Unobtainium is the metal at the top of every Naval Architect’s wish list. It’s a perfect marine material; light, strong, stiff yet flexible—it’s as inert as gold, but costs only pennies per pound. Sadly, like the search for El Dorado, this metal quest remains more alchemy than chemistry.
For now, aluminum, especially the alloy 6061-T6, is the solid performer. It singlehandedly upstaged spruce as the mast material of choice, and for decades it’s done its job admirably. The alloy isn’t perfect, but by understanding its vulnerabilities, and mitigating those negative characteristics, the functional lifespan of an aluminum spar can be measured in decades not years.
Yes, carbon fiber spars are in many ways the next step forward. But for those intent on being cost effective and not in the hunt for a few tenths of a knot increase in boat speed, aluminum remains the cost effective alternative. In a future issue we’ll focus on carbon’s influence on spars, hulls, rigging, and sails.
Most metal masts are made from long, cylindrical billets of aluminum alloy. Each tube section is created using a powerful ram to force a heated billet (400-500 C) through a set of dies that squeeze and shape the billet into the cross section and wall thickness of a specific spar. Lots of lubricating release agent and 15,000 tons of ram pressure are used to reshape the malleable aluminum.
Billet residue is captured and recycled, while the tube shape undergoes quenching as it moves off on the runout table. The next stop in the line involves a process that draws (pulls) and straightens the tube section.
Once fully cooled, it goes through a T6 heat tempering process that elevates tensile strength from 35,000 to 45,000 psi. Lastly, spars can be anodized, painted, powder coated, or left uncoated. Some masts are extruded in half sections and machine-welded together lengthwise.
There are other aluminum alloys that are better suited to welded hull construction or used for metal casting purposes, but 6061-T6, containing small amounts of silicon, magnesium, and other trace elements, delivers the strength, stiffness and lightness that’s vital when it comes to making spars.
The “T6” alloy is weldable, but doing so anneals and weakens the area that’s welded. This is one of the reasons why, when splicing two sections together, a doubler is added internally that overlaps the junction. Excess heat buildup during the plug welding process that joins the sections is kept to a minimum. Some manufacturers mechanically fasten the junction using machine screws or heavy duty pop rivets.
Unfortunately, aluminum isn’t quite the sequel to tomorrow’s Unobtainium. Aluminum, like steel alloys, show a proclivity to oxidize. But in the case of most steel alloys, oxidation is an ongoing process that only reaches completion when the object in question has become an unrecognizable pile of rust.
Bare aluminum, on the other hand, reveals a very different oxidation curve. A shiny new piece of aluminum develops a speckled, gray, oxidized coating that actually becomes a protective layer, preventing further oxidation. Ironically, this means that the ugliest looking mast in the marina, that non-anodized, unpainted one with the aesthetic appeal of dirty socks, is about as well protected from further deterioration as the spar on the gold-plater with the automotive finish. This is the reason why most commercial mariners restrain from painting the deck and topsides of their aluminum workboats.
The root cause of this aesthetic injustice is the way moisture, especially salt water, finds every microscopic void or coating imperfection and causes aluminum to oxidize around voids and spread beneath the paint layer. By the time blisters appear and paint begins to flake, the sub surface is covered with aluminum oxide and starting to pit.
There are several ways to tame the effect of chloride-rich seawater. But when it comes to a failing painted surface, thorough prep work is essential. Modern epoxy primers help hold corrosion at bay, and single and two-part urethane coatings seal the surface. Hard-anodized or powder coated spars are even better protected, but cost more and are more complicated to refinish when they finally fail.
Galvanic corrosion is aluminum’s second major nemesis, and it stems from an electrical interaction rather than oxidation. Metals are rated on a galvanic corrosion scale that places less reactive (more noble) metals at one end and more reactive (less noble) ones at the other end.
Platinum, beryllium and magnesium lean against one of the bookends of this scale. Magnesium, a plentiful element, is strong and light, 35 percent lighter than aluminum, but way too reactive in the marine environment. Platinum and gold sit at the opposite bookend of reactivity and are so inert that all other metals become anodic in their presence. The metals that lie in between these are relatively ranked according to their behavior in an electrolyte such as seawater.
