Controlling Headstay Sag
A key "Go Fast" for any racer is the ability to control headstay sag and genoa shape. However I find that at the mid-fleet level this is often a concept not fully understood or applied correctly. For this article I am talking about conventional masthead rigs. While the same principals apply for fractional rigs, and masthead rigs with swept aft spreaders, the techniques are enough different to warranty an entirely separate discussion.
First it is important to understand that any headsail is designed for a specific range of headstay sag. This amount will have been determined by your sailmaker based on the wind range that the sail is to be used in, the actual amount of sag that is present across that range, and the amount of control available on the boat. To get the most out of your headsail you need to control the sag across itï¿½s range as conditions change. In the light end of the range you will want to increase sag to make the sail fuller and the entry rounder; on the upper end you will need to reduce sag to make the sail and entry flatter. Headstay sag is a function of the area of the sail, the amount of wind pressure on the sail, and the amount of tension in the headstay. As wind speed increases it exerts more pressure on the sail which will cause the headstay to sag more, if more tension in not applied to the headstay. As the headstay sags back it puts more shape into the headsail, just the same as decreasing forward mast bend puts more shape into a mainsail.
The main adjustments you have for controlling headstay sag on a masthead boat are the runners and or checkstays. They are NOT for controlling mast bend. If all you are concerned about is mast bend you donï¿½t need runners, just use more backstay when you want to bend the mast more, and use less when you want less mast bend. Runners and checkstays certainly limit the amount of mast bend you have but the reason you use them is to transfer more or less load from the backstay to the headstay. Imagine sailing along in 10 knot of breeze, the main is trimmed just right, you have good speed, but you arenï¿½t pointing well. The genoa seems to be too deep and is too round in the front. You can put on more backstay tension which does reduce the headstay sag, but now the mast is bent too much and the main is too flat. To correct for this you tighten your runners until you pull the mast back to its original bend; now the mainsail looks just the same, but the headstay has less sag and your genoa is flatter. In the same way if the wind lightens and you need more power you ease the backstay and the runners to keep proper mainsail shape but increase sag in the headstay.
I see too many people pull their runners tighter in light air and loosen them in heavier air because they are trying to use them to control mast bend; and this is just the opposite of what they should be doing. In general; the heavier the air, the tighter the backstay, the tighter the runners. Headstay tension and sag are critical elements of maintaining proper sail shape. Keeping the entry and depth of you headsail correct in changing conditions is at least as important, if not more important, than changing the shape of you main. Well sailed boats that finish in the top of their fleets are constantly adjusting their runners, checkstays, and backstay to optimize headsail shape; and you should be too!