Fractional rig

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A J/24 showing its fractional rig.









Throughout the 20th century, most sloops of less than 20 feet in length have had fractional rigs.

Many (perhaps most) sailing sloops from the first part of the 20th century were fractional rigged. The increased prevalence of fractional rigs on sloops in the early 20th century probably coincided roughly with the disappearance of the gaff rig. One possibility for its increased prevalence during this era is that, relative to a masthead rig, it allows the designer to provide the boat with more sail area without using a bowsprit or a boom that extends beyond the stern, and without demanding that the sails be of a very high aspect ratio. This was important because the sailcloth available in those days made it difficult to construct high-aspect sails.

As sailcloth improved in the second half of the 20th century, it became possible to construct sails of a higher aspect ratio. It therefore became more practical for sloops to be designed with the simpler masthead-rigged mast. Most production sailboats from the '60s and '70s were masthead-rigged sloops. Masthead-rigged sloops typically carry a larger spinnaker than fractional-rigged sloops, and therefore enjoy a speed advantage when racing downwind.

Fractional rigged sloops are starting to become popular again, especially for high-performance racing boats. A fractional rig allows the mast to bend more easily, which in turn allows more adjustment to the shape of the mainsail, especially when sailing upwind. Many people believe that a fractional-rigged sloop is faster upwind than a similar masthead-rigged sloop, especially as the wind strength increases. Most of the newest generation of fractional-rigged sloops fly their spinnakers from the top of the mast. These boats are said to have "masthead spinnakers", and this development gives the boat very good performance both upwind and downwind in all wind strengths.

Maintaining Forestay Tension[edit]

An inherent problem of a fractional rig is its inability to exert as much tension on the forestay as a similar masthead rig. The reason for this is because the attachment point of the forestay is not directly opposite to the backstay. Under certain conditions, the moment arm that results from these offset attachment points can be used to advantage, as this is an effective method of inducing bend in the mast, which in turn flattens the mainsail. If the distance between the attachment points is too great, a mast of average stiffness will be unable to apply optimum forestay tension, unless one of the following mast-stiffening design features is used:

  • Jumper Stays—These additional stays and their associated jumper struts limit the amount that the upper portion of the mast can bend, and thereby enable tension to be effectively transferred from the backstay to the forestay. Jumper stays were common in the first part of the 20th century. Their biggest disadvantage is weight and windage aloft.
  • Running Backstays—These are supplemental temporary backstays (one on each side) that attach opposite of the forestay. They are very effective at generating forestay tension, but make the boat more difficult to tack because one running backstay needs to be released and the other one applied each time the boat tacks.
  • Swept back spreaders—Swept back spreaders (or rather their associated shrouds, which attach to the deck significantly aft of the mast) provide significant fore-and-aft support to the mast, and thereby increase tension in the forestay. The biggest disadvantage of this scheme is that it makes it difficult to deliberately slacken the forestay for the purpose of making the jib fuller in light winds.

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