Shroud tension question...

[mdevour]

This is a design question of sorts. I don't assume I can get an answer here, but I will appreciate your opinions.

I'm dealing with a Bermudan rig on a 24' sloop.

The original design specifies a mast rake of several degrees (set by a measurement from the mast truck to the transom), and a range of shroud tensions depending on how heavy the air is. A middle of the road setting would be, say, 400 pounds on the uppers and 200 pounds on the lowers.

The original mast was replaced by the previous owner with one made from the next larger size of aluminum extrusion.

My question: What would be the general effect of the stiffer mast on the desirable shroud tension settings? Is it reasonable to set it up the same, or should they be adjusted up or down?

Details of the boat and rig, should you need them:

The boat displaces 2200 lbs., has a beam of 6' 10", and draws 3' with a 675 lb. fin keel

The aluminum mast is about 26' tall, mounted in a tabernacle on the cabin top. The boom is about 10' long. The stays and shrouds are wire.

It typically carries a 130 sq. ft. main and 110 sq. ft., 180% genoa.

The backstay attaches to the top with the main halyard block and the tops of the jumper stays.

The forestay, genoa halyard block, jumper struts, and the top end of the outer shrouds attach at the 75% point.

The spreaders for the uppers, the bottom of the jumpers, and the lower shrouds attach at about 40% of the mast height.

The uppers and lowers attach to a single chain plate on each side, about a foot aft of the mast.

I hope that thoroughly describes the problem. I'll answer any questions I can if that's not enough information. :eek:

Rule of thumb answers are as welcome as detailed ones. Now that I have the mast rake re-set and the turnbuckles sorted out so I can actually set up the tensions, I'd appreciate any guidance you could offer me.

Thank you,

Mike D.

[Bob Cleek]

I always enjoy such questions. Exactly how do they suggest you measure the shroud tension? You could rig a scale in line with the shrouds, but then you have to take it off to set up the rig. They once made this little device that clipped on the stay and supposedly measured the tension, but what a crock!

Your rigging tension should be no greater than necessary to keep the mast straight and secure when under sail. The longer the cable (e.g. uppers) the tighter it will be, since the longer wire will stretch more. You don't have to worry about it if you adjust the tension under sail. It will work itself out. The leeward shrouds should be slack. Your headstays should not sag to leeward much, although most all do a bit in a blow. That's all there is too it. The important thing to remember is not to set them up too tightly. That is the most common mistake.

[mmd]

Rigging shops will have a device called a tension meter that test the deflection of the rigging wire over a known distance under a given force, and from that data the tension in the wire can be calculated. I used to use a similar device for checking aerial support wire and guy wire tension when I used to build aerial CATV systems. A simple device, really, but quite expensive - not your usual home-hanyman tool. You might be able to rent one for a half day or day. The one shown below is a digital unit, the one we had was an analog one with a dial guage.

http://www.dillon-force.com/products/images/mainquickchk.jpg

P.S. - The shroud tension should stay the same even though the mast section is larger. Be sure to frequently check the mast for straightmess ("in-column") while you are setting up the shroud tension. The object is to make sure the mast stays in a straight line, regardless of shroud tension and mast angle. S-bends are a serious no-no. Lay on the deck with your head at the mast stem and sight up along the mast both on the side and the forward axis of the mast. Close enough is not good enough, especially if you race or sail in heavy weather.

[ 08-15-2005, 09:14 PM: Message edited by: mmd ]

[mdevour]

Bob,

This is the tool I have:

Loos PT-series tension gauges (http://www.loosnaples.com/Loos2/PT.htm)

http://www.loosnaples.com/Loos2/tension.jpg

It's what people who are racing this class recommend, and whose settings they use as reference.

I'm not racing, but I figure I might as well use the class specs as general guidance in setting things up.

Working with it so far, it seems reproduceable and sensitive enough for my taste. I make no claims as to its absolute accuracy, however. ;)

I am leaning toward the middle to lower end of the recommended tension range. I will check the mast for straightness under way. The lee upper shroud does ease off visibly.

Thank you,

Mike D.

[mdevour]

Originally posted by mmd:
Rigging shops will have a device called a tension meter that tests the deflection of the rigging wire over a known distance under a given force, and from that data the tension in the wire can be calculated. Yep, same concept as the inexpensive Loos gauge. Again, no claims as to accuracy! :cool:

The shroud tension should stay the same even though the mast section is larger... The object is to make sure the mast stays in a straight line, regardless of shroud tension and mast angle. ... Close enough is not good enough, especially if you race or sail in heavy weather.My friend, the previous owner, always talked about how seaworthy these boats were, and I think he was beefing up the rig with heavier air in mind.

I'll work in this general range and make sure the mast stays straight. I suspect it won't be very hard, as the design is not over canvassed nor highly stressed.

Mike D.

[Ron Williamson]

Whatever you do,don't climb the mast without guying it to something solid.The boat will heel from your weight and could sink at the dock.
DAMHIKT.
R

[Ian McColgin]

I've hauled my 250# up smaller boats to adjust jumpers, so I'd not worry about going up that far. Just do it in a calm place and make sure your deck crew (if you're not working solo) stays put centered at the base of the mast. Certainly reaching the turnbuckles on the jumpers, which might be almost reachable if you stand on the gooseneck anyway, will present no overwhelming balance problems.

