rigging. I found the explanations from experts baffling regarding tension.
Eg. Seldon, do the caps as tight as you can without being a gorilla?? Another rigger, intuitively tight???
Basically I took advice from handful of contributors on this forum. I tightentened them hand tight, then another two turns with spanners. I went sailing, put her on approximately 15 made sure the mast was in column and the leeward shrounds were on the edge of being slightly lose to tight.
It hasnt fallen down yet. Touch wood.
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Old 17-05-2017, 17:18   #4
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Re: How much to tension the lower shrouds?

Sweet looking boat.

Usually you tighten shrouds to about 20% of their breaking strength +/-5%. Enough so that when close hauled in 20kts the leeward shrouds aren't loose, or slack. As being too loose is almost as bad as is too tight.

However, if things are that far out of tune, it's time to retune the whole rig. Since tightening up your lowers may only cure one of the mast's symptoms of ill health. Given that where one problem is so graphically evident, usually there are also some which are hidden.

Also, it never hurts to go up the rig & inspect everything regularly anyway, so now's a good time to do so. Which, if you're not experienced in such things, try it anyway after reading a bit of Nigel Calder. And take a camera along to snap pics of everything, for later analysis, as well as for sharing for feedback. Then you might want to have a pro take a poke at things. And pay for his time to explain everything he found, what the options for fixing them are, ditto on the causes, all while paying him for this time (make it clear in advance that you want to do this).

And for anyone else reading the thread, the boat has swept spreaders, with diamond stays. No permanent backstay.


PS: When was the last time the rig was pulled for a full overhaul, or tuneup? And how old's the rigging?
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Old 17-05-2017, 22:44   #5
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Re: How much to tension the lower shrouds?

Maybe if you found another boat that also has a loos gauge you could get a starting point and then write it down?
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Old 18-05-2017, 08:05   #6
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Re: How much to tension the lower shrouds?

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Originally Posted by UNCIVILIZED View Post
Sweet looking boat.

Usually you tighten shrouds to about 20% of their breaking ...... was pulled for a full overhaul, or tuneup? And how old's the rigging?
The rig was manufacture and stepped in 2010 and has never been worked on since. However, the yacht is in Trinidad and part of the work we need done includes the mast coming down, some electronic stuff changed on the mast, etc. That will be the perfect opportunity to properly inspect and if need be, replace the rigging.
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Old 18-05-2017, 08:48   #7
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Re: How much to tension the lower shrouds?

Assuming that there's some specific tension you're looking for. How do you measure what tension you're at?
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Old 18-05-2017, 09:22   #8
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Re: How much to tension the lower shrouds?

You could put in an inch or so of prebend in your mast which strengthens it.

I did this on my old Bristol mostly out of habit from racing Beach Cats for years. I did it on the beach cats for sail shape and to strengthen the carbon fiber mast. More prebend as the wind increased etc

Anyway, on this old Monohull, I put 100 lbs more on the loos gauge on the forward lowers than all the other stays which gave the mast about an inch or so of prebend.

The forward lowers I have at 550 whereas all other stays are at 450 or so.

On my beachcats, I used the diamond wires to adjust the prebend. 700-1000.

I ended up just staying near 700 on the diamonds which gave me maybe 1 1/4" of prebend. I have forgotten what spreader rake I was at to accomplish this though. You'll have to experiement should you try this approach
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Old 19-05-2017, 04:43   #9
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Re: How much to tension the lower shrouds?

An interesting thread. I have a masthead rig on a Jeanneau 43ds. Had all standing rigging renewed last year after 12 years. The riggers said i should re-tension by one turn after 200 nm (which i did after stopping at 4,000 nm). I could do it on the cap shrouds but only 1/2 turn on the lowers because the lowers were really tight. No idea what the tension is now but it is "tight". No flopping of the leeward shrouds when sailing in F4-6. When the riggers setup the tension they never used any Loos gauge. Just by feel. Jeanneau suggests that the mast bend should be 1/2 of the forward-aft measurement of the cross-section of the mast and that is what i have.
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Old 19-05-2017, 06:11   #10
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Re: How much to tension the lower shrouds?

It's pretty impossible to say. First off, a loos gauge won't fit that wire.

Secondly some/many cats aren't stiff enough to fully tension all play out of the rig. The boat just bends more.

If the mast is rotating on the step by 1/8" it does sound like the step or mast butt could be worn possibly due to inadequate tension over time.

On a boat this size, you really need a qualified person to evaluate the rig under sail and preferably have that same person inspect the mast, rigging and step during refit.

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Old 19-05-2017, 12:13   #11
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Re: How much to tension the lower shrouds?

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Originally Posted by Boatguy30 View Post
It's pretty impossible to say. First off, a loos gauge won't fit that wire.

Secondly some/many cats aren't stiff enough to fully tension all play out of the rig. The boat just bends more.

If the mast is rotating on the step by 1/8" it does sound like the step or mast butt could be worn possibly due to inadequate tension over time.

