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Rig tuning process for dummies please

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Fleet
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Initially Posted - 03/30/2011 : 04:42:31

I'm trying to understand how to set my rigging properly and I'm getting confused on how to properly start. I'm going by EP's tuning guide, setup for medium air and let's use that as a start. Do I start with all the upper and lower shrouds hand tight? Take slack out of the backstay and adjust the forestay for 10" to 12" rake measured at the deck? Now tighten the upper shroud around 26 - 28 per Loos while verifying the mast is straight? Now I need 1" prebend. Is that bend forward or back? How do I measure that and which lower shroud do I tighten to achieve the 1" bend?

Fleet
Kamikaze
1983 Hull #397
Lake Guntersville, AL

Edited by - on

joearcht
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Response Posted - 03/30/2011 : 06:09:05

I went through the same agony after purchasing my boat and tried setting the rake using a plumb bob as some suggested. I was not satisfied with the results or accuracy. I then learned from Chris Mckillip and/or Harstick sails that the proper length for the forestay was 31'-10" pin to pin. Once that was set I've been happy with the results. I took careful measurements at the turnbuckle so I can repeat the setup after a mast take down. I have the Loos gauge, but I've not relied on those readings and don't even know what they are right now.
I set the cap shroud by trail and error trying to keep the strain on the system as low as possible. I think the tightness is relative to the winds you are setting for and what is needed to keep the tip of the mast from bending off too much. The pre-bend is the question I've mostly agonized over. Some information I've read has lead me to believe the bend is forward and some aft. I think part of the problem I've had is defining what "bend" means. For the sake of clarity, I'd define bending aft to mean that the center of the mast is aft of the line between the mast tip and base. So what I've done is tighten the aft shrouds which puts the bend toward the aft of the boat. I then just hand tighten the forward shrouds (you don't need the forward shroud fighting the aft shroud for the pre-bend). To measure the pre-bend I just pulled a halyard tight to the base of the mask and gauged its distance from the face of the mast at mid-height. I'm still not sure if this is right, but I've been able to get the boat to go fairly well this way so I'm sticking with it until better information comes along. It may be that the sail can be cut for the bend to go either way, I just don't know for sure. maybe someone else will add to this thread and clear this up once and for all.

Edited by - on

SailCO26
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Response Posted - 03/30/2011 : 07:28:03

I've always called the bend "forward" as in our case it's really the middle of the mast that's moving and not the top. ie the center of the mast is farther forward than the top and base, or the bend at the center is toward the front of the boat, or the bend at center is away from the main and toward the headsail.

Shrouds:
I set with the backstay off and the forestay measured at 31-10 (I also make a mark on the boom with a weighted halyard so I can rough-check this with the stick up, but rarely actually do anything with it); shrouds hand-tight PLUS whatever it takes to get the stick square side-side (use a halyard with a weight hung over each side - it should contact the deck at the same place on both sides). I do not set this based on Loos numbers, just hand-tight PLUS what it takes to center. On race day shrouds are adjusted at the dock for the expected conditions using the Loos gauge to keep both sides equal.

FWD lowers (pre-bend):
[Yes, pre-bend with the FWD lowers. Otherwise you're pre-bending the mast with the bend to the AFT! You dont REALLY want that, right? :)]
Measure pre-bend by holding the main halyard tightly in the sailtrack at the base of the mast. Eyeball the distance between the halyard and the aft edge mast at the greatest point. Add pre-bend by tighening the fwd lowers (afts should be slack at this point). Sight up the sailtrack in the mast from the base, using the P or S lower to keep this straight. This fwd pre-bend will also keep the mast from inverting or pumping in heavy chop.
Edit: btw, I intentionally didnt mention a target distance for this pre-bend - that depends on the cut of the main (ref your sourcing loft) as well as the age/condition.

AFT Lowers:
[On a masthead rig with square spreaders the backstay really just tightens the snot out of the headstay and compresses the mast into the deck, but with the fwd pre-bend the mast will be encouraged to bend further forward - again that's the center of the mast more forward than the top/base. Under sail this will both stiffen the luff of the headsail and flatten the center of the main - both useful things in higher winds.]
Once the fwds are set, put max crank on the backstay. Tighten the aft lowers to just past hand tight (setting the max bend amount here), again using P/S to keep the sailtrack straight. Release the backstay and recheck the mast column. If out, use the Fwds to re-adjust then back to max backstay and adjust the Afts. You should be in column both backstay off and max.

Once in the water I may make some fine tuning to the pre-bend if the main looks too loose or too tight, depending on the age/condition of the sail.

Jim

Edited by - SailCO26 on 03/30/2011 07:36:26

Fleet
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Response Posted - 03/30/2011 : 13:31:41

Interesting. I don't have the time to take down the mast and set the proper length for the forestay. Can I cheat right now and adjust the forestay for 10" to 12" of aft rake? Its interesting that one is using aft prebend and the other is using forward prebend.

