Masts and rigging

Mast and rigging

Masts and booms are defined as "spars", stays and shrouds form what is known as "standing rigging", that is the category of equipment which holds the sails, while the term "running rigging" groups other equipments (halyards, sheets) which have the function of continuously adapting the sail configuration to the changing wind conditions. Excessive rig deformation, allowed by a non-sufficient system stiffness, has the negative effect of changing the expected pressure distribution on the sails, decreasing the propulsive efficiency of the boat. On the other hand a certain amount of flexibility is necessary to allow the mast to be bended in order to allow the sail to have a proper shape relatively to the sailing condition. As a consequence, mast and rigging should have "reasonably resistant" section. Because the mast is the leading edge of the mainsail, a large section has the effect of creating a high pressure area behind the mast, neutralizing a significant portion of the main sail, thus reducing the total propulsive force and rotating it athwartship. In addition, the rig system has a very high centre of gravity and an increase of its weight has negative effects on stability and on the capability of the boat to "stand" the wind. This can be counterbalanced only by increasing the keel weight, and so the total displacement of the boat. Excluding unstayed masts which are predominantly used on vessels under 10 meters, sailing yacht spars are sustained by a three-dimensional rigging system made up by shrouds in the transverse plan of the boat and stays in the longitudinal one. Stays and shrouds are connected to the boat in correspondence of proper reinforced hull points.

mast-and-rigging

Common locations for headstay and backstay are the bow and the stern, while shrouds are secured athwartships the mast by chainplates. Both shrouds and stays are connected to the top of the mast in case of a masthead rig, and below the masthead in a fractional rig. Diagonal shrouds are connected near the spreader roots and, in oder to avoid higher compressive loads, angles below 10-12° are not recommended. In the longitudinal plane space availability allows the stay angle to open up to 30° and more while, in the transverse plane, maximum shroud angles are limited by the reduced hull breadth. To avoid long unsupported spans that may cause buckling phenomena, masts are then fitted with spreaders, in a number to keep the shroud angle over 10°; the highest spars can have up to 6 spreader levels. Shrouds can be continuous or discontinuous; the continuous solution consists of full-length shrouds, with constant section, from the mast attachment point down to the chain plates. The discontinuous solution consists in separate spans from two sets of spreaders connected at the spreader end with mechanic links. In the longitudinal plane aft of the mast, the mainsail requires unconstrained space so that it becomes difficult to set support points for the mast at intermediate heights. The way the mast is supported depends on the type of rig: in a masthead sloop the mast is sustained by a forward headstay and an aft backstay, while in a fractional sloop the mast is sustained by a forestay and, aft backstay attached to the top of the mast and by running backstays attached in correspondence of the forestay. In the cutter configuration the mast has an additional support ahead, a babystay and, optionally, running backstays after.

The presence or not of running backstays depends on the nature of the yacht: in a cruise yacht it is preferable to avoid the runners in order to make the boat easier to be handled, whilst it is necessary to set them on a racing yacht in order to better trim the mast and achieve best performances. For all the considered configurations spreaders can be set in line with the mast axis or aft swept in order to give ad additional support in the longitudinal plane. Aft sweep of spreaders greater than 15° often negates the need for runners. The type of arrangement heavily influences the performances of the boat and the strength of the mast as well. So it is very important to consider adequately the proper configuration in view of a verification of spars and rigging. Masts can be either deck-stepped or keel stepped. Deck stepped masts are used in boats which need to be trailed or to pass beneath low bridges on channels, because masts can easily be raised without needing a crane. For large sailing yachts keel stepped mast is preferable, mainly for its higher resistance with regard to bending, compression and buckling. This is due to the higher efficiency of the lower end constraint and to the contribution of the through-deck passage, which can be considered an additional constraint. On the other hand the mast below deck represents a considerable encumbrance for cabin layout and it heavily influences the interior layout.

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