Tuning the Carbon Mast

[Justin Scott]

I have had several emails about tuning the carbon rig so I thought it was time to collect people's thoughts and start a new thread. We have some previous threads on "rig tension and chocks" and "Headstay tension and sag". They are getting old and we have learnt some lessons since then.

Ray Sanchez-Pescador writes:- " I also read in the Forum bits and pieces for tuning the mast to the new sails.  Any more finalized thoughts on shroud pressure and forestay length for different wind conditions?"

Here is my take, but all opinions welcome.

Forestay length we dont change for different wind conditions. Once I got the length that gave me neutral helm, then it works in all wind conditions. I will measure my forestay and post it. As an experiment, I did try lengthening one breezy day to see if it opened the slot but the pointing deteriorated so much that it was not worth it.

Uppers need less adjustment than the aluminum rig. The carbon rig is more "self tuning"

Because the chain plates are so much futher aft and because the carbon rig can flex, the uppers seem need less tension than the aluminum rig needed. In most conditions I have been sailing with 27-29 on uppers. In really big breeze, I crank up to 35+, soley to protect the rig downwind.

I have my mast butt in the most aft position or second most aft position (on Bennett boat) and I look for 2" of prebend in most conditions and 3" in big breeze.

Lowers are really loose. I can hold them together with the uppers about 3' up from the chainplate. When I am sailing, I look to see that windward lower has tightened and mast has bent fwd nicely along mainsail luff. Too much bend = too flat at front of main, so tighten lowers. Too little bend = too deep main and backwinded by jib, so loosen lowers.

I chock the front of my mast BUT the chocks are not so tightly wedged as the aluminum mast. I have found that if I chock it really tightly, I get a little mast inversion low down on the mast. Mast chocks do not control headstay tension as much as they did with the aluminum mast.

[Steve LeMay]

We had a REALLY tight forestay on #22 for the big winds. Basically, we counted threads showing. Uppers had 5 showing, lowers were in the 6.5 slot and the Forestay had 3-4 threads showing. We felt great on point and speed. Saw 6.0-6.2 uphill ( GPS ) as a polar. If we dropped speed, we "burped" the main and Jib together and dropped the bow just a little. This really got the boat out of the 5. somethings. As the day lightened, we took 10 turns off the forestay and felt like that was a good thing.

Question: How do you guys keep from getting crushed in the 5-8 kt range? We got waterlined to death by the FT 10's and an 11 metre. Tried to drop the bow but that was much worse a death.

[Felipe Payet]

Because the chain plates are so much futher aft and because the carbon rig can flex, the uppers seem need less tension than the aluminum rig needed. In most conditions I have been sailing with 27-29 on uppers. In really big breeze, I crank up to 35+, soley to protect the rig downwind.

Thanks for the tips, Justin.  When you say 27-39, which Loos gauge is that with?  I assume the newer PT1 "pro" model?  (I ask because I have the old style LS91, so I'll be doing some converting).

Regarding mast chocks, how much thickness in chocks do you end up putting in? (I'm curious about the fore-aft location of your mast in the partners).

And finally, yes pls. do post the length of your forestay (or perhaps easier, jib halyard hoist measurement, and/or main halyard hoist measurement back to an easy to identify point on the transom, like the rear top edge of the rudder tower?)

Thx again,
Felipe.

[Justin Scott]

Question: How do you guys keep from getting crushed in the 5-8 kt range? We got waterlined to death by the FT 10's and an 11 metre. Tried to drop the bow but that was much worse a death.

5-8 knots is sweet spot for both point and speed for OD racing. Dont drop the bow at all. Provided there is not big chop, you can go max point and maintain speed.

You are at two disadvantages in PHRF. 1) At that windrange, waterline is going to count. Thats why the Ft 10 and 11 meter owe you time.  2) The Viper is deliberately designed to be easy and tactical to sail upwind. A wide slot gives the boat a very forgiving groove upwind and plenty of power even at max point. If I was going to modify the boat at all for PHRF, I would consider a second set of inboard jib tracks. Not as fun or as easy to sail but in flat water at that wind range ...

You didnt read it here. One Design Class illegal, but would help sail to the rating.     

[Steve LeMay]

I agree on the inboard tracks. We fooled around with " barberhauling" the jib in with the weather sheet and it paid BIG but for small periods of time, like 30 seconds. That would confirm a tighter sheeting angle. Have any of the sailmakers REALLY looked at luff curves and bend characteristics for the new rigs? I am sure Hyde has but has a US builder looked? I called an old buddy @ Quantum Charleston and he was hinting @ a Viper program. With as bendy as these top sections are, I would guess it is going to take several shots at getting it dialed in.