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SailCO26
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Response Posted - 03/30/2011 : 14:15:08

That'll get you in the ballpark.

Re fwd/aft pre-bend, if it works use it. I can't imagine intentionally doing that (I've seen masts "invert" and go aft, but IMHO that's basically what the lowers are there to prevent). But apparently it works for Joe.

Edited by - on

GaryB
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Response Posted - 03/30/2011 : 18:51:10

I'm by no means anywhere near an expert but I'm pretty sure I remember reading an aft-bend can break the mast under conditions that would not break it if it's got forward bend.

I do know that forward bend is the correct direction. Once you set the pre-bend, if you run a line from the top of the mast to the base on the aft side of the mast the center of the MAST should be forward of the vertical line.

I'm sure others will chime in shortly and clarify this for all of us.

Edit to clarify mast instead of boom.

Edited by - GaryB on 03/31/2011 21:13:50

Fleet
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Response Posted - 03/31/2011 : 03:42:21

The length of Capri boom is approx 9.5 ft. So, if I understand correctly, the center of the boom (approx 4.75 ft from the gooseneck) will be just forward of vertical line from the top of the mast? That seems way excessive, especially with EP is calling for a 1" prebend and a 10" - 12" rake.

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SailCO26
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Response Posted - 03/31/2011 : 06:22:09

Whoa! While 4.75 would be half of 9.5, where does that center point being forward of a plumb line from the mast head come from?

10-12" of rake is plumb measured from the masthead to the deck (ie straight down). Thus the mast head will be 10-12" farther aft than the step, by that same measurement - but not 4.75'. That's windsurfer kinda rake angle! ;)

As the boom is higher up the mast than the deck/step, measuring rake at the boom would be a number slightly LESS than 10-12".

Jim

Edited by - on

joearcht
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Response Posted - 03/31/2011 : 18:18:31

I'm just getting more and more confused. The following is what I got from the EP sails tuning guide:

"The aft lowers will control the amount of pre-bend in your mast. For one of our mainsails we recommend 1” of pre-bend unless you are sailing with a very light crew and need a little flatter mainsail. Maximum pre-bend should be 1.5”. The forward lowers are set with just enough tension to balance against the aft lowers so that the middle of the mast stays centered. It should not fall off to leeward, or bow out to weather."

Unless I'm measuring everything all wrong, when I tighten up on the aft lowers first my mast bends aft (center of mast is aft of a line between the mast tip and mast base}! Now one thing I really have not checked out yet is what happens to this pre-bend when I put the backstay on hard. Does the mast straighten out or does it invert and the bend goes forward? I'm sailing with an Old North sail and do not have the original design specs from the previous owner. Last season was my fastest season yet, but if there is more speed to have I sure want to figure it out and I really don't want to lose a mast if I'm torquing it wrong.
Am I interpreting the EP tuning guide correctly?

Edited by - on

SailCO26
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Response Posted - 03/31/2011 : 18:27:31

I've read that too, but it doesnt make sense. The only thing I can figure is that by "aft lowers will control the amount of pre-bend in your mast" they mean LIMIT the amount of pre-bend (or really backstay induced bend). If you think about it, pre-bend going AFT will increase the fullness of the sail - more aft bend (IF that's what you could accomplish by loading the backstay) would only cause MORE draft in the sail, NOT flatten it - right?

The lowers balancing the middle is spot on, with the FWDs being used w/o backstay and the AFTs being used full backstay.

If you do a search in this forum, this has been covered before. In fact, if I recall correctly a couple of springs ago Harry (EP Sails) came on himself and explained what to do. Search for posts by "EPSails".

Edit: Here's what he had to say (bold mine):
<blockquote id="quote">quote:<hr height="1" noshade id="quote">
Originally posted by epsails ((http://www.catalina-capri-25s.org/forum/topic.asp?TOPIC_ID=17540))

Basically your headstay length sets the amount of mast rake you have, the backstay controls how much tension you have on the headstay and how much you bend the mast, the cap shrouds keep the top of the mast centered side to side. In a rig like the Capri 25 that doesn’t have in-line lower shrouds it is the combination of the tension on the forward and aft lowers keep the middle of the mast from sagging to leeward and help to control mast bend.

Pre-bend in a mast does two things; it helps set the mast to fit the mainsail you have, and it makes the mast more stable by reducing mast pumping and giving it increased lateral stiffness. While you can induce some pre-bend with just shroud tension that is really not an option unless the rig has swept aft shrouds and spreaders. In a Capri 25 you would have to have the cap shrouds extremely tight to really induce pre-bend and that is not recommended because having the caps that tight would then require having both lowers set up very tight to keep the middle of the mast from sagging to leeward as the wind increases; and having them that tight would greatly limit how much control you have of mast bend.