Anyway, gauges are ok for replicating the rig in initial tune-up. They are very nice in scientific fleet tuning. They are a great engineering tool in load measurement and in setting up static structures like shore towers. And gauges keep me from setting up quite so loose as usual, but they don't really save that much time. To tune correctly, you'll need to sail a little anyway, so here goes without gauges:

Get the jumpers set on the ground tight enough to put in just a little bend - just discernible. Sight the mast to ensure this bend is only forward - not at all side to side.

Since you mention a tabernacle, let's assume it has a proper pivot. Put the mast's heel in, hook up the lowers with turnbuckles a bit looser than they will end up. Depending on the boat's geometry, it may well be possible to hook up the uppers and backstay as well. Just make sure they are all a bit loose.

Bend the shackle end of the jib halyard to a line long enough to go through a shackle at the stem where the jib tacks and back to the mast base. Make sure the jib halyard and everything else are secure.

Have a pole with a crutch or notch end handy in the cockpit. Get under the mast near the transom and hoist to shoulder height. Grab your pole and get it under the spreaders. You may have to let it trail in the water at first. Use the pole to heave the mast up till it's pressed forward against the stays. Trim on the jib halyard's tagline and belay that off. Now the rig will stay up while you tune.

Get the mast fore and aft first. Since the backstay is "measured" perhaps you've got no turnbuckle. So, hook up the jib stay and tighten till it takes the jumper-set bow out of the mast.

Next, get the top of the mast centered athwartships. I use the main halyard for this. Make sure the lowers are slack and just fiddle the uppers till you have an even amount of main halyard to each chainplate.

I start a bit loose, always. In this size boat you'll be about right if you use a wrench or pliers or (traditional turnbuckle) a fid to hold the stay still but actually turn the turnbuckle by hand only at first. If you can get to centered and hand taught, that's nice. Then use your fid for mechanical advantage to add perhaps one full turn for initial tension.

Once the uppers are set, trim the lowers to hand taught such that the mast stays in column. Sighting along the track is best. Once in column, add perhaps a turn and a half to each side - always sighting to make sure the mast stays straight.

Now go for a sail. Note the weather helm and mast bend on each tack. Fix the weather helm and fore and aft bend in the mast first. If anything, the mast can have a little bow concave on the main luff. This will flatten the sail. Stare carefully. You can increase or decrease the mast's bow with jibstay tension. If you must make way too much bow to get enough tension to keep the jib luff from sagging off, then get to a quiet place to anchor and get up the mast to do the jumpers. If you must do that, slacken your upper and lower shrouds and start the athwartships centering from there, all over again.

Shouldn't be a big deal, as with the stiffer mast just a tiny initial bow should have given you enough room for adequate jibstay tension.

The lee shrouds will be a bit slack on each tack. I do NOT take all the slack out on one tack and then flop over as that will be way too much. Take a half turn on each leeward turnbuckle, tack and take a turn on the other pair. Now sight the mast and feel the lee shrouds. Tighten a little at a time till the lee shrouds are just barely loose in a Force 3 or Force 4 gentle to moderate breeze.

Then anchor and measure that the mast is still centered amidships and has no lateral bow at rest.

If all is well, get back underweigh and crack a beer.

G'luck

[Ian McColgin]

I've hauled my 250# up smaller boats to adjust jumpers, so I'd not worry about going up that far. Just do it in a calm place and make sure your deck crew (if you're not working solo) stays put centered at the base of the mast. Certainly reaching the turnbuckles on the jumpers, which might be almost reachable if you stand on the gooseneck anyway, will present no overwhelming balance problems.

Anyway, gauges are ok for replicating the rig in initial tune-up. They are very nice in scientific fleet tuning. They are a great engineering tool in load measurement and in setting up static structures like shore towers. And gauges keep me from setting up quite so loose as usual, but they don't really save that much time. To tune correctly, you'll need to sail a little anyway, so here goes without gauges:

Get the jumpers set on the ground tight enough to put in just a little bend - just discernible. Sight the mast to ensure this bend is only forward - not at all side to side.

Since you mention a tabernacle, let's assume it has a proper pivot. Put the mast's heel in, hook up the lowers with turnbuckles a bit looser than they will end up. Depending on the boat's geometry, it may well be possible to hook up the uppers and backstay as well. Just make sure they are all a bit loose.

Bend the shackle end of the jib halyard to a line long enough to go through a shackle at the stem where the jib tacks and back to the mast base. Make sure the jib halyard and everything else are secure.

Have a pole with a crutch or notch end handy in the cockpit. Get under the mast near the transom and hoist to shoulder height. Grab your pole and get it under the spreaders. You may have to let it trail in the water at first. Use the pole to heave the mast up till it's pressed forward against the stays. Trim on the jib halyard's tagline and belay that off. Now the rig will stay up while you tune.

