Re: masthead versus fractional rig?
I apologize I wrote this for another purpose and is a bit long, but it discussed the pro's and cons of masthead vs fractionally rigged sloops, as well as the cutter rig.
Sloops and Cutters are the most common rigs being produced today. In current usage these terms are applied quite loosely as compared to their more traditional definitions. Traditionally the sloop rig was a rig with a single mast located forward of 50% of the length of the sailplan. In this traditional definition a sloop could have multiple jibs.
Cutters had a rig with a single mast located 50% of the length of the sailplan or further aft, multiple headsails and in older definitions, a reefing bowsprit (a bowsprit that could be withdrawn in heavy going). Somewhere in the 1950's or 1960's there was a shift in these definitions such that a sloop only flew one headsail and a cutter had multiple headsails and mast position became irrelevant. For the sake of this discussion I assume we are discussing the modern definition of a sloop and a cutter.
Historically, when sail handling hardware was primitive and sails were far more stretchy than they are today, the smaller headsails and mainsail of a traditional cutter were easier to handle and with less sail stretch, allowed earlier cutters to be more weatherly (sail closer to the wind) than the sloops of the day. With the invention of lower stretch sailcloth and geared winches, cutters quickly lost their earlier advantage.
Today sloops are generally closer winded and easier to handle. Their smaller jibs and larger mainsail sailplan are easier to power up and down. Without a jibstay to drag the Genoa across, sloops are generally easier to tack. With less hardware sloops are less expensive to build.
Sloops come in a couple varieties, masthead and fractional. In a masthead rig the forestay and jib originates at the masthead. In a fractional rig, the forestay originates some fraction of the mast height down from the masthead. Historically, sloops were traditionally fractionally rigged. Since the earliest wind tunnel tests (1920’s) it has been understood that fractional rigs with minimally overlapping headsails tend to generate the most drive per square foot of sail area. This efficiency advantage has to do with a variety of factors such as sail interation and minimized tip vortexes.
But fractional rigs make sense for a variety of more practical reasons. Their smaller jibs are easier to tack and in a building breeze, they are easier to depower and then reef down to a snug masthead rig. Because virtually all boats develop some weather helm with heel angle, reefing the mainsail, while leaving the jib, makes sense in terms of balancing the helm. But also because the jib represents a smaller portion of the overall sail area, one jib can often function across an extremely wide range of windspeeds. Fractional rigs generally place a lower stress on their hulls and often get by with lighter rigging and hardware for an equal structural safety margin.
Today, modern fractional rigs are often proportioned so that they do not need headsails that overlap the shrouds making them even easier to sail. One of the major advantages of a fractional rig is the ability, especially when combined with a flexible mast, to use the backstay to control mast bend and sail shape. Increasing backstay tension does a lot of things on a fractional rig: it tensions the forestay which in turn flattens the jib, and opnes the head of the jib. Increasing backstay tension induces controlled mast bend, which flattens the mainsail and opens the leech of the mainsail. This allows quick depowering as the wind increases and so allows a fractional rig to sail in a wider wind speed range without reefing, or making a headsail change than a masthead rig, although arguably requiring a bit more sail trimming skills.
To a great extent, the smaller jib on a fractional rig eliminates the need for a jibstaysail, with its added hardware, complexity, and the assocated diffuculty tacking a genoa around the jibstay, as might be found on a cutter or a sloop which had a jibstay when intended for offshore use.
In the past fractional rigs used to require running backstays. But today better spar materials and design approaches have pretty much eliminated the need for running backstays. That said, fractional rigs intended for offshore use, will often have running backstays that are only rigged in heavy weather once the mainsail has been reefed. The geometry of these running backstays typically allows the boat to be tacked without tacking the running backstays and larger fractionally rigged race boats will often have checkstays.
Masthead rigs came into popularity in the 1950's primarily in response to racing rating rules that under-penalized overlapping jibs (genoas) and spinnakers and so promoted bigger headsails. Masthead sloops tend to be simpler rigs to build and adjust. They tend to be more dependent on large headsails and so are harder to tack and also require a larger headsail inventory if performance is important. Mast bend is harder to control and so bigger masthead rigs will often have a babystay that can be tensioned to prevent pumping and induce mast bend in the same way as a fractional rig does. But dragging a Genoa over the babystay makes tacking a bit more difficult and slower. While roller furling allows a wider wind range for any given Genoa, there is a real limit (typically cited 10% to 15%) to how much a Genoa can be roller furled and still maintain a safely flat shape. As a result, masthead rigged boats generally require more sails in their sail inventories and more frequent sail changes to address the same wind range as a fractional rig.
Masthead sloops which are intended for offshore use are often fitted with a jibstay which allows them to fly smaller sails in heavier weather. Depending on the specific rig design, these often require running backstays as well, with a geometry which needs to be tacked in heavy air. The smaller headsails on fractionally rigged sloops generally eliminate the need for this intermediate jibstay. That said, as fractional rigs have been used on larger boats and with larger fractions (i.e. 15/16th) it has become more common to see jibstays added on frac's intended for offshore use.
Masthead rigs generally carried larger spinnakers, which in the past gave them an advantage on deep reaches and when heading dead downwind. But modern fractional rigs, often carry masthead spinnakers eliminating this former advantage.
Cutters, which had pretty much dropped out of popularity during a period from right after the end of WWII until the early 1970's, came back into popularity with a vengeance in the early 1970's as an offshore cruising rig. In theory, the presence of multiple jibs allows the forestaysail to be dropped or completely furled, and the resultant combined reefed mainsail, and the full staysail, results in a very compact heavy weather rig (similar to the proportions of a fractional rigged sloop with a reef in the mainsail). As a result, traditionalists often cite the cutter rig as the ideal offshore rig.
While that is the theory, it rarely works out that the staysail is properly proportioned, (either too small for normal sailing needs and for the lower end of the high wind range (say 20-30 knots) or too large for higher windspeeds) and also is either made of a sail cloth that makes sense as a heavy weather sail but which is too heavy for day to day sailing in more moderate conditions or out of a sail cloth too light for heavy going. When the jibstaysails are proportioned small enough to be used as heavy weather sails, cutter rigs will often develop a lot of weather helm when being sailed in winds that are too slow to use a double reefed mainsail. Like fractional rigs, cutter rigs intended for offshore use, will often have running backstays that are only rigged in heavy weather once the mainsail has been reefed. Unlike the fractional rig, when the running backstays are deployed, the geometry of these running backstays typically requires that the running backstays be tacked whenever the boat is tacked.
Cutters generally make a less successful rig for coastal sailing. Typically, because of their offshore intent, cutters tend to have snug rigs that depend on larger Genoas for light air performance. Tacking these large Genoas through the narrow slot between the jibstay and forestay is a much harder operation than tacking a sloop. As a result many of today's cutters have a removable jibstay that can be rigged in heavier winds. This somewhat reduces the advantage of a cutter rig (i.e. having a permanently rigged and ready to fly small, heavy weather jib).
Cutters these days generally do not point as close to the wind as similar sized sloops. Because of the need to keep the slots of both headsails open enough to permit good airflow, the headsails on a cutter cannot be sheeted as tightly as the jib on a sloop without choking off the airflow in the slots. Since cutters are generally associated with the less efficient underbodies that are typical of offshore boats this is less of a problem that it might sound. Cutters also give away some performance on deep broad reaches and when heading downwind because the Genoa is sailing in the bad air downwind of the staysail, and they generally can only fly smaller spinnakers as well.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies
Last edited by Jeff_H; 12-09-2013 at 02:40 PM.