Can We Trust Plastic Boat Parts?

When a plastic part on our boat breaks, we often deem it “damn cheap plastic” and toss it away. In some parts of the world, that broken bit winds up in the whorl of floating plastics known as the Pacific Garbage Patch, and practically everywhere else (even our bodies), as part of the uncountable trillions of micro-plastic particles slowly entering the food chain. Sadly, the reason the part broke is often our own fault. Even parts made with the highest-quality materials can fail when misused. As with any material used on our boat, we should recognize the characteristics of various plastics, how they can fail, and how we can repair them.

Plastics offer many great features desirable to sailors. Plastic parts are inexpensive, don’t corrode or rot, and offer almost infinite possibilities for engineers and designers. We often take these benefits for granted. Take the ubiquitous YKK zipper, for example. For years, we cursed the corrosion-prone metal teeth on our zippers; now seagoing zippers have plastic teeth that are surprisingly durable and never corrode. When we look a little closer, we see that many plastic parts offer great advantages over the metal and wood components they replace.

The failures, though, are real. Old sailboats are falling apart bit by bit, and many of those failures are molded plastic parts for which no spare is available. UV rays bring the demise of many plastic components, but part of the problem is that we treat plastic exactly like the metal part it replaced. Overtightening screws that fasten into molded plastic parts is one of the most common mistakes people make. A plastic part will fail under torque values 20 times lower than a metal one. If we simply go easy on the screwdriver when assembling plastic parts, we’ll potentially save hundreds of dollars over the life of our boat.

To the untrained eye, many plastics look the same, so they are treated as if they are interchangeable. They aren’t. Just like substituting brass for bronze, exchanging polypropylene for nylon can cause real problems.

The webbing straps and padeyes used to secure boat batteries offer a good example of why the type of plastic matters. Nylon will melt when exposed to battery acid, and a polyethylene strap that is put under even a modest load will creep (deform) over time, allowing the battery to slide. Polypropylene straps, on the other hand, resist battery acid and have very little stretch. But polypropylene has its own Achilles heel; ultraviolet (UV) rays. The floating polypropylene rope attaching your Lifesling to the stern rail must be meticulously protected from sunlight by a polyester webbing cover or it will deteriorate to dust in just a few years.

Let’s take a closer look at strengths and weaknesses of the most common plastics found on sailboats.

HDPE AND LDPE

Both high-density polyethylene (HDPE) and weaker, more elastic, low-density polyethylene (LDPE) resist almost any chemical, explaining why our world is awash in plastic bottles. As a load-bearing material on boats, however, polyethylene (PE) has several problems. Popular HDPE products like Starboard will crack under sustained load, making it a poor choice for backing plates, bow platforms, and anything under tension or compression. Some PE products will resist UV degradation, but exposed spacers and washers above deck will begin to crack after five years in the sun, if not sooner.

PE’s main attraction is its slipperiness. HDPE is suitable for large, low-speed bearings, such as rudder bushings, but if the bushing sees any sunlight (almost inevitable on deck) it will crack.

PE is commonly used in water tanks, waste tanks, and portable fuel tanks. We don’t recommend plastic for large diesel fuel tanks, which should be made of metal. Plastic tanks are well-suited to small gasoline-powered boats that often get wet, but the inability to thoroughly clean out these tanks and limited options for molding baffles (needed to prevent sloshing) makes them undesirable for large tanks (see “Diesel Fuel Tank Replacement,” PS May 2007).

PE’s slipperiness makes it difficult, if not impossible to repair. You can get some sealants and flexible epoxy products to stick to PE by first sanding, then flame treating the surface, but it’s a challenge. Plastic welding is also an option. (see “Repairing Plastic Tanks,” PS December 2018).

Bottom line: Although useful for holding tanks, small water tanks, and bushings that are kept out of the sun, PE is a plastic we avoid if we can. If PE is the only option, plan on replacing exposed parts every few years, or as soon as they show signs of weakness (fading, stiffness, brittleness, cracking). Once they break, fixing is often too much trouble.

POLYPROPYLENE

Polypropylene is very similar to HDPE, but it is a harder plastic and has even less UV resistance. Commonly used in fishing nets and traps, it is a prime offender in ocean pollution. From a sailing perspective, it’s only useful for making floating lines, which must be closely monitored for UV degradation.

Bottom line: A avoid polypropylene products. If you want line that will float, consider adding recycled balsa or cork floats to the line.

