Shroud Tensions

The shrouds should be operating equally in each tack ... so therefore you should set up with the Loos gage to be reading the same for each side. If this doesnt solve the anomaly, then begin to look for chainplate, chainplate base and mast step 'issues'.

Probably the best comprehensive 'mast and rig tuning guide' on the 'net is from Seldén Masts.
This guide is downloadable from: http://www.seldenmast.com/page.cfm?id=6740 (choose your language)

RichH said:

The shrouds should be operating equally in each tack ... so therefore you should set up with the Loos gage to be reading the same for each side. If this doesnt solve the anomaly, then begin to look for chainplate, chainplate base and mast step 'issues'.

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I agree 100%. The rig must be balanced on both sides first using the Loos gauge as a guide. Then check the shrouds (and forestay) after a couple of days to see if the tension you dialed in is still there. If not, find out why and address it. The rig is only as good as its support in the hull and deck.

The rule of thumb I've seen for shroud tension is 20% of the wire breaking strength on the uppers and 10% on the lowers...but this is just a rule of thumb. The true test is sailing hard on the wind with full sail and the boat about to be overpowered. This is the point of maximum load on the rig. If the geneoa cars are adjustable, make sure they're in the same position on both sides of the boat.

Examine the leeward shrouds while on one tack and then the other, then bring the boat upright (heave-to or head up) and tighten the shrouds that were slack as needed. Make small adjustments, as adding tension on one side also adds tension on the other side. Go back to sailing hard on the wind, examine the shrouds again and repeat the process until the leeward shrouds are no longer slack on either tack. You now have as much tension in the shrouds as you need and no more. Measure the tensions with the Loos gauge and record them in your log book for future reference.

These are still not absolute settings, as sail condition and crew weight also affect the load on the rig. However, you now have a reproducible point of reference for your rig. This is very useful if you trailer the boat and lower and raise the mast frequently. You can reduce rig tension if you're going to be away from the boat for a while then reset it quickly when you get back. If you leave the shrouds fully tensioned, periodic checks with the Loos gauge can also warn you of deterioration in the rig and its support points.

RichH said:

Probably the best comprehensive 'mast and rig tuning guide' on the 'net is from Seldén Masts.
This guide is downloadable from: http://www.seldenmast.com/page.cfm?id=6740 (choose your language)

Click to expand

RichH,

The Selden guide is excellent. The person who helped me with rig tuning must have read it.

If the tensions arent kept constant, the elasticity and flexure of the rigging wire can cause it to 'relax' and change its dimension.
New wire especially, needs to be stretched out to the intended loading several times until it begins to take its final dimension.

Glad that you solved your problem.

Shroud tension Oday 272 LE question

Oday 272 LE - when I stretch a halyard down front of back side of mast it appears mast has a bend in the middle - bowed with bulge toward starboard side. I used shroud tensioning to get the mast straight - maybe not the best method. When sailing in 10-15 kn the lee side shrouds are both loose. Wanderer 138 in the response to prior post recommend shrouds on both sides be tight under stress of wind. Hmm - not sure what to do to correct mast bow and apply proper shroud tension. Suggestions? The manual says "hand tight" turnbuckles.

"hand tight" in most 'mechanical situations' usually means the very tightest or very MAXIMUM that a normal male human with a normal hand strength can *possibly* turn it.

Get a gage or read the (above listed) Seldén guide for tuning without a gage. ;-)

That Selden tuning guide is very comprehensive, thanks for posting that. Also, the ruler used to check cable stretch was very interesting.

Please correct me if I'm wrong, but won't the tightening of one side also tighten the other corresponding shroud?

Unless they're not each pulling at 180 degrees to each other. Never mind!

to Justsomeguy - it seems to me that tightening top vs lower shrouds can rebend the mast which is bowed in the middle near lower shrowds. It appears to have worked but not all shrouds are equally tight according to the gauge and the lee side does not limp on both upper and lower lee sides in winds 10+ kn.

Reading the selden files, they appear to favor actual measured cable stretch or a tension meter. I seem to recall something similar in an aircraft manual, but I cant put my finger on it.

I dont know if would have a different effect on your mast, but its worth a try following their recommendations. Maybe its bent?

justsomeguy said:

Please correct me if I'm wrong, but won't the tightening of one side also tighten the other corresponding shroud?

