old north tuning guide

[Mike Lebov]

anyone have a link to the old north tuning guide?

thanks!

Mike

[Peter Beardsley]

I think they've taken down the old tuning guide, so unless someone has a PDF of it that they can scan in, you may be out of luck.  But the main differences were changes to the mast butt position (old guide has it max aft), headstay length (old guide has it as max length), and spreader angle (old guide has it at 290-295mm). 

[Justin S]

I'm not sure about the concept of an "old tuning guide" for "old sails"

My mother had a sign outside her stables which read
We have horses for adults.
Ponies for children
And for People who don't like riding. We have horses who don't like being ridden

[Fletcher]

Here's one from 2017 with the 290mm spreader angle
north-viper-tuningguide-R03.pdf

[John Leyland]

I'm not sure about the concept of an "old tuning guide" for "old sails"

My mother had a sign outside her stables which read
We have horses for adults.
Ponies for children
And for People who don't like riding. We have horses who don't like being ridden

I've ridden one of those horses that don't like to be ridden until the tree branch took me off :-)

[Justin S]

Here's one from 2017 with the 290mm spreader angle

290 of sweep and 33 of tension on the uppers in 18-25 knots of breeze was a horrible combination and is still a horrible combination.  Digging up old tuning guides is a really bad idea.

As I remarked earlier old tuning guides for old sails is like giving a horse that does not like being ridden to a rider who doesn't like riding.  Its a bad idea.

We have learnt a lot about the boats in the last 5 years, and older sails need more help not less.

[GeoffDuckworth]

I'm not arguing, just curious what made that a horrible combo?

[Peter Beardsley]

I'm not arguing, just curious what made that a horrible combo?

With that setup, the rig is arguably too soft and prone to inversion downwind in big breeze, which is the most common cause of otherwise rare mast breakages.  The more spreader sweep you have and/or the tighter your upper shrouds are, the stiffer and safer your mast will be.  That said, sailmakers have cut the luff curves of their current mainsail designs to have spreader sweep of 290-295 mm by and large - maybe some around 300 mm, so deviating from that has its own set of issues from a performance standpoint, and a change to the class rule as to a new mandatory minimum spreader sweep (which I know a couple of people are debating proposing) could also cause sails to be obsoleted and a bunch of mainsail redesigns.  There's never a reason to have less than 290 mm of spreader sweep, and in general teams should not be sailing in big breeze with uppers that are too loose or lowers that are too tight. 

[Justin S]

That said, sailmakers have cut the luff curves of their current mainsail designs to have spreader sweep of 290-295 mm by and large - maybe some around 300 mm, so deviating from that has its own set of issues from a performance standpoint, and a change to the class rule as to a new mandatory minimum spreader sweep (which I know a couple of people are debating proposing) could also cause sails to be obsoleted and a bunch of mainsail redesigns.  There's never a reason to have less than 290 mm of spreader sweep, and in general teams should not be sailing in big breeze with uppers that are too loose or lowers that are too tight. 

Without claiming to have all the answers to tuning a mast. Here are a couple of tidbits of information:

1.  I measured 11 boats sweep at Annapolis. The range with one exception was 300-330. The one exception was illegal so I will leave that out of the equation. (I have purchased some insurance on that boat because I think the return will be better than bitcoin).
The median was 310 +/- 3 mm.
I did not measure Peter B's mast.
2. We have set up Dragon Lady with 360 mm, because we wanted a noticeable experiment.  All of the existing inventory work well. In two conditions, more sweep seems noticeably faster. In other conditions, it seems about the same.
3. I had a beer and chatted with Ched Proctor who designed the upwind sails for North.
The sailmakers did not design the sails  to a specific spreader sweep. They designed the sails to a specific amount of prebend. There was no science to 295. Once they had the sails going fast, they wanted customers to be able to replicate their settings. The mast they had been working with had 295 of sweep, the loos settings gives the mast the right amount of prebend if you have the exact same mast butt position, same sweep etc..
If Ched was allowed to play with spreaders, he would increase sweep and lengthen the spreaders.This would lead to different rig tensions but it would not obsolete the sails.
Asked about the speed advantage of more sweep, it did not surprise him that it appears faster upwind in light air. The challenge in the Viper in light air is to induce some prebend in the rig to maintain flow. In light air, a straight mast creates a deep main which light air cannot stay attached to. At the same time, you want a soft headstay. Ched tried various ideas to induce some bend in the rig including a line from the bow to the mast with 4:1 purchase which the TC advised was going to be illegal.  More sweep will allow light air prebend at a lighter up rig tension and thus a softer headstay.
As to the clearly perceived difference downwind in moderate planing conditions, we talked about this, without a clear conclusion but whether through tight uppers or more sweep, anything that allows you to open some twist in the main without "snaking " the headstay is a good thing. 
4. Applying a similar logic, without the benefit of the right conditions to test the increased sweep, we have pondered the impact of more sweep when sailing upwind in the top end of the windrange. We speculate that we can achieve the same amount of pre-bend with better rig control and a tighter headstay.  Hopefully we shall get a chance to see. 

New sails will invariably be faster than old sails...but a change in spreader sweep is unlikely to make your existing inventory obsolete.

[Peter Beardsley]

I'm looking forward to sailing against anyone at Noroton next week who has 330-360 mm of sweep and seeing how that goes - bearing in mind that a lot of teams will be new and it won't be a perfect test.