Mainsail Trim: Adjustable Backstay

I did not learn to sail on vessels with adjustable backstays. It wasn’t until I acquired Skiron that I started to understand this important way of controlling both the main and headsails. Now, I would feel at a loss on what to do if I were sailing a vessel without the important functional control.

My adventures with the backstay started in response to a video of sailing across the SF Bay that I had sent to the previous owners of the boat. They noted forestay sag and shaking and suggested that I might remedy that situation if I tensioned the backstay. The next time I experienced such, in wind just over 20 kn, I tensioned the backstay and noted a dramatic settling down of the the vessel, less weather helm, and better pointing abilities. Clearly, I had some reading to do followed by some experimentation under way. I learned a lot. I will focus on the effects of the adjustable backstay on the mainsail in this entry and then on the effects of the headsail in a future post.

The adjustable backstay runs from the stern of the vessel to the aft part of the masthead.

The backstay should be tensioned in heavy winds and mild to moderate sea states. When tensioned, the top of the mast is pulled down and backwards…….this puts a bend in the upper half of the mast, with the bend facing the bow. When this bend takes place, it stands to reason that the upper two-thirds of the luff of the sail will be pulled away from the leech. This flattens and depowers the mainsail. The draft of the sail moves aft so the halyard must be tensioned to move it forward again. In my opinion, the outhaul should be tensioned whenever tensioning the backstay so that the lower third of sail is in unison with top two-thirds. It is important to watch those tell tales streaming back from the main and adjust the mainsheet as is needed.

Sighting up the mast one can see the bend induced by tensioning the adjustable backstay.
Adjusting the backstay while underway.

In lighter winds and heavier sea states the backstay should be detensioned. When the backstay is detensioned, the mast bend is diminished, the draft becomes fuller, more rounded, and moves too far forward. The halyard should be detensioned to move the maximum draft toward aft. The outhaul will likely need adjustment. The end result is more power and a vessel that is easier to steer but pointing abilities are sacrificed.

I’ve found that appropriate use of the sail controls can effectively deal with weather helm and stabilize the vessel. I’ve even been able to sail on without reefing in winds of 20-25 kn using the controls to my advantage. I do reef, especially, when winds of that magnitude are sustained. I have learned, however, that in the microclimes of the SF Bay Area, if you reef at 17 kn of wind you will soon have 12 kn of wind in a wind shadow feel underpowered, and then experience gusts to 22 kn. These other ways of controlling the mainsail, to power up or depower when necessary, make much more sense in a rapidly changing windscape.

If you have completed this series of mainsail trim blogs you might be wondering how I like to adjust the mainsail to sail downwind. Here is my general approach: tension the boom vang; move the traveler to leeward; play out the mainsheet. If the downwind course will be for more than an 30 to 45 minutes or so I also like to detension the halyard, the outhaul, and the backstay. In following seas with a significant swell I employ a preventer.

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