Backstay Adjusters

By Stephen Jensen:

Back stay adjusters are one of the more important controls on the San Juan 21. No racer should be without one. By tensioning the back stay you bend the upper portion of the mast aft, this flattens the upper ½ of the mainsail and opens the leach. It also increases the head stay tension, reducing the head stay sag and fattening the jib as well. This depowers the rig and can reduce heeling forces by about 5 degrees. That can be important on our somewhat tender boats. A Jib downhaul is a good tool to measure headstay sag. Tightening the downhaul line while the jib is up will show the distance between the forestay and the downhaul line.


This picture is of the back stay loose and mast straight. Note the amount of draft in the main and the distance the jib downhaul line is from the forestay.



In this pic, the back stay adjuster is full on and the mast is bent back.  Note how the main is flatter and the draft position is further aft and the forestay sag is reduced.



With the side effect of the draft moving aft, pulling some Cunningham is usually needed if you keep the tension on. The back stay adjuster is pulled on when the wind starts to overpower the boat, usually when the boat heeling over 20 degrees.  Remember to tension the Cunningham to move the draft back to its desired position.  When going down wind, you want to ease the back stay all the way off.  The reason is not to give the main a fuller shape, because when the boom is all the way out the bend in the mast is not flattening the sail shape, but  the forestay sag powers up  the jib for downwind running.


There are a couple of styles of backstay adjusters.

The easiest is just a 4:1 or 6:1 set of blocks. Some go so far as 12:1. A safety wire line is needed in case the blocks or line fail so the mast won’t fall forward.  The downside of this style (and it is minor) is that the attachment point is off to the side of the hull so it pulls the mast to the side.

This is the standard adjustable backstay using a 6:1 block setup on Charmed Juan. This unit is un-cleated by lifting up on the line. It replaced a fiddle block 4:1 system that was un-cleated by pulling the line down.  This caused problems because there wasn’t much room available to un-cleat.





A Split Backstay adjuster solves this problem and allows you to route the control lines to both sides of the cockpit so it can be adjusted from either side. When you are heeled way over and your backstay adjuster is at the back of the cockpit, and on the low side, you will appreciate having the adjuster closer to you.  Some of the faster skippers use the backstay adjuster as a primary control like the main sheet.


Here is a split back stay system on Wooglin. The control lines are lead to the cockpit on either side so the helmsman can adjust the back stay from either side.