When it comes to marine applications, there aren’t many platinum thru hulls, but silicon bronze is a pretty good compromise between cost and corrosion resistance. It’s rank on the galvanic scale is toward the more noble end and it behaves as a cathode to less noble metals like zinc, brass, and aluminum, which become anodes in the proximity of more noble metals.
Unfortunately, when dissimilar metals are in direct contact, all it takes is a little rain or morning dew to set up a temporary galvanic cell. Salt spray finds all the nooks and crannies on a sailboat and as the water evaporates it leaves behind crystalized sodium chloride (NaCl). Each raindrop, wave splash or drop of dew rehydrates the electrolyte. And as every galvanic cell demonstrates, wherever two or more dissimilar metals are immersed, a current flows and the less noble material (anode) corrodes causing electrons to flow toward the more noble metal (cathode). The net result is pitting and eventual destruction of the anode.
This prolonged, double-barrel assault on an aluminum spar is most noticeable in areas where dissimilar metals make contact.
There’s an old superstition about putting a couple of silver or copper coins under the mast step, just before stepping the spar. It may have been a good luck charm in the days of iron men and wooden masts. But today, placing a copper penny or silver eagle in a wet mast step completes a highly reactive galvanic cell and creates a corrosion experiment of the first order. The right answer is to do everything possible to separate dissimilar metals. Putting a Delrin strip or dielectric PTFE tape between the hardware and the mast wall really helps.
When installing larger stainless steel hardware on a mast, it’s easy to cut out a gasket from a sheet of 30 mil thick Teflon. Also be sure to use Tef-Gel or a similar dialecrtic grease or sealant on all screw threads.
Once the mast has been unstepped, positioned horizontally on horses and the headsail furling gear removed, it’s time to take a close look in all the nooks and crannies where things can go wrong. I prefer a bottom up approach and group the mast into four related subsets: base, column, spreaders, and masthead. If the mast is going to be painted, postpone this DIY inspection until all the rigging and hardware has been removed. In either case, scrutinize the spar, hardware and rigging attachment points, especially where high loads are focused.
It helps to have a good magnifying glass, a pick, knife and small scraper on hand to expose and inspect oxidized areas. Place a piece of contrasting color masking tape on each point of concern as you progress toward the masthead. Once the inspection is complete, use a digital camera or smartphone to document the more serious issues. These snapshots provide a record of the location and extent of all corrosion, deep pitting and any cracks emanating from fasteners or hardware. Also record all dents or other impact damage and any sign of ongoing abrasion. Serious damage can be caused by misled wire running rigging and the cycle loading wear linked to variations in tension. Naturally, all standing and running rigging should be thoroughly inspected at this time— a topic of a future article.
AT THE BASE
Keel-stepped masts aboard many cruisers and racers are hidden below the cabin sole and reside in a wet, corrosion prone, bilge ambiance. And it’s another reason why, when a mast is unstepped, the entire support structure, step and the heel fitting deserve a close look. Check for signs of corrosion and make sure the hardware that fastens the heel fitting to the grid or other transverse and fore-and-aft support is in good shape. This structure supports compression loads and also must respond to changes in backstay tension and side loading, not to mention the shock loads of a beat to windward in heavy seas. This is also the time to do what I call spar-oscopy. Take a compact LED flashlight and tape it to the end of a long, thin PVC tube or bamboo fishing pole that will be used to look at the mast interior.
This jury-rigged light on a pole, allows you see signs of internal corrosion and gives you a chance to locate abrasion points where halyards have been misled or are rubbing on hardware. A narrow spot beam will illuminate much of the inner wall of the mast, and if the running rigging has been replaced with thin messengers and the spreader “dog bones” (cross connecting supports) have been removed, you will have a clear sight line up the spar. This is a good time to sort out any halyard overlaps.
Riggers also look for an ailment called “elephant foot.” It’s a descriptive name for the partial crumpling of the spar near the base of the mast, It’s caused by over-compression and/or a wall section that is too thin. This wrinkling is usually just above the mast step, and it indicates a condition just shy of complete failure. It can be linked to prolonged ponding to windward with excessive backstay tension and overpressuring mast jacks. In some cases a new section can be spliced into the spar. By if it’s an older mast and other significant signs of deterioration are present, it may be time to opt for a new spar. Don’t bet the farm on an “it hasn’t failed yet” assumption; hire a skilled rigger to advise on the tough calls.