PLASTICIZED POLYVINYL CHLORIDE

Polyvinyl chloride (PVC) is commonly used to make flexible plasticized products like vinyl windows and fabrics. Flexibility, good UV resistance, and transparency make it a suitable base material for all flexible windows. It can also serve to screen out harmful ultraviolet rays (see “A Surefire Sunburn Defense,” PS September 2023).

PVC’s fire resistance and excellent insulating properties make it the most common electrical wire insulation. Affordable cost and an ability to bond to many fabrics make it a favorite material for waterproof upholstery and awning material on boats. Long-term bonding, however, can be a challenge, because the gooey plasticizer can migrate into the bond area. Whatever adhesive you use, it must be flexible; epoxy will just peel away.

Bottom line: Clear PVC fabrics and windows provide durable service. For windows that resist fogging, look for window products that are coated with polyurethane.

PVC PIPE

PVC pressure pipe is ubiquitous in plumbing, though flexible tubing is generally preferred for long lengths on boats. The most common type is schedule 40. Industrial piping is typically schedule 80, which is 30-50 percent thicker and carries pressure ratings that are proportionally greater. Schedule 80 is also 60-100 percent stronger against impact and leverage. The heavier pipe is sometimes specified on boats because it is more rugged, not because the greater pressure rating is required.

Hard PVC can be bonded with many glues, but pipe cement only works well in socket fittings designed for pipe. Repair options for parts include Plexus, G-Flex, and polyurethane adhesive sealants.

Bottom line: If you have PVC pipe that might be subjected to impact or significant strains, schedule 80 pipe and fittings provide an increased safety margin.

PVC APPEARANCE BOARD

Widely sold under the brand Azek, PVC appearance board is hard PVC with tiny air bubbles blown into it. The bubbles reduce weight and increase toughness.

Bottom line: Before you pay more for Starboard, consider Azek. It’s very resistant to UV, and works well with paint, sealants, and adhesives (see “Is Azek the new Starboard?” PS December 2021).

NYLON

A type of polyamide, hard nylon is strong and resistant to deformation under sustained load (creep). It absorbs more water than polypropylene, and will swell when it’s wet. It is vulnerable to acid and bleach. It is not a good choice for electrical insulating components that might get wet. Avoid using nylon through-hulls anywhere near the waterline. UV degrades hard nylon, so unless the through-hull is painted, it will crack within 5-10 years. Marelon or other carbon-fiber reinforced materials are a much better choice for plastic through-hulls; the carbon filler improves UV resistance and the fibers prevent cracking.

Bottom line: Above or below the waterline, do not use plastic through-hulls that are not specifically designated for use below the waterline.

ACETAL PLASTIC

Sold under the familiar brand name Delrin, acetal plastic is strong, stiff, wear resistant, and is commonly used for ball bearings and races on marine blocks and travelers. Nylon has similar properties, but it is more flexible and has greater impact strength. Both plastics can be reinforced with fibers. This increases strength, stiffness, and resistance to creep and fatigue required for many applications, including deck hardware and plumbing.

Bottom line: Acetal plastic has been a welcome innovation, replacing metal bushings and bearings in deck hardware that would be subject to corrosion. Machinable and available in different densities, the material is a good choice when a custom bushing or bearing needs fabricating. Look for UV resistant formulas for outside use.

EPOXIES

Epoxy-based plastics have excellent bonding characteristics and can sustain a relatively high temperature range, but they are brittle. Epoxy components are generally easy to repair and extremely versatile when fiber reinforced.

Bottom line: Epoxy is a good choice for parts that you will want to bond in place. The material needs to be protected from UV, typically with paint or clearcoat with UV blockers.

COMPOSITES

The above generalizations can be misleading, because many of the plastics aboard are composites of two or more materials, creating a product that is much stronger and more durable than the original polymers when used alone. In fact, the difference between some composites and their main ingredient is greater than the difference between steel and iron.

For example, polyester resin is brittle and weak, but combined with fiberglass cloth it is the essential ingredient in our boat hulls. Likewise, fiber reinforcement turns brittle or weak resins into strong, stiff plastic parts, like blocks and through-hulls.

TYPES OF PLASTIC FAILURES

Since there are so many different plastics, each with its own unique, often hidden, vulnerabilities, you need to be careful about which plastic you choose for which job and how you use it. Misapplication or misuse are causes of failure that we can control.

Chlorine. Exposure to chlorine is bad for many plastics (as it can be for stainless steel and aluminum). ABS tap water piping commonly failed in homes because of chlorine exposure. Cleaning your marine head with chlorine bleach will quickly ruin the elastomers in the head valves.