Unless they're not each pulling at 180 degrees to each other. Never mind!

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The forces will have to balance out - so changing one shroud's tension will affect the others. With so many shrouds on the boat and the boat not a perfect rigid body - it is not likely to be an easy item to figure out.

OC

luby said:

to Justsomeguy - it seems to me that tightening top vs lower shrouds can rebend the mast which is bowed in the middle near lower shrowds. It appears to have worked but not all shrouds are equally tight according to the gauge and the lee side does not limp on both upper and lower lee sides in winds 10+ kn.

Click to expand

Yep. My mast has some funk to it as well, straightened to a slight curve.

So far, so good.

so here is what I found out.
The back stay is off center to starboard. Therefore if you tighten up too much, the mast bends in that direction also.
Snug up the back stay first. I went to 27 on the loose guage, then do the uppers at 30 and the lowers at 27. It worked for me.
Anything more on the back stay and, and it started going to starboard.
bob

My Catalina 270 has two spreaders. I can not find a good answer on what the middle shroud tension should be. Seldon says my upper shroud should be at 15% and the lower shrouds should be less.

Every boat I've ever owned came with a mast rigging guide. It is very basic.... set the rake, get the mast straight, adjust the tension.... therefore, when you get to the tension adjusting stage....first at the dock, then under sail.... for every adjustment you make on one side, you must duplicate it on the opposite side or else........ the mast will no longer be straight. You do not need a Loose gauge to straighten your mast, or even set your tension for that matter. I suggest you keep the gauge in the toolbox until you get the mast straight, then bring it out as a tension guide. No matter what the gauge tells you, if the mast looks crooked, it's crooked and you must get it straight.

The gauge is especially helpful for racing sailors who may adjust their rig tension per the daily conditions, or trailer sailors who use turnbuckles instead of pin stop shroud adjusters. But, you do not need it to adequately tune your mast.

LakeOntario270 said:

My Catalina 270 has two spreaders. I can not find a good answer on what the middle shroud tension should be. Seldon says my upper shroud should be at 15% and the lower shrouds should be less.

Click to expand

For a plain vanilla rig tension set up, dont forget to include 'pre-bend' of the mast. For a rig with one spreader 'set', the normal 'prebend' or the amount of forward bow is about 3/4" of bow, for multiple spreader sets 1/2" forward bow per spreader set. The prebend will help stiffen the mast which will help alleviate mast pumping and induced vibrations; plus, your sailmaker usually always cuts the mainsail expecting this 'prebend' to be present. Without prebend a mainsail will set with more draft than what was designed ... causes a bet more heeling and slower (powered -up) sailing. Without prebend, when sailing in extremely high wind strength, the mast can bow towards the stern of the boat thus allowing more draft and a significant 'power-up' of the mainsail ... prebending keeps the mast straight under the more extreme conditions, instead of bowing 'aft'.

So to answer your question about lower shroud tensions and to include 'prebend' ....
Set up with the forestay and backstay somewhere near 12-15% tension, set the main side stays (cap shrouds) at near 12-15% tension ..... then, set the tension in the lower stays to about 10% tension (equally, working up to that 10% by small increments - the goal is to not induce any bend in the midspan of the mast .... yet).
Then, again in small increments of adjustment, tighten up both forward lower shrouds while releasing tension in the aft lower shrouds (readjusting so there is NO 'side-ways' bend) until you get an approximate 3/4" forward 'bowing' of the mast (the bowing will be 'towards' the front of the boat). The aft lower shrouds should probably be somewhere at approximately 8-10% final tension and the forward lowers may be as high as 18% ... depending on the latent stiffness of the mast. Boat manufacturers use masts from various mast manufacturers, so the tension in the lower shrouds to arrive at the usual and proper 'prebend' is arbitrary.
To measure the prebend, use your mainsail halyard stretched between the top of the mast and the bottom of the mast, you (should) know the halyard diameter so simple 'eyeball' the space that develops at the midspan between the halyard and the mast. If youre using a tension gage, check for the forward lowers to be equal in tension and the same with the lower aft shrouds, ... record the amount of tension in especially the lowers so you can easily return to those tensions if for any reason you change the tensions in the future.

Then of course, go sailing on a hard beat to windward and make any changes to tension in especially the lower shrouds in order to keep the mast (on a side to side basis) straight, record and/or change your records for future use.

;-)