At first glance, the mechanical challenge linked to stripping hardware from a mast seems rather simple. All you need are a couple of screwdrivers and you’re ready to go. Unfortunately, the gods of galvanic corrosion have placed another obstacle in the sailor’s way.
The threads of those stainless steel screws attaching hardware to base plates or to the mast wall itself have become so corroded they are likely to be screwdriver-proof. Part of the blame goes to original hardware installers, who gave little attention to coating threads with an anti-seize compound and the effect it would have on future maintenance.
So after some years or decades, when it’s time to see what lies underneath the hardware, my first step is to clean all oxidation, paint and grime away from the screw slots and make sure that the chosen screwdriver fills the entire slot. A snug fit is the goal. Then, if a good counter clockwise twist fails to elicit any rotation, it’s time to add a wrench to the screw driver and deploy a good deal more torque.
If this also fails to loosen the bugger, I go to plan B before I ruin the screw slot. Step one is to use a pick to scrape away oxidation around the screw head perimeter. The next step is to douse the area with a penetrant such as PB Blaster, CRC’s Ultra Screwloose, Knocker Loose Plus, Gasoila Free-All or a similar product (see Inside Practical Sailor blog post, “More Boat Tips: Unsticking Stuck Nuts and Bolts”).
Before once again applying torque to the problem, I spend some time using a drift pin and a small ball-peen hammer to tap each chemically soaked fastener. Afterwards I add more penetrant around each screw head. Instead of immediately reverting to a brute force approach, which more often than not leads to a broken fastener or a damaged screw slot, I let the penetrant do its thing and return the next day with my portable impact driver and assortment of screw driver bits. The small Makita impact tool applies a pulsing torque. Combined with a little penetrant and a lot of patience, I’ve found this tool to be very effective on stubborn fasteners. Screw diameters of ¼ inch or less are not hard to snap so use pulsing torque is far better than more leverage and brute force.
If the screw slot is damaged it’s time to switch gears and be ready to drill out the head of the screw and pull the hardware off the remaining stud. A stud remover fitted to a socket wrench works better than vise grips when it comes to backing out a headless screw. But it requires a half-inch or more of the screw stem to be exposed.
The secret to drilling off the damaged head of a screw involves the use of a drill bit made for stainless steel. Place it in the chuck of a low-speed drill that delivers ample torque at slow speeds. Those using a dull bit and a high-speed drill are likely to work-harden the stainless steel screw head, making it even harder to drill. Applying cutting oil that both cools and lubricates a bit will make drilling more effective.
ALONG THE COLUMN
A sailboat mast is like a long electrical fuse: one bad spot and the show is over. Critical failures are usually linked to standing rigging failures and can occur at toggle or tang attachment points, on the spar itself or at spreader tips and roots. Upper shroud tang fittings, near the masthead, need a close look. Check clevis pin holes for elongation and Tball or stem ball cups for deformation.
Sight along the open spans of the spar, where no hardware is attached. It should be free of abrasion marks and signs of halyard shackle damage. It’s surprising how many painstakingly applied paint jobs are ruined by halyard slating cause by poorly set halyards. During this part of the inspection also check exit sheaves, winch bases/pads, mast steps, the bow light, radar bracket and other attached hardware.
The gooseneck fitting and boom vang points of attachment are highstress areas and prone to developing stress cracks. Just below this union, forces converge at the mast partners, the reinforced area where a keelstepped spar passes through the deck. Check here for stress-related damage as well as corrosion issues. If you find signs of extensive pitting or stress cracks, a cosmetic repair can be more harm than help. Have a local rigger with a good reputation take a close look at what you have uncovered.
The mainsail mast track should be straight and the slugs, slides or cars that run in or on them should slide freely. Take an extra slide or car and hand test the track, identifying any points where friction increases. Problems are often caused by burred or dented metal, oxidation in an internal track or misalignment at track joints. Most of these issues are easy to resolve while the spar is horizontal and access is optimized. In-mast or in-boom furling systems each have an inspection and maintenance routine outlined by the manufacturer. Maintaining optimum reliability revolves around following these guidelines. Care should be taken to avoid keeping paint and primer from hampering track function.
