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How should I set up a sail for heavy air?

2019 June 1

Dear Boat Doctor,

I own a Caliber 40 and have been working my way down the Windward Islands, planning to spend hurricane season in Grenada. The trip has been great, but we’ve been caught a little off guard by the wind in the islands. I have the boat set up for coastal sailing but I am not convinced I have the headsail optimized for the sustained higher winds of the Caribbean. How do I dial in the halyard tension and headstay sag to get the boat trimmed right?


Peter Favor

via email

Dear Peter,

You’re right to take a close look at your setup now that you’re sailing in consistently heavier air.  In a nutshell, you want to depower your sails in heavier air,  so you can flatten out the boat, point higher and ultimately sail faster with more comfort.

Let’s start with the headstay, which is probably a static adjustment on boat like yours. In stronger winds, the headstay will sag to leeward. This is important because your sail maker designed your sail with a certain sag in mind. If that sag is greater than what the sailmaker designed, the sail will have a lot of depth and a round entry, a slow speed, powerful wing shape. If there is less sag, the shape will have a fine entry, think of a low lift, high speed wing like a fighter jet. A flatter sail is desirable when you have plenty of wind and wish to point higher and depower the sail.

Your sail was likely designed with a 2% to 3% expected headstay sag, this is roughly a foot. Sag can be really hard to see, the easiest way to judge it is to setup a spare halyard right next to the deployed sail and go sailing in about 15 knots of wind. You will easily be able to measure the sag against the straight halyard.

You’ll set the headstay tension with the backstay largely, though if you can’t get enough tension you may need to shorten the headstay turnbuckle too. You likely won’t adjust this underway (without a backstay adjuster) I’d set it about a foot and roll with it.

Halyard tension impacts the position of the maximum draft of the sail. If the draft is too far aft, the sail will not have much forward drive and will just induce heel. Too far forward, and the sail will also lose forward drive.  You want the maximum draft positioned at roughly 30% back from the luff of the sail.  Further complicating things, the draft moves aft as the wind increases, meaning more wind requires more halyard tension to maintain the same shape.

I would start with a halyard tension that removes all the scallops from the luff, set this at about 15 knots and work from there. In heavier air, you’ll add tension, in lighter air you’ll ease.

Don’t be afraid to roll up a little genoa too, you should be able to carry a full 120% up to 15 knots, but you’ll need to take out successive bites as the wind increases.