J/30 Class Association Forums J/30 Forum Racing Tools and Techniques Headstay Sag

How much headstay do you typically have at max wind range for #1 and max backstay tension on a beat ~ 14 knots TWS, smooth water/min. chop?

And how much bend do you induce in the upper section of the mast under these condition?

Mast is set up as follows: step full forward, blocked max aft at the partners, max headstay length, uppers at 43, lowers 45. Prebend is about 2".

Last year, I was timid about pulling max backstay, since I did not have the thru-bar installed. Installed it over the winter, so feel more comfortable going full backstay, but it sure looks like a LOT of bend, and I still have what looks like too much sag.

Ran across this rigging guide awhile back. http://www.alohaowners.com/pages/projects/rig/rig.htm
Puts big emphasis on how little backstay tesnion has to do with forestay sag on a frac. rig.
How do you like your though bar? I just finished installing mine, will go for first sail tonight.

Ok - so to much pre-bend will prevent you from adjusting the forestay. Mast rake tightens the forestay not pre-bending the mast. Pre-Bending the mast makes the mast shorter and forces lower section of the mast forward (think of squeezing a small straw between your fingers it bulges and gets shorter). For kicks loosen your rig to take the prebend out and leave equal pressure on the shrouds (try 25 on Loos) see how tight your forestay is now and then you should see that the backstay is more effective. You may also need to adjust the length of the backstay to make it more/less effective. Then tighten up the rig and watch the forestay so it does not go to slack. If it is too slack then tighten your lowers to remove the pre-bend.

The right amount of sag is based on the cut of your sail, windspeed and halyard tension.

David
#397

[This message has been edited by dbows (edited 05-21-2008).]

[This message has been edited by dbows (edited 05-21-2008).]

Thanks, the guide confirms my own experience in testing the impact of backstay tension on headstay sag. Also, now I understand why the main does not flatten so much putting on backstay.

My thru-bar went in fine this spring. I found it helpful to drill holes around the edge of the cutout (inside the lines, obviously!) to remove a lot of material fast and for less grinding with the Dremel disc. As one would expect, no noticable difference under sail, but a whole lot more confidence in putting backstay tension on.

Have basically set up as you suggest, with very little pre-bend, 1" - 2". Lowers are at 45 Loos, keeping the middle of the mast aft. Mast step is full forward, mast blocked back in the partners for max rake.

Perhaps I am expecting a straighter headstay than possible. Last night, in 18 knots on a beat, we saw about 3" sag. Our tacking angles were good ~90 deg, so maybe we are OK.

The straw analogy applies more to a masthead rig, where the center of the mast is more free to move forward. Looking at the loading on the mast, and considering the tapered section above the forestay, I don't believe there is much bending going on in the middle. The lowers go up only slightly in tension when I put the backstay full on at the dock, perhaps one Loos #.

Great discussion ! To answer your last question, 3 in of sag is not much at all, that's pretty tight. However, your tacking angles in 18 kts should be way less than 90, more like 80. You've basically got the concepts right, tighter rig especially lower makes tighter headstay. Here's my quick, easy tuning guide, when sailing upwind your caps (uppers) on leeward side should be just barely loosening (flat water tighter, choppy looser ). The lowers should be tight enough to keep the mast in column side to side ( light air , a bit of leeward sag side to side ). This will put you in the ballpark but you've got test and retest and record your own settings and numbers when you are fast.

The other trick is don't use the backstay unless you really have to. Keep tightening the rig, tightening the mainsheet and tighening the vang, especially mainsheet.

One other thing to mention is that your prebend needs to match the luffcurve of your main. The prebend numbers you guys have been discussing are pretty low and may not match how your main was cut. Good luck and keep up the great discussion.

mmark - you appear to be a new name on the forum. Welcome! What boat / hull do we identify you with?

I am new to the forum. Thanks for the welcome. If you look at the article about last year's NA's on the front page, you will see who I generally sail J 30's with. While I own and sail primarily a star and a finn, I've always liked the J 30's as I started sailing them in college when they were new ( and so was I!). Love the boats and love the people in the class.

The referenced tuning article made me say hmmm. Nothing wrong with the description or ideas. In fact it sounds like how we used to set up. Then I notice it is reprinted from Sail August 1979! Nothing new under the sun eh? After switching brands last fall after many years refining our tune, we basically have started over--with assistance from our sailmakers. Generally looser on the shrouds, more mast pre-bend, tighter headstay, and much more adjustments depending on wind speed and sea state. Twenty-five years and still learning...

Bob - so who is Beebop's latest sailmaker that requires the retune? What do you see different on the cut?

On Rhapsody, I've got max class headstay, mast base 1" from step and the "every day" sails as Quantum Main & Jib. The tune that works for them has the uppers tighter than the lowers. The "new sails" are North. I've played with different setups and last weekend raced with equal settings 1000# upper & lowers with light air predicted. The boat seemed to move ok, but as the wind picked up from 8 kts to 18 kts (not per prediction...), I didn't like the shape of the main. The luff was real baggy and the leach looked tight. Almost like the sail was cut fuller towards the mast. Any pointers for settings people like with North Sails?