Search for causes of abrasion, eliminate the dings and dents from halyard shackles by solving lead problems. And be on the lookout for hairline cracks emanating from fasteners on the leading edge of the mast. Modern spar design accounts for backstay tensioning that induces bend in the mast to adust headsail shape. This bending results in an intentional tension increase on the spar’s leading edge, adding new stress to a column already in compression. Small cracks emanating from fasteners on the leading edge of the mast can be enlarged as the mast is intentionally bowed.
Every sailor who’s painted anything on their boat has plenty of tips to share. But when it comes to useful insider advice, pay more attention to the pros who have learned what works over many years. The good news is that although paint brand allegiance may vary, generic mast prep and painting techniques have a high degree of correlation.
When it comes to the first step in the prep process, every expert sings the same refrain. Remove the hardware if possible, especially if there’s any sign of blistering or paint failure around the edges. If there’s no sign of any corrosion at all, and the fasteners are likely to snap rather than release, carefully prep and tape around the hardware.
Sand, wire brush or sand/soda blast all areas where corrosion has pitted or left the surface covered with white aluminum oxide. Take a close look at the heel of the mast and the mast step itself. Both need to be free of corrosion and not damaged by metal loss or physical damage. The same goes for the area where spreaders, stays and shrouds attach. The masthead fitting also deserves close scrutiny. Inspect the aluminum around where the sheave axle(s) attach. A corroded aluminum masthead truck, with deterioration around the support for headstay or backstay toggles, can spell disaster. This corrosion inspection is a good time to catch pending problems.
In most cases, OEM painted spars hold up quite well, especially those that have been carefully prepped, epoxyprimed and LPU top coated. Eventually, weathering causes the gloss to disappear, but the paint retains excellent adhesive quality. If you’re facing such a challenge and there’s little or no sign of physical damage or corrosion around hardware, there’s nothing wrong with simply renewing the top coat.
In such cases, begin with a wash and/ dewax cleanup, sand with 220/320, remove dust, tape off hardware, solvent wipe and apply of two coats of the same (or similar type) topcoat, scuff-sand between coats.
However, if there are dings, scrapes or areas where corrosion has damaged the coating or areas where paint adhesion is failing, a decision must be made between spot repairs and complete mast redo. The latter involves removal of most or all of the hardware and stripping off every bit of the old paint. A spot repair approach is much less labor intensive, but if corrosion is rampant, spot repairing can be counterproductive.
During the prep process it’s essential to clean and degrease the surface before doing any sanding or other abrasive work. I prefer to use the solvent/cleaner of the paint manufacturer I’ve chosen. Clean cotton rags work best, and by meticulously wet wiping the surface you eliminate contaminants that can be forced into the substrate during sanding.
In the case of a repair and recoat effort, once the corrosion and flaking paint have been removed, feather in the adjacent painted mast surface with 60- 80 grit paper to achieve a toothy grip for the epoxy primer that follows. When doing a spot repair, this taper zone becomes an important test of one’s ability to feather an edge and hide the old to new paint junction. Seamless blending of the primer sets the stage for a successful, smooth transition spot repair. If, as you sand the boundary, the old paint continues to flake rather than allow you to feather the edge, It time to switch gears and consider removing all the paint.
An important step in painting aluminum is to get an epoxy primer on a freshly sanded and clean surface as soon as possible. When painting an entire spar, It helps if you can set up a way to hang the mast at waist level so it can be rotated in order to access all surfaces efficiently.
Interlux recommends doing the degrease wipe down with their 202 Solvent Wash prior to sanding. Then prime the spar using their InterProtect 2000E/2001E, thinned 15-20% with their brush or spray reducer. It’s a user friendly epoxy primer and easy to sand. Two coats makes the 60-80 grit sanding marks disappear. Both single-part Bright Sides and two part Perfection deliver a smooth glossy finish. The former is easier to apply and the latter is more durable and long lived.