DEET. N,N-Diethyl-meta-toluamide (DEET) is a strong solvent that can melt and render many common plastics permanently sticky. Vinyl windows are particularly vulnerable. If you touch your dodger window after applying bug juice, you’ll leave a permanent handprint. DEET turns vinyl armrests into sticky magnets for dirt and grime. Keep DEET out of the cockpit, away from vulnerable plastics, and wash your hands after application. Though not as effective against mosquitoes, we like catnip spray because it won’t harm plastics. It is actually more effective against biting flies, too.

UV damage. As with skin cancer prevention, covering up your plastics is the most effective way to prevent UV damage. Mainsails should be covered, and roller-furling sails should have UV covers. Use awnings and covers to protect clear vinyl windows. We use covers to protect the plastic components in self-tailing winches; the covers also keep ice and bird droppings out of the winch-handle sockets.

Paint is a good choice for epoxy and other plastics that are vulnerable to UV damage. Waxes and protectants are often recommended, but our testing and research has found that waxes and protectants do little to ward off UV rays. Waxing does reduce oxygen and ozone damage by sealing the pores, but for long-term protection against plastic’s big enemy, UV rays, you must apply the coatings so frequently that it becomes impractical.

Carbon black is the most effective UV blocker for plastics, and this is why the plastic parts of pulleys, winches, hatch frames, and tires are nearly always black.

Creep. Unlike metals, all plastics creep (deform) under sustained load. Fiber-reinforced composites can nearly eliminate creep. Some plastics creep rapidly, like HDPE (Starboard). Even under very low loads, some plastics can suffer creep failure, evidenced by cracks that appear gradually over time.

Cracks caused by creep failure are not the result of cyclical loading, as is common in metals. They are the result of the polymer chains rearranging themselves. A metal screw only slightly over-tightened in a plastic thread can lead to this type of failure. In our view, all plastic threaded parts need a warning sign against over-tightening. Plastic fasteners are oversized to better distribute forces on the surface and threads. Any fastener threaded into plastic should be lightly snugged, but not tight.

Water absorption. All plastics absorb moisture, which can lead to hydrolysis, a breakdown in the chemical bonds in a polymer. Some of the more moisture-absorbent polymers include polycarbonate, nylon, and polyester. Polyurethane caulk can also fail due to hydrolysis. (PS documented 3M 4000UV’s hydrolysis-related failure in 2020, see “PS Seeking Reports of 4000UV Failure,” PS April 2020). The stronger ester linkages found in vinylester resins are more resistant to hydrolysis, which is why it is preferred for gelcoat.

User error. Although we call this “user-error,” in many cases the misuse that led to the failure is something the manufacturer should have anticipated. For example, plastic fairleads will quickly wear if the line is pulled at too great an angle. To prevent wear, we need to correct the lead angle or use a metal fairlead.

In another example, plastic levers on cam-type rope clutches can break when we try to completely lock down the handle on a clutch that is already gripping a loaded line in its cam. One way to avoid this is to make sure there is no load on a rope clutch when you lock it down. All of the load should be taken by a winch until the clutch is completely closed. The tilt-tile locking mechanism on Lewmar rope clutches do not have this problem (see “Rope Clutches Tighten Their Grip,” PS November 2014).

PLASTIC IN PRACTICE

A routine survey of any contemporary boat will surely reveal potential plastic trouble spots. Here are a few areas to include in your own plastic check.

Through-hulls. Plastic through-hulls have their converts. We’re on the fence. Marelon and TruDesign through-hulls can provide reliable service, but white-nylon through-hulls do not have sufficient UV protection. You can expect white nylon through-hulls to fail within 5-10 years. Some plastic through-hulls are shaded by plastic clamshells, but if the clamshell falls off or is removed, they become vulnerable.

A typical failure occurs when a long, unsupported plastic through-hull gets knocked by something heavy in a locker and shears, allowing water to flood in. You can prevent this by using flanged seacocks to spread the force and by storing only lightweight items in lockers where through-hulls are located, but best advice is to shun white nylon through-hulls entirely, even above the waterline.

Luff hardware. On smaller boats, nylon sliders or slugs on the mainsail luff are a potential weak point. The slippery nylon reduces friction as they slide on the mast track, but a sudden load can break the slider bail—the u-shaped eye on the slider where the sail attaches. Ronstan and Bainbridge make slugs with stainless-steel bails, and these are preferable. For external and internal sliders, reinforced All-Slip sliders from Bainbridge are a good choice. A tightly fitted sail cover will prevent premature failure. Plan on replacing luff slides every three years—even though they can last longer. When the first one fails, replace them all.