On a boat this size, you really need a qualified person to evaluate the rig under sail and preferably have that same person inspect the mast, rigging and step during refit.

Jeff Goff
Talking about bending the boat, this was the main reason why we build the boat with epoxy, S Glass/Carbon cloth and Balsa. She is stiff - damn stiff! There is no wear on the step or mast butt - at least nothing yet. However, the lower shrouds are much loser than prior to her cross Atlantic voyage voyage.

Getting a qualified person - this type of work seems to be a bit of voodoo science mixed with experience and gut feel. This is what concerns me more than anything else.
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Old 19-05-2017, 12:18   #12
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Re: How much to tension the lower shrouds?

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Originally Posted by thomm225 View Post
You could put in an inch or so of prebend in your mast which strengthens it.

I did this on my old Bristol mostly out of habit from racing Beach Cats for years. I did it on the beach cats for sail shape and to strengthen the carbon fiber mast. More prebend as the wind increased etc

Anyway, on this old Monohull, I put 100 lbs more on the loos gauge on the forward lowers than all the other stays which gave the mast about an inch or so of prebend.

The forward lowers I have at 550 whereas all other stays are at 450 or so.

On my beachcats, I used the diamond wires to adjust the prebend. 700-1000.

I ended up just staying near 700 on the diamonds which gave me maybe 1 1/4" of prebend. I have forgotten what spreader rake I was at to accomplish this though. You'll have to experiement should you try this approach
There is already a huge amount of pre-bend in the mast - a little more and it will surely pass for a boomerang. But it was startling enough for us to questioned Sparcraft about it at the time. They assured us it is correct. But it does not seem that the lower shrouds are responsible for the mast pre-bend. It seems to be from the swept spreaders and its rigging.
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Old 19-05-2017, 16:29   #13
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Re: How much to tension the lower shrouds?

Quote:
Originally Posted by Revelations View Post
There is already a huge amount of pre-bend in the mast - a little more and it will surely pass for a boomerang. But it was startling enough for us to questioned Sparcraft about it at the time. They assured us it is correct. But it does not seem that the lower shrouds are responsible for the mast pre-bend. It seems to be from the swept spreaders and its rigging.
Yep, I should have read your post more closely, but I was on the phone and talking with someone nearby while reading your post and typing the answer

My last two catamarans (Nacra 6.0 and Nacra F-17) had rotating masts that sat on Teflon balls as I remember so if we had the prebend about right and the shrouds piano wire tight we considered that A-OK

Example:

https://www.google.com/search?q=nacr...=1495232751657
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Old 19-05-2017, 17:04   #14
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Re: How much to tension the lower shrouds?

Quote:
Originally Posted by Revelations View Post
There is already a huge amount of pre-bend in the mast - a little more and it will surely pass for a boomerang. But it was startling enough for us to questioned Sparcraft about it at the time. They assured us it is correct. But it does not seem that the lower shrouds are responsible for the mast pre-bend. It seems to be from the swept spreaders and its rigging.
On that type of rig the lower shrouds are there to prevent the middle of the mast from pumping forward. Tightening them will actually reduce the pre bend.

If the front of the mast is moving, but not the back, it would suggest to me that the mast may be raked aft too far, and isn't sitting flat in the mast step. How's the forestay tension?
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Old 19-05-2017, 17:09   #15
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How much to tension the lower shrouds?

IMO any rigger that does not actually measure the rig tension should be avoided! Otherwise it's a guess. Guessing on expensive structures is not professional. There is a great book available for this - Ivan Dedekam's Sail and rig tuning. If the rig is non rotating, like this one, and a cruising boat, here are some basics, if there is no tuning guide for the model concerned.
Forestay length sets mast rake. Start with 3 degrees. Now, with all shrouds hand tight on clean threads: Make sure mast is vertical - measure Masthead to chainplates on each side. Do it again to the toe rails - is the boat symmetrical?
Now, 15% of break load for the wire. Measure the elongation over a 2m length, elongation reflects load, and can be found online, from the wire manufacturer, or in Ivar's book. Use a steel tape measure, 2m above the swage on each cap. Then, tighten each one evenly till the desired elongation is reached. Check the mast bend - on this rig induced by spreader angle. It should never exceed 1.5 x D ( mast dia, or depth fore/aft). Sight up the sail track on the mast - if it's not in column, it's quite easy to see looking up the track. Tighten the lowers to 10%, ensuring the mast stays in column. Now go for a sail. With the boat fully powered up, the mast should remain in column. Adjust lowers to keep it there, remember same number of turns each side! It's ok if the leeward cap shroud is unloaded when fully powered, just before you'd have to reef, but it should never be loose and flopping around. Loose shrouds can shock load, with dire consequences!
Finally, some mast twist is normal - high end sailing instruments have compensation for this.
There is a great bit in the back of Ivar's book about preload on the rig, and why it actually reduces rig load when sailing. Unloaded rigging just means you have used up the preload.
A proviso here - it can also mean the mast step structure and attachment is flexing - not unusual in a multi.
All just my opinion and experience. Use this info at your own risk. :-)
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