Pettit offers a complete lineup of aluminum paint and prep products. Their approach kicks off with a solvent clean and a medium grit emery cloth sanding. When the residue has been removed, a thin coat of #6455 Primer should be applied. Two hours later, EZ Prime #6149 is applied and when it’s cured and sanded with 220 (repeat if necessary). Finish with two coats of Easypoxy.
Awl Grip recommends an initial cleaning with their surface cleaner T340 followed by a vigorous Scotchbrite scrubbing with Deoxidine and a thorough rinse to remove all residue. When dry prime with 30-Y-94 and within 3-6 hours, without sanding, apply 545 epoxy primer. Sand 220/320 and top coat with Awl Craft 2000.
If the spar was previously anodized precede the above with a 10-minute wash using a 33% solution of natrium hydroxide. Don’t let the solution dry on the spar. Immediately water-rinse and follow the prime and paint process above.
Spreader junctions are like a dangerous highway intersection, a point where competing forces interact and where there are no traffic lights to tame the flow. Rigging tension on the windward side of a sailboat cause compression loads to increase in the windward spreader(s) and decrease in the leeward spreader(s). Discontinuous standing rigging optimizes wire/rod diameter in each panel section, but it also complicates spreader tip hardware. All too often, spreader boots or a well-meaning taping effort, ends up looking like a response to an ankle injury. Even worse it creates a moisture-holding corrosion bath that enhances galvanic corrosion and oxidation. The goal is to avoid going overboard with padding and tape and making sure that water will not collect around spreader tip hardware.
Spreader bases are another realm of serious concern due to cycle loading, multidirectional forces and dissimilar metal contact. Swept back spreaders, especially those that eliminate the need for a backstay, cope with even greater loads. So when the rig is un-stepped, check how the spreader attachment was engineered. Was a doubler added to the mast wall and/ or were cutouts installed and hardware added to connect spreader pairs? In either case, corrosion in key load path areas can greatly decrease the spar’s ability to cope with the fluctuating loads. It’s no surprise that masts often break just above a set of spreaders.
AT THE MASTHEAD
Once launched, it’s hard to see what’s going on at the masthead. This means that when the spar is down it’s time to get a really close look at the mast truck and its associated fittings. Begin by disconnecting the standing rigging and checking the geometry of every hole that supports a clevis pin. The rule of thumb is: round is good, elliptical is bad. This goes for the tangs that connect upper shrouds to the spar as well as the holes in a welded aluminum masthead fitting. The loss of an upper shroud while beating to windward usually brings down the mast, so extra attention in this area is time well spent.
Carbon spar manufacturing mimics the engineering pioneered in the aerospace industry. They have become an essential component In the most competitive ranks of sailboat racing and caught on with cruising sailors who own lighter, more performance oriented sailboats.
Most spars are built on metal mandrels by carefully aligning layers of prepreg unidirectional and multi-axial carbon fiber from masthead to heel. Intermittently, a debulking process is used to squeeze the layers together, and after the laminate schedule has been carefully aligned, it’s placed in an autoclave. Here the epoxy prepreg in the carbon material becomes viscous and cures under controlled heat and air pressure. These materials are expensive, the labor is time-consuming and the quality control must be rigorous.
One of the major advantages of carbon mast building is the ability to engineer the layup to coincide with the load paths and stresses in the structure. Finite element analysis has helped identify how and where forces are transferred through the tube section. Weight is saved by only adding material where it is needed.
A cruising boat designer may opt for extra reinforcement that increases the safety factor by raising the breaking point of the material. Racing sailors have validated the performance uptick associated with carbon spars. Carbon/epoxy laminates do not suffer from corrosion but they are anything but immune to UV light. It’s one of the reasons a white primer and LPU topcoat is the sensible finish.
Minor impact damage and abrasion from poorly led running rigging is fairly straight forward to repair. But damage linked to sailing loads that cause major cracks in the laminate or interlayer delamination is another story altogether. In these cases, the spar builder or a composites shop engineer has some tough decisions to make. The big challenge is when a high-tech laminate bundle fails it’s very difficult to scarf in a new section that will handle all the loads in a manner that’s equivalent to, let alone, better than new. Some insurance companies put restrictions or higher premiums on coverage of carbon masts.