Composite FittingsJan 1, 2003
Not many years ago, composite masts were a rarity. They were seen mostly on racing boats where problems of design and construction were worked out each time a mast was built and used. Now, composite masts seem to be standard equipment on everything from conservative cruisers to BOC race boats.
The acceptance of composite (or carbon fiber; the terms are often used interchangeably) masts in recent years is due to their reliability, strength, and considerable ability to reduce weight aloft, greatly enhancing a boat's stability and sailing performance. However, these spars use very specialized material. How does one go about attaching fittings to such a mast? Can fittings be added by the average sailor or must specialists perform these tasks?
Most sailors are accustomed to aluminum masts. And one of aluminum's positive characteristics is its wall strength. Regardless of a fitting's orientation on an aluminum mast, support will be provided by the mast wall. (When attaching items to an aluminum mast, however, isolation of dissimilar materials is crucial. Destructive electrolysis will develop if aluminum and other metals, such as stainless steel, are allowed to come in contact.)
Most riggers, and some sailors, are proficient in drilling, tapping, bedding, and mounting hardware onto an aluminum mast. Proper bolt sizing and use of a sufficiently large backing plate are important considerations. Aluminum mast strength is gauged by wall thickness and tube shape, the values for which are listed in easily used engineering tables. Ian Nicolson's Boat Data Book lists recommended mast wall thicknesses, mast section shape, and weight per foot for aluminum masts. For example: a 50-foot boat would normally use an aluminum mast with a wall thickness of .25 inches and an elliptical shape of 12 inches by 8 inches, weighing approximately 10 lbs per foot. This information is provided in tables because of aluminum's uniform molecular structure and standardized mass production techniques.
Composite masts, unlike aluminum, are not extruded from an ingot, but built up in a mold, similar to a boat's hull. Their construction technique, combined with the characteristics of carbon fiber, and the coring material (if used), make mast selection, fitting attachment, and repair greatly different from that of aluminum.
Depending on the manufacturer and intended mast use, composite masts can be built in a variety of ways. Generally, the procedure is to place carbon fiber material, similar to fiberglass cloth, in a mold to which epoxy resin is then added to saturate and bond the carbon layers together.
Each manufacturer has its own technique and theory on mold design and construction, as well as the temperature and pressure necessary to produce the strongest and lightest mast. Autoclaves and vacuum-bagging techniques are used in various combinations to produce a carbon fiber structure. In essence, a composite mast needs to be free of voids, have adequate curing of epoxy resin, and have its carbon fiber material properly oriented to support mast loads. Fiber orientation
Material orientation is a key feature, for, unlike aluminum masts, which show support equally in all directions, a composite mast may not be designed to handle side loads imposed by a later addition of a winch or other piece of hardware. Some composite mast manufacturers use a honeycomb coring material to increase wall strength, similar in concept to coring a boat's hull. Coring is often used in composite booms, where high side loads and multiple loads from sheets, vangs, and lifts can all be acting at once.
When hardware is fitted to a cored mast or boom, care must be taken to provide adequate backing plates to spread loads and to apply bedding.
Composite masts and booms are usually sheathed with a covering of fiberglass to insulate the composite from chafe, contact with metals, and the sun's ultraviolet rays. (The resin used in composite masts is degraded by exposure to UV light.)
Most America's Cup boats are equipped with composite masts which, to save weight, are not painted and appear in their natural black state. Being exposed to UV light during rigors of the America's Cup competition does not make for mast longevity. Keeping this fiberglass or paint coating intact is vital to protecting carbon fiber material. This is most important at the base of a mast, where an aluminum or fiberglass shoe provides support and is usually in contact with salt water, which finds its way to this low area.
Mast structures, regardless of material, are primarily designed for the stresses of local buckling. When a mast is supporting sails and rigging, it is the downward, compressive forces that a mast wall must carry. On aluminum masts it is uniform wall thickness and mast shape that accepts these loads; where side loads are imposed, at spreader bases and at deck level internal compression sleeves are usually installed.
On a composite mast, since it is laid up from carbon material which is injected with epoxy resin, wall thickness can be tailored to meet calculated loads. Wall thickness is usually greatest at a mast's base and least near its top. Thickness is increased at spreader bases, mast partners, rigging tangs, and other areas of high or eccentric loading. Carbon fiber material that is added to accept these loads is usually tapered into a mast wall to help distribute stresses.