  Also to really mix it up, has the class looked at adopting laminated mains ie: Pentex? same price as dacron and a crap load lighter. Sure dont want to rock the class as it is growing like a virus! Sailing Anarchy can prove this.. Lots of skeerred competitors and interested parties!!!!!

[Jay Harrell]

has the class looked at adopting laminated mains ie: Pentex?

The posted class rules already state:

Mylar with Polyester scrim and Dacron are specifically allowed for main and jib. Nylon is specifically allowed for the spinnaker.

doesn't Pentex fall under "Mylar with Polyester scrim"?

[Justin Scott]

Sail materials are a detour for this thread. We can start a new one on sail materials. Briefly, because we have an "approved sailmaker" process. We liase with the sailmakers. We talk cost and durability. They propose materials that they can build with that are profitable for them at the price levels that we think are reasonable for the Viper. If all agree, then we approve that material. A material can be added if it comes down in price.  Hence Pentex and Mylar with polyester scrim has been approved because it is now so reasonable in price. We believe that having our sailmakers participate in the rules process is a good thing.

[Justin Scott]

So here are some of the measurements I promised you:-

Headstay length:- measured from center of cottor pin at tack to furthest point of tang at head, is 298 (757 cm) inches.

Spreaders:-

Length. Measured from where trailing edge of spreader touches mast, to the shroud.  26 1/2 inches (67.5cm)
As a quick approximate check, this should be 27 1/2" from leading edge of spreader to the shroud (it is more accurate to measure from the trailing edge.)

Angle (Important to have the same spreader length as I have in order to tune angle). From shroud to shroud at the spreader tips, my measurement is 48 1/4 inches. (122.5 cm)
I achieved this angle by having the small inner screw in the most fwd position and the large outer screw in the most aft position. Ie max sweep. The spreaders themselves have a series of holes on the inner edge. I found that you cannot use the aftmost hole, so I believe I am in the 2nd aft hole. This was the max sweep that I could achieve.

I found that by having greater spreader sweep, I achieved more pre-bend and more support for the spinnaker downwind with lower rig tensions. The lower rig tension allows the mast to "do its stuff" upwind.



I still owe you the exact measurement for my mast butt position.

When you have this measurement....this will be the set up I had for Charleston. It was effective in light air and breeze.   
 

[Ray Sanchez-Pescador]

Justin,

I finally had the opportunity to set up the mast per your numbers and go sailing.  I attached the Ronstan adjusters for the lower shrouds directly to the U-bolt because adding a small shackle meant that there was no way to add any pressure to the lowers even at the 9 setting.  Without the shackles, at the number 3 whole, I was able to hold the lowers next to the uppers about 3 feet above the U-bolt.  A 27 reading on the loos gauge gave me almost 3" in prebend.  The mast but was set all the way aft.  The forestay length and the rest of the measurements were set to your measurements.

We had a bit of trouble hoisting the new main sail as the cord seemed to fit tightly on the track (we'll be using silicone next time we hoist the sail).  In spite of placing a very small knot to attach the halyard, the luff of the sail looked loose, and adding cunningham pressure pushed the draft quite a bit forward.  ???

When we went sailing, the boat indeed had a light weather helm reminiscent of what I had before with the aluminum mast.  I raised the jib from the tack about 2.5" in order to get the telltales to fly with the cars all the way forward - it probably isn't enough.  I also noticed that the draft of the new main is quite deep, even with 3" prebend.  Is that your experience too?  If I reduce the prebend to 2" as you indicate, the draft will likely deepen.  ???

All in all the sails look great, the mast seems to behave well and the boat feels fast.  Thanks for your help with this process.  Next week we'll try our spinnaker.  We'll see how we do when we go to the HSR and meet other Vipers.

Ray

[Alan#38]

My mast inverted with the spin. up in over 25 kts of wind (the gust we were in may have been higher).  I posed a question to SuperSpars about how much tension I can have on the upper shrouds to avoid this and they sent the question on to Paul Young.  His reply is below:

*********************************************************

From: Paul Young [mailto:[email protected]]
Sent: 12 July 2008 02:58
To: Super Spars (Sales)
Cc: [email protected]
Subject: Re: FW: Shroud tension for Super Spars carbon mast - Viper 640

The mast needs to be set up with the spreaders angled far enough back to get 4 " of pre-bend measured at the spreaders when the rig is tensioned up on dry land. The new chain plates have to be fitted in the new position to achieve this.

