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Backwinding: Mainsail luff curve

Yesterday when sailing on Battlewagon, I noticed that their mainsail never backwinded, even in relatively strong wind.  In particular, I noticed that is was significantly less deep than ours.  It had a shallow draft — much more shallow than on PERSPECTIVE.  There was about an inch or two of pre-bend in the mast.

And that made me think about the connection between pointing, backwinding, pre-bend and the design of the mainsail.  It seems to me that we didn’t have as big a challenge with backwinding prior to this season.  But now that the forestay is shorter and we are pointing higher (especially with the flat #2 and #3 headsails), we have this annoying backwind in the mainsail.

Intuitively, we all want to clean it up, and we believe that there is more speed to be found when we do that.  But, the steps we take to clean it up come at the expense of the mainsail leech shape, and generally we don’t gain any speed — sometimes we sail faster with a pronounced bubble in the main.

Rig tuning is clearly a factor, as I wrote recently.  But what if the mainsail is simply cut too deeply?

This morning, this idea popped into my head, and it made me think about how difficult it had been to point with the new Doyle #3 headsail before it was re-cut and the forestay shortened.  That headsail had been designed with a deep shape to give the boat lots of power, assuming that it was the only headsail the boat would have.  The main we are using now (also by Doyle) was also designed at the same time with the same designers, making the same assumption:  no overlapping genoa, therefore lots of power needed from the mainsail.  In other words, the sails were built for power, not pointing.

Before last season, Gil and I measured the Dole mainsail and headsail  and the original Quantum #3.  Sure enough, there was a huge difference in the luff curve of the two headsails.  When we switched to the original #3, we could point higher.  With that in mind, we used the Quantum #3 until it fell apart (it had been damaged by the sun during the first season — another story).  And based on that experience, I had North sails recut the Doyle #3 until it is the nice flat headsail we have today.  (and that also lead to shortening the forestay).

But we never measured the original Quantum mainsail to compare it with the current Doyle mainsail. So, this morning I did that.  Here’s the comparison:

Bingo!  The original Quantum mainsail (blue ink) has 2 1/4″ less luff curve than the newer Doyle mainsail.  It is definitely shallower than the Doyle mainsail.

I’d like to try the experiment of using this mainsail in the upcoming races to see if we can tame the backwinding without compromising speed.  Yes, we’ll need some pre-bend too, but maybe only half as much.

Some additional thoughts

  • The Quantum main is in very good shape.  Unlike the #3, it was not exposed to UV light.
  • If this works out, our standard sail plan may well be the Quantum mainsail and the #2 genoa, shifting down to deep powerful sails (main and genoa) only for very light winds, when power is more important than pointing.
  • We’re likely to have less weather helm with a flatter mainsail — better pointing, better balance, better speed.
  • I’ve noticed that Top Gun has two mainsails — a new black one for windy days, and their older transparent one for light wind days.

Let’s give it a whirl and see how it goes.



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