Additionally, mast tracks, goosenecks, mast caps, radar brackets, and mast winch bases can all be built as part of a mast. By building and bonding these components to a mast, metallic fasteners, point loading, and corrosion are eliminated.
If a fitting is added to a composite mast after construction, mast wall thickness, load orientation, and method of attachment all have to be considered. When dealing with a composite mast the attachment of winches, padeyes, cleats, tracks, line stoppers, and all manner of gear must be moderated.
Many times rivets are used to mount hardware on an aluminum mast when inside access cannot be had. Rivets can be used on a composite mast but they must be non-corrosive. Drilling and tapping relies on aluminum's strength to accept tapped threads. Carbon does not have this type of strength. If threads are tapped into a carbon mast, they will soon give-way under load, allowing slight movement of the attached fitting, which increases wear and loading, ultimately leading to failure.
Composite fittings Ideally, a fitting on a composite mast should be built of carbon and bonded, with epoxy or other adhesive, to a mast when it is built. If a fitting is added later it should be mounted with through-bolts that are isolated from that carbon fiber laminate. Mast wall thickness and the orientation of carbon fiber material must be determined prior to mounting gear, otherwise there could be insufficient and incorrectly oriented carbon material to support fittings. There are cases of winches and stopper blocks added after the mast was designed and built actually pulling out of carbon masts when put under load. Composite mast companies often say their ideal mast is one containing no metals, just carbon fiber. This is theoretically possible, since, with proper design and forethought, all mast components can be fabricated from carbon and then bonded or laminated to a mast tube. Spreaders, tangs, tracks, goosenecks, brackets, and even mast steps can be formed from composite. Mast tracks are ideally suited for being built into or bonded to a composite mast, since an aluminum track is quite heavy and requires numerous mechanical fasteners to hold it to a mast. Some of the gains in weight reduction aloft provided by a composite mast are lost when a heavy aluminum track is bolted to it. By building a composite track into a mast, significant weight is saved and the track is much stronger since it is an integral part of the mast.
For example, a storm trysail track added to a composite mast after manufacture would require significant mast wall reinforcement and significant consideration to the type of fasteners needed. Titanium and monel are the preferred metals for attaching fittings, since they react the least with carbon. Stainless steel is the next choice.
Composite masts are not always built as one long tube but often are sections butted together or two halves splined together, depending on required mast height. In butt joint construction, sections, usually 50- to 60-feet in length, are scarfed together to provide the required mast height. Bonding these lengths together is done using epoxy adhesives that are also used in aerospace applications. They have been proven to produce permanent, flexible joints that are resistant to water, humidity, and solvents. Manufacturers' specifications for these adhesives claim up to 5,000 PSI in shear strength.
Composite masts are also constructed by forming fore and aft shells which are then scarfed together lengthwise. An advantage to this technique is access inside an entire mast. Inspection and installation of items requiring through-bolting is easier when a mast's inside is exposed. Bonding internal tubes for wire runs or halyards is also easily executed with fore and aft shell construction.
Once two or more lengths are butted together or two halves scarfed together, sheathed in fiberglass and painted, it is difficult to tell where sections have come together. Prior to attaching or rearranging fittings on a composite mast's exterior, location of these joints should be known. Drilling a hole through a joint might not be appropriate for prolonged mast life.
Most composite mast manufacturers say that joints and scarfs are stronger than the mast itself, but there is no need to compromise scarf and butt areas by drilling holes (which removes wall material) and attaching gear (which adds stresses). Masts have long been an ideal place to attach winches, cleats, and other fittings for controlling running rigging. With composite masts it is best to either build a mast with all attachments and support pieces made of carbon and installed during construction, or mount gear, like winches and line stoppers, on deck around a mast's base. A properly designed composite mast would normally have provisions for two or more mainsail, headsail, and spinnaker halyards, as well as multiple topping lifts, and lazy jacks. These lines can all be led to deck blocks, then to winches, line stoppers, or cleats. Carbon fiber masts provide noticeable improvement in sailboat performance and stability. Their construction and material qualities also make them very different from aluminum, requiring careful examination and consultation with the builder before drilling holes and adding gear.