Get the mast fore and aft first. Since the backstay is "measured" perhaps you've got no turnbuckle. So, hook up the jib stay and tighten till it takes the jumper-set bow out of the mast.

Next, get the top of the mast centered athwartships. I use the main halyard for this. Make sure the lowers are slack and just fiddle the uppers till you have an even amount of main halyard to each chainplate.

I start a bit loose, always. In this size boat you'll be about right if you use a wrench or pliers or (traditional turnbuckle) a fid to hold the stay still but actually turn the turnbuckle by hand only at first. If you can get to centered and hand taught, that's nice. Then use your fid for mechanical advantage to add perhaps one full turn for initial tension.

Once the uppers are set, trim the lowers to hand taught such that the mast stays in column. Sighting along the track is best. Once in column, add perhaps a turn and a half to each side - always sighting to make sure the mast stays straight.

Now go for a sail. Note the weather helm and mast bend on each tack. Fix the weather helm and fore and aft bend in the mast first. If anything, the mast can have a little bow concave on the main luff. This will flatten the sail. Stare carefully. You can increase or decrease the mast's bow with jibstay tension. If you must make way too much bow to get enough tension to keep the jib luff from sagging off, then get to a quiet place to anchor and get up the mast to do the jumpers. If you must do that, slacken your upper and lower shrouds and start the athwartships centering from there, all over again.

Shouldn't be a big deal, as with the stiffer mast just a tiny initial bow should have given you enough room for adequate jibstay tension.

The lee shrouds will be a bit slack on each tack. I do NOT take all the slack out on one tack and then flop over as that will be way too much. Take a half turn on each leeward turnbuckle, tack and take a turn on the other pair. Now sight the mast and feel the lee shrouds. Tighten a little at a time till the lee shrouds are just barely loose in a Force 3 or Force 4 gentle to moderate breeze.

Then anchor and measure that the mast is still centered amidships and has no lateral bow at rest.

Edited to add - Wish I could delete the double post. Sorry. Need coffee.

[ 08-16-2005, 07:34 AM: Message edited by: Ian McColgin ]

[John E Hardiman]

M. G. Devour;

You didn'y say wether this was an open trailer sailer or not, but from the size of the rig and the comment about different rig "tunes" I willing to guess it is. I agree with Ian that the rig should be set up by eye first. Make sure that the masT is not only straight, but centered in hull (a ribbon tape measure on a two-blocked halyard and measure to the chainplates works). Then measure the shroud tensions for reference. If it is a trailer sailer then this should ensure that you can get the rig back in the same "tune", however I would install levers on the shrouds and forestay so I didn't have to touch the turnbuckles (or pins if you have them). Most serious racers have different "tunes" for different race conditions on a boat this small as you noted. That is because the open hull on such a small boat distorts under the rig load (a larger boat distorts as well, but does not make such radical changes to tension). As Stuart Walker says, get a little black book and write it all down.

[rbgarr]

He said the mast is mounted on a tabernacle on top of the cabin, so she's not open.

[Ian McColgin]

Yeah, the over 25% total weight bulb keel is another hint.

But she could still be drysailed. Let us know if you raise and strike the mast a lot or just once a season.

One does tune for various weather and even for points of sailing. The lateral tune should not change much, but fore and aft might. As you get to know her, you'll find that bowing the mast and tightening the jib luff can both be done by tensioning the back stay and are desireable the more you go to weather and the windier it is. Next, you'll get a hydrolic backstay . . .

But for the start, don't just assume that the guy who had her before knew everything. If she's got a heavy weather helm in a moderate breeze at say 10 - 15 degrees heel and if she's correctly trimmed with crew weight aboard, you'll want to slack the backstay and lowers and trim the jib stay to pull the rig forward.

G'luck

[John E Hardiman]

Originally posted by rbgarr:
He said the mast is mounted on a tabernacle on top of the cabin, so she's not open.Sorry, missed that part redface.gif , but a Wylie Wabbit or a Santana 22 (Edit or a T-bird) is about the stated size and they are trailer sailed so....

[ 08-16-2005, 11:31 AM: Message edited by: John E Hardiman ]

[mdevour]

John,

No it's a decked cabin boat. Yes, there's a lot of folks racing it, and they do all the things you mention. ;)

My goal is to make sure it works and doesn't break. I'm not looking for the last nth of performance.

Ian,

Thanks for all the detailed advice.

It is trailerable, but I don't have a trailer. So it's once a season for now.

Actually, all I'd need to be able to take it apart in a hurry is to put a lever in the headstay. Do the thing with the genoa halyard and forked pole to control it coming down...

There is a tackle on the backstay. They advise you tighten it for pointing in heavier air, and ease it off the wind.

I assume the previous owner did not know what I do about the original rig. He changed everything, and I'm putting it more to rights by the design.

I'll have to see how bad the weather helm is. These boats are supposed to have the mast raked that far, and they're pretty good performers. I've always been able to balance the boat with the genoa trim. We'll see how it goes now.

Now if we can get any winds besides "light and variable" I can take her out for a shakedown! :rolleyes:

Thanks,

Mike D.