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Rig tuning and backwinding

One of the ingredients of our success yesterday evening was almost certainly some renewed focus on rig tuning.  So far this year, I dialed in something approximate and have left it at that.  It was a conversation with Doug Folsetter that got me back to thinking about it.

I described to him the fact that our mainsail was getting heavily backwinded above about 8 knots of true wind speed.  First he said what others have said — that it’s okay to sail with the backwinded ‘bubble’ in the luff of the mainsail.  It can be a very fast mode.  But then he went on to describe how he’s tamed that situation on his Viper 830 — a tighter rig tuning with significant pre-bend.  After making these adjustments and fine tuning for a couple of years, they are now able to keep their mainsail powered up in conditions where it used to be ‘inside out’.

Why?  Well, I’m no expert, but here is what I’ve been able to piece together.

  • When the wind fills the genoa, the forestay pulls the mast to leeward at the hounds (where the forestay and the upper shrouds meet the mast), unless there is enough resisting tension in the upper shrouds.
  • When the wind fills the mainsail, the mast bends to leeward all along its length.
  • This effectively increases the depth of the mainsail along the mast
  • Air flow in the gap is insufficient to support that much depth in the mainsail, whereas there is plenty of air flow on the windward side and the net effect is an inverted pressure gradient near the mast, that creates the bubble.
  • The bubble extends aft until the airflow coming off the outer surface of the genoa meets the leeward face of the mainsail, and the desired pressure gradient is restored
  • Tightening the backstay in these conditions is only partly effective, because it tends to bend the mast laterally as well as forward and aft

So, to combat the mast bend to leeward, the shrouds have to be much tighter, beginning with the uppers but also including the mids and lowers to keep the mast in column laterally, when the sails are under load.  And once this is established, then the backstay adjuster will induce mast bend in the forward/aft direction, flattening the luff of the mainsail.

And, to flatten the luff of the sail, the mast should be pre-bent, using the shrouds to induce some forward and aft bend even when the backstay is loose.

Last night I tuned the rig differently than we had done previously this year:

  • very firm uppers, lowers and mids
  • no pre-bend

The results were positive — with the backstay adjuster somewhere near half stroke we could balance the boat well, even though we may have had too much headsail area, and we could remove most of the bubble most of the time using plenty of twist on the mainsail and managing gusts with the traveler.

But I want to build from this to see what happens with some pre-bend, by softening the mid and lower shrouds 5 mm each compared to last night.  I’ve built a new tuning chart that I think should accomplish this.  We’ll start applying it and keeping notes.


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