The second guide concerns the way that the sail fits the mast. Any sail will crease when the mast bends and the leech is pushed into the belly of the sail.
There is also a shortening of the distance between the mast blackbands. If the luff curve fits the mast bend, the creases will disappear when the cunningham is tensioned. Sometimes, however, these diagonal creases do not all disappear in the body of the sail no matter how hard the luff is tensioned. Obviously, then the luff curve and mast do not match. These creases are most persistent at the various hard spots in the sail. Often the tension line between the inner end of the bottom batten and the clew extends well into the sail. The point where the persistent creases, if extended, would reach the mast is the area of incompatibility. Usually this is in the area by the spreaders.
By standing in front of the boat it is very easy to see why the creases are there. The curve of the fullness is not fair and there is a distinct change in the profile. This marks the change over point between the area where the luff curve is not being used up. In other words, the sail is either too flat above the line or too full below the line for that particular mast.
The solution is to progressively stiffen the mast by lengthening the spreaders and angling them forwards until with light to medium cunningham tension the creases disappear.
The final guide is perhaps more visible on larger boats where the sails are usually flatter and the masts stiffer. Sometimes the luff curve of the sail and the bend of the mast do not match so well that all available fullness is used up. The sail becomes so flat that it cannot support the roach and it pivots to leeward above the straight line from head to clew. As the aft edge of the sail bends away, the inner end of the battens stick up slightly in a unsightly `S` bend. Power will be lost and in particular off-the-wind speed will be poor. The solution is to straighten the mast more and, perhaps, tighten one or two of the leech seams outside of the head clew line.
Experimenting with the mast is best done outside the race situation. In the race itself too many things are happening, making it impossible to concentrate on the work that the spreaders are doing. Try differing rig tensions because the slacker the rigging the less work the spreaders will do, and if possible try to see your boat from outside to calculate from a distance the effect of the changes.
This also has the advantage of showing you what your boat looks like to the opposition. Of course, the more organized it looks the greater the demoralising effect it has.
Many classes do not limit the position where the shrouds enter the mast. They merely restrict fore-triangle height. Heavyweight crews, or those with flexible spars, should consider raising the hounds to support the top of the mast.
Raising the hounds started when the spar manufacturers began to taper the tops of the masts more and more. Although this may be good from a windage point of view, it created and still creates, great problems. Boats that have new masts, but not new sails, found that they often went slower. The tops were simply bending too much for the luff curve and the sails lacked power. Extra luff round in the upper luff soon cured that. Even so, once the hounds have been left behind there is nothing the driver can do artificially to stiffen up the top of the mast. It all depends on the stiffness of the section.