With in-line cap shrouds pre-bend is induced by having a fulcrum that the mast is bent around, either chocks in the mast partner with a keel stepped mast or the forward lower shrouds with a deck stepped mast. To measure pre-bend you have to pull on the back stay just enough to take the slack out of the headstay. With the backstay just tensioned you can make the forward lowers tighter until you get the amount of pre-bend you want. After setting the forward lowers the backstay is used to control both headstay tension and mast bend, and the aft lowers are used to control how much mast bend you will get. As the wind increases your headstay will sag more and make the genoa fuller. This is countered by pulling the backstay tighter which increases the headstay tension and decreases the headstay sag. At the same time it bends the mast more which flattens the mainsail. However you will quickly reach the point where the mast is bent as far as it can go without distorting the mainsail and making it too flat, and that will limit how much tension you can get on the headstay. The aft lower shrouds counter this bending component by limiting how far the middle of the mast can bend forward. They need to be set up tight enough so that they keep the mast from over bending for you mainsail when the backstay is pulled on to its maximum.

Just a note on pre-bend; class rules limit the mid-girth of the main, how wide it can be across the middle. If you build a sail for more pre-bend the sail has to have more luff curve to fit the bend. For every inch of luff curve you add to the front of the sail you have to move the leech roach in an inch to keep the mid-girth the same. Since leech area adds much more performance in helm balance, pointing, and off the wind exposed sail area we build our mains with a minimum of luff curve and as much roach as we can. Our sails are designed to have only ½” to ¾” of pre-bend, just enough to stop the mast from pumping too much.

Harry Pattison

<hr height="1" noshade id="quote"></blockquote id="quote">
I'd like to say once again THANKS to Harry and EP Sails for not only continuing to support this fleet but being active with the owners as well.

Jim

Edit II:
Forgot to mention the North sail. I used to be a North junkie myself and originally had NS inventory on my CP25 - the boat was purchased from another North wh0re. This year the last one (spin) is finally being retired! That said, the NS main I had (which was probably 99-00 vintage) liked only about 4-6" of rake.

Edited by - SailCO26 on 03/31/2011 18:42:24

Ericson33
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Response Posted - 03/31/2011 : 20:11:39

I am working tonight so I will make this short and sweet.

1. loosen everything - use your halyards to steady the mast.
2. take the boom off
3. get something to wedge under the mast and step, also a peeny goes in there.
4. raise a small line attached to a plumb bob all the way to the mast head, set the mast rake at the gose neck to 10 to 12 inches. set wedges under mast, leave bob on the mast for now.
5. hoist a 50' tape up to the mast head.
6. measure side to side to get the mast in column, use same base point on each side, tighten side stays.
7. my final number on loose guage was 32-34 - mast is now set side to side.
8. tighten forward lowers to bring center of mast forward at the middle to 1". Mine was at 28 on loose guage.
9. hand tighten the aft lowers to 22. when its windy you tighten the aft lowers before heading out.
10. now its time to adjust the head stay, I had mine loose in light air, about 4" of slack up the extrusion.
11. backstay will flatten the forsail by making the headstay flatter at the luff.
12. for a genoa - say 150% the genoa cars are on the small tracks right at the cockpit maybee 1'-0" from the genoa winches.
13. keep the slot open, leave the main sheet alone and adjust the traveler in puffs.
14. the genoa drives this boat, the masthead rig is simple and when setup is easy to sail well in light to mid-air range.
15. heavy air - reef the main and get a 100% jib, body weight is a must.
16. keep the boat flat, keep from moving from side to side, keep the boat flat.....
17. in light air, stay on the low side, it will help the boat track better.

Keep your head out of the boat, look for better air, other boats and have fun.

Chris

Edited by - on

GaryB
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Response Posted - 03/31/2011 : 21:18:00

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by GaryB
 I'm by no means anywhere near an expert but I'm pretty sure I remember reading an aft-bend can break the mast under conditions that would not break it if it's got forward bend.

I do know that forward bend is the correct direction. Once you set the pre-bend, if you run a line from the top of the mast to the base on the aft side of the mast the center of the MAST should be forward of the vertical line.

I'm sure others will chime in shortly and clarify this for all of us.

Edit to clarify mast instead of boom.
<hr height="1" noshade id="quote"></blockquote id="quote">

I mistakenly mentioned "Boom" in my earlier post. I meant to say "Mast".

Edited by - on

joearcht
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Response Posted - 04/01/2011 : 05:23:37

Thanks Jim and everyone else,
I'm going to re-tune and I expect to blow away the competition now!
The secondary explanation from Harry and Chris's summary are clear enough.
I just got really turned around by the EP tuning guide and my own lack of knowledge.
I'll give it a try and see what happens.

Edited by - on

Fleet
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Response Posted - 04/03/2011 : 04:39:41

Modified my rigging yesterday, just before the big race and what a difference. With noticeable forward pre-bend, I was now able to point almost as high as the big boys. Prior to the adjustments, I was always 4-5 degrees lower than the bigger boats. Now I'm probably within 1-2 degrees of them. A little more fine tuning and I may be able to make up the difference. Thanks guys.

Edited by - on

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