In heavier air say 25 knots, the prebend can be increased to 5" by winding on the shrouds and the mainsheet should have a knot tied in it so that the boom stops short of the shrouds by 3". This stops the mainsail leach going forwards of 90 degrees to the centreline, and the mast losing the aft support component of the vang.

I have not measured the rig tension, so cannot give a Loos setting. The tension is not important since the upper shrouds join the mast above the forestay, as the wind gusts the shroud loads build up downwind, and the forestay goes slack, The tension when sailing always exceeds the tension measured on dry land.

What is important is that the mast simply bends forwards more as the shroud loads increase, and   what happens when the prebend is sufficient.


regards,
Paul.

[Alan#38]

I've talked this over with the local North Sails sailmaker.  He believes that I should be blocking the mast on the aft side to keep the mast from inverting.  I'd like to hear what others think about this approach before I try to rig something in this regard.  If I do something like this it would be a strap around the mast at the deck level.  I don't know whether a strap that low would do any good since there's practically no leverage at 2 feet above the base of the mast.

[Chris Shaughnessy]

FWIW my #76 came with a strap at decklevel - it wraps around back of mast and then goes into a jamcleat so you can tension it. Can send you photos if you want to try a retrofit.

[Alan#38]

Yes - I'd really like to see it.  This mast inversion problem is my biggest area of concern at this point.

Interesting that they just started adding this.  It shows that Kerry Poe at North here in Portland REALLY knows what he's talking about (but then I already knew that).

Thanks Chris - my email is: alans -at- schampsystems.com

Unless you just want to attach to the Forum.

[Ray Sanchez-Pescador]

Interesting.  I'd like to hear what Justin and Paul are saying about the strap thing.  Seems like one more rubber band to something that could be a problem.

Here is what I did, which I think matches Justin's setup.

1.  Mast butt full aft
2.  Spreaders bolted to achieve max aft sweep
3.  Added 2" to forestay (first carbon mast fleet order, thus too short)
4.  Measured prebend by using main halyard from top of mast to gooseneck - farthest point is somewhere around the shrouds.  No blocks at the deck.  Resulting Loos Gauge (black) number for 2" prebend is 25.
5.  Determined what pressure would result in 5" prebend and got 32.
6.  After the mast was tuned, I added enough blocks to the front of the mast at the deck only to fill in the gap (no pressure) - note that you need to take off about half the blocks when you increase your tension from 25 to 32.
7.  My lower shrouds are probably too long - I installed the Ronstan adjuster directly on the u-bolt and I have to rig the shroud on the #2 hole so that I get any tension while sailing.  As it is, I can run up the lower shroud next to the upper using my hand and I can go over 3 FEET above the deck before I can't keep them together.  If I added a shackle as it has been suggested, I wouldn't be able to tie down the lowers at all.

Unfortunately, I haven't had good winds to test the setup.  We'll be in Huntington Lake next week and we'll have to go through trial by fire.


Best regards,
Ray
USA 49

[Justin Scott]

I've talked this over with the local North Sails sailmaker.  He believes that I should be blocking the mast on the aft side to keep the mast from inverting.  I'd like to hear what others think about this approach before I try to rig something in this regard.  If I do something like this it would be a strap around the mast at the deck level.  I don't know whether a strap that low would do any good since there's practically no leverage at 2 feet above the base of the mast.

You were right. The strap low down has little effect. It helps put up the mast single handed.Rondar supplied it on its first boat. I guess it could marginaly help reduce fwd bend but pretty insignificant.
The primary ways to prevent too much fwd bend when sailing down wind are:-
1. Enough tension in the uppers. 30-35 on the loos gauge.
2. Prebend. I'm now sailing with of 3" -4"with the Hyde main .
3. Spreaders max aft.
4. In big breeze tight cunningham (it acts like the string in a bow) just before windward mark
5. BUT above all, not easing the mainsheet out too much. The leech of your main is your back stay.   

Agree with everything RS says. My lowers were also too long. I shortened them. I think the 2nd and 3rd round rigs had shorter lowers.
Also i dont love the shroud adjustors on the lowers. I note the class rules allow turnbuckles. I will probably go that route in the future.