Many of us will be aware that to trim our sailboat we have various options open to us. This includes sheets, halyard tension, car position, vang or kicker and traveller position. In addition, we can change a sail’s shape and efficiency by adjusting leech tension and create or reduce draft by adjusting the outhaul.
In addition to trimming sails using these control lines, racing boats in particular and smaller keelboats with limited capacity to reef will also rely on the backstay and cunningham in order to change the shape of the sail and power and depower the rig.
The backstay on many yachts is only adjustable by an experienced rigger and requires a deep knowledge of the subject. However, other vessels will have an adjustable backstay meant to be adjusted to suit conditions and point of sail. The adjustment might be by way of a simple pulley system, much like the main sheet mechanism. Other backstays may be hydraulic and they are operated by a pump and release valve. Larger cruising vessels might have an electrically powered backstay.
Adjusting the backstay will either increase the bend of the mast (backstay tension on) or decrease bend of the mast (backstay eased). It’s usual to ease backstay downwind and put on backstay tension when going upwind.
If one over tensions the backstay, it can have a significant effect on the boat, depowering the mainsail. This may well negate the need for a reef. Over-tensioning an adjustable backstay and/or easing or dumping the vang will significantly depower the boat.
Downwind, you would usually keep the vang under tension and ease the backstay although it’s important to realise that the primary purpose of the backstay is to brace the mast and stop it falling forward or breaking!
Fractionally rigged yachts (where the spreaders are spread back and the forestay does not go to the top of the mast) will usually have additional bracing in the form of a running backstay. These are used to help brace the mast below the masthead, where the forestay meets the mast or where an inner forestay might be used. Again, adjustment of the running backstay can alter mast bend and have an effect on boat speed.
Remember, when adjusting rig tension it’s important to release tension gradually so as not to create large ‘shock forces’ which will, over time, induce significant wear and tear to the rig.