UK Sailmakers' Encyclopedia of Sails
4.9 - Jib Top
The Jib Top is a distance racing sail used for reaching when sailing with the wind forward of the beam, but not so far forward as to be sailing on the wind. Its draft is noticeably greater than that of a genoa and is positioned forward since the sail is not used to go upwind. It has a very high clew so that it can be set properly with the sheet eased, something a low-clewed genoa can’t do as well.
On a typical overlapping genoa, the leech is twice as long as the foot. Jibs on boats with non-overlapping headsails, the ratio of the leech to the foot is closer to 4:1. When the genoa sheet is eased, the load on the longer side of the sail is affected more than on the shorter side. Therefore, when the sheet is eased the clew lifts and the top of the sail twists open. As the top twists open the upper part of the sail luffs and reduces the driving force of the sail.
Excessive twist results in loss of power and distortion of the slot between the headsail and the main. Too much twist is typically cured by moving the jib lead forward, which in turn causes the whole back of the genoa to converge with the boat's centerline, which increases heeling and causes backwinding of the mainsail. By raising the clew on the jib topsail, the foot gets longer and the leech gets shorter. The closer in length the foot and leach get to each other, when the sheet is eased there will be less twist, better retention of power and more optimized air flow between the jib topsail and the main.
There are some other advantages to the high clew:
It's a great sail for races with reaching or running starts because the helmsman can see everything that's going on under the jib.
2. Not catching water
Once a genoa is eased enough so the foot is out over the water, catching waves as the boat pitches can be a big problem. Water is a lot heavier than air and if the sail catches a big enough wave, it can blow up. The high clew of the Jib Topsail usually eliminates this problem.
3. Room for a staysail
The big open area under the Jib Topsail is ideal for setting a genoa staysail, increasing effective sail area.
A less obvious but equally important feature of the Jib Topsail is its design shape. Because its purpose is to reach and not point, it has a very full, powerful luff. Think of the front edge of a spinnaker or a light No. 1. This powerful front end develops more forward thrust than a flatter "go-to-windward" sail.
BOATS WITH NON-OVERLAPPING JIBS
Many of today's new racing boats are designed with sail plans that have non-overlapping jibs. The J/122, First 40s, XP44s and TP52's are good examples of this design concept, and they all run into problems jib reaching. The regular jibs on these boats are extremely high aspect sails and the twist problem described earlier gets even worse. In addition, the head angles of such sails are so narrow that they have very little area up high so the top of the sail is almost useless on a reach.
A jib top with its higher clew, has a wider head angle. A wider head angle produces more area up high, which increases the power of the sail. See the diagram. Even though Jib Topsails for rigs with non-overlapping headsails are narrower than optimal, they are still a good sail to have in the boat's distance racing inventory for conditions where a Code Zero would make the boat over powered or when the boat is sailing a reach too close for a Code Zero.