Leach-line cleats. Typically made of nylon, leach-line clam cleats are light, cheap, and don’t corrode to leave stains on the sails. But they are vulnerable to UV damage and often don’t last as long as the sail. They are good for racing and smaller sails, but on larger boats we prefer aluminum leach-line cleats.

Bushings. Many boats rely on nylon bushings at rudders and hinges, and after 15 years in the sun they disintegrate. (Bushings on the hinges of folding trimarans like the Corsair are a good example.) Wear is not the problem; UV rays cause the flanges securing the bushings to disintegrate. If the bushing you need is out of production, McMaster Carr and specialty suppliers sell plastic bushings of all sizes, but you might need to do some trimming. PVC pipe bushings can be adapted for rudders on smaller boats. A machine shop familiar with plastics can create custom bushings out of UV stable acetal such as Delrin.

Cleats. Many small sailing skiffs and daysailers are sold with plastic composite cleats. When new they are usually strong enough for dock lines, but as they age and become brittle, they will inevitably crack. Aluminum is a much better option. For big cruising boats, silicone bronze and stainless steel are preferred.

Cam cleat fairleads. We’ve had Harken cam cleats with integral fairleads on two boats. Ropes chafed through one, and the other broke when it was stepped on. We switched to a fast-release model with a stainless steel bent rod fairlead which has held up fine. A fairlead is not always a good application for plastic.

Backing plates. We sometimes see boats with Starboard-brand plastic backing plates. While the material won’t rot, it will gradually creep. This will cause the bolts to loosen, leaks to appear, and if the deck core has not been sealed, delamination and deck rot will follow. Fiberglass is our preferred material for backing plates. Stainless steel and aluminum are both good options, but stainless steel is expensive and aluminum can corrode.

Floating winch handles. Plastic winch handles don’t ding the deck, but they flex, and we find they don’t fit as well into pockets that are designed for stainless handles. We broke an 8-inch Lewmar winch handle (it delaminated into two halves). We cleaned it up, glued it back together with Plexus, and it has been fine ever since, though it is now a secondary handle, and not used as much.

Masthead sheaves. The advantage of a plastic sheave is that it requires no bearings, so there is no chance of seizing, and there is less halyard wear. The disadvantages are that it will eventually crack, the result of UV damage. Plastic sheaves are acceptable on boats up to about 35 feet, but they should be replaced every 10 years. If you have metal sheaves, clean and lubricate them every few years. Ignore them at your peril. The last thing you want is for the halyard to jump out of its sheave and become jammed when you need to reef.

High-load blocks. We’ve experienced crushed plastic bearing and sheaves in several high-load turning blocks, all related to halyards. While we are fans of composite blocks for applications where light weight and low friction are a priority, metal is preferred for high-load blocks. Very high-load blocks—masthead sheaves, genoa leads, and some deck organizers are better served by non-ball bearing systems with primarily metal parts. Low friction rings can serve as turning blocks, but you will need to limit the number of turns and use slippery Dyneema rope, with covered tails for grip and handling as needed.

Hatch boards. Plastic seems like a good choice for a hatch board, but not all plastics are strong enough. Starboard is popular because it doesn’t rot, requires no paint, slides easily, and is simple to fabricate. Unfortunately, Starboard is weak and can warp. Lexan is acceptable. Medium density overlay plywood (MDO), solid wood, composite, and aluminum are also good choices (see “Making Sense of Plywood Choices,” PS December 2023/January 2024).

Safety handrails. Given plastic’s vulnerabilities, we don’t recommend the material for safety handrails. Teak looks lovely, but its condition can be hard to evaluate.

Bimini/dodger hardware. Plastic hinges are commonly used in the metal frames for Bimini tops and dodgers. You can expect any plastic parts in the hinges to fail.

CONCLUSION

Plastics have a lot of advantages and a lot of potential. They are light, inexpensive, and slippery. But compared to 316 stainless or bronze, the materials are 8-10 times weaker and 35-50 times more flexible.

They are not vulnerable to corrosion in the conventional sense, but they are vulnerable to UV, heat, and some chemicals. The application must be suitable and the design must consider the material properties. If we are going to depend on them, we need to learn to use them and repair them.

TECH SPECS: PROPERTIES OF COMMON BOAT PLASTICS