Marine engineers pick polyamide from a four-axis problem, not a single datasheet: grade family (PA 6, PA 6/6, PA 11, PA 12), moisture-driven dimensional drift, continuous service temperature, and whether the part is a load-bearing structural housing or a sliding wear component.
Field reality: PA 6 and PA 6/6 still own the cost-driven gear and bushing market because of their high tensile strength and ~220 degrees C / ~255 degrees C melting points, but they absorb 2.5 to 3 percent water by weight in humid service and lose stiffness as a result [S2]. Long-chain PA 11 and PA 12 absorb roughly an order of magnitude less water, which is why they are the default for cable jackets, through-hull fasteners, and valve seats that sit wet for years [S2][S3].
Grade family and what changes between PA 6, PA 6/6, PA 11, and PA 12
PA 6 and PA 6/6 are by far the most used polyamides globally, selected for their performance/cost ratio across automotive, industrial, and marine structural parts [S1]. PA 6 is polymerized from caprolactam, has a melting point near 220 degrees C, and is slightly more flexible and impact-tough in thin sections than PA 6/6 [S2]. PA 6/6 melts at roughly 255 degrees C, runs stiffer at room temperature, and is the workhorse for structural brackets, gears, fasteners, and load-bearing clips [S2].
Long-chain nylons are the marine specialty tier. PA 11 and PA 12 are produced from longer monomer chains, which is the structural reason they absorb less moisture, resist environmental stress cracking better, and survive fuel and hydraulic fluid exposure without going brittle [S2][S7]. For continuously submerged hardware, deck fittings exposed to salt spray, and any part where a 0.1 percent swell translates to a leak path, PA 11/PA 12 is the default starting point rather than PA 6/6 [S3]. Glass-filled PA 6/6 (typically 30 percent GF) is the route when you need the heat and stiffness of PA 6/6 but want to pull the moisture-expansion coefficient down; carbon-filled grades are the next step when static dissipation or extra wear life is needed [S2][S4].
Moisture absorption: the marine-specific failure mode
Moisture absorption is the single largest spec delta between polyamide grades in marine service. Unfilled PA 6/6 in a humid or wet environment absorbs 2.5 to 3 percent water by weight, producing measurable dimensional change and stiffness loss [S2]. PA 6 sits in a similar band; PA 12 sits roughly an order of magnitude lower on the same axis, and PA 11 lower still, which is why long-chain grades are picked for fluid-contact hardware [S2][S3].
HDPE is the relevant benchmark, not a substitute: HDPE has zero water absorption and floats at ~0.95 g/cm³ vs. PA 6 at ~1.14 g/cm³, but it softens above ~80 degrees C and creeps under continuous static load, so it is wrong for any structural marine housing that runs hot or carries a sustained load [S3]. The marine spec answer is to match absorption to the duty cycle: PA 12 or PA 11 for wet dynamic seals and submerged bearings; moisture-conditioned PA 6/6 (or glass-filled PA 6/6) for above-deck structural parts that are designed at the as-molded, conditioned dimensions, not the dry-as-molded dimensions [S2][S4]. Conditioning the resin to equilibrium moisture before machining is standard shop practice for tight-tolerance marine bushings, because designing to the wet state avoids the part growing after install.
Service temperature, wear, and where each grade actually fits on a vessel
PA 6/6 and glass-filled PA 6/6 are the right answer for marine gears, valve actuators, and pump housings exposed to engine-room ambient above 80 degrees C, with continuous service typically rated up to ~150 degrees C for unfilled and higher for glass-filled variants [S2][S4]. PA 6 fills the same slot at a lower temperature ceiling, with the advantage of better impact resistance in thin sections, useful for snap-fit clips and small covers [S2].
For self-lubricated bearings, wear pads, and slider blocks on deck machinery, cast PA 6 and oil-filled cast PA 6 grades (such as the LFX-type self-lubricating cast nylons used for conveyor and crane wear parts) are the standard because their low friction and high abrasion resistance remove the grease-maintenance burden [S5][S6]. Below the waterline, on through-hull fittings, anchor rode rollers, and any hardware that lives in saltwater, PA 12 and PA 11 dominate because their low water absorption keeps dimensions stable and their chemical resistance covers fuels, hydraulic fluids, and mild bilge chemicals [S2][S7]. A related marine material decision, the metal side of the same problem, is laid out in the cast iron selection for marine engineering spec map, which covers the load-bearing housings and valve bodies that the nylon wear parts ride inside.
Reinforcement, compounding, and ASTM test methods you actually need on the print
Marine spec sheets should pin the resin, the reinforcement, and the test method together. The base resin (PA 6, PA 6/6, PA 11, PA 12) sets the moisture and chemistry profile; the reinforcement sets the stiffness and creep behaviour; the ASTM test method sets what the numbers in the datasheet actually mean [S6]. Glass-filled PA 6/6 at 30 percent GF is the common structural spec; carbon-filled grades are specified where static dissipation matters (e.g. fuel-handling hardware) or where extra wear life is required on a high-cycle bearing [S2][S4].
Compounding options beyond reinforcement are part of the marine toolkit: impact modifiers raise cold-temperature toughness for deck hardware in northern waters, heat stabilizers push continuous service ratings up, and flame-retardant packages are required for cable carriers and certain interior parts under vessel class rules [S4]. For sliding wear specifically, internal lubricant packages (oil-filled cast PA 6, MoS2-filled grades) are picked over neat resin because they extend dry-running intervals and lower the coefficient of friction against steel shafts [S5][S6]. marine valve trim is one of the highest-payoff places to specify a self-lubricating cast PA 6 seat rather than a metal-on-metal pair, because the polymer absorbs minor misalignment and runs quieter.
Comparison: PA 6, PA 6/6, PA 11, PA 12, and a cast grade side-by-side
The table below is a first-pass filter for marine selection. Numbers come from the cited comparison guides and the underlying resin data; treat them as typical unfilled values unless the spec is glass-filled. [S3]
PA 6 vs. PA 6/6 vs. PA 11 vs. PA 12 (typical unfilled):
1. Tensile strength: PA 6 = high; PA 6/6 = higher (room-temperature leader of the four); PA 11 = high but lower than PA 6/6; PA 12 = medium, lower than PA 6 and PA 6/6 [S2][S4].
2. Melting point: PA 6 = ~220 degrees C; PA 6/6 = ~255 degrees C; PA 11 = lower than PA 6/6; PA 12 = lower still [S2].
3. Moisture absorption (saturated, ~50 percent RH and above): PA 6/6 = 2.5 to 3 percent by weight; PA 6 = slightly higher than PA 6/6; PA 11 and PA 12 = roughly an order of magnitude lower, the marine-relevant delta [S2][S3].
4. Dimensional stability in wet service: PA 6/6 = medium; PA 6 = medium; PA 11 = high; PA 12 = highest of the four, the default for tight-tolerance wet hardware [S2][S4].
5. Continuous service temperature (unfilled): PA 6/6 around 150 degrees C; PA 6 around 120 to 130 degrees C; glass-filled PA 6/6 higher; PA 11/PA 12 lower than PA 6/6 on heat but with far better wet stability [S2].
6. Cost band (qualitative): PA 6 and PA 6/6 sit at the low end; PA 11 and PA 12 sit at the high end, often 2x to 3x the price per kg, which is why the marine practice is to use long-chain grades only where moisture or chemical resistance actually demands them [S1][S4].
The general engineering context for this trade-off, including where nylon sits against acetal, PBT, and PEEK on the same axes, is covered in the broader engineering plastic reference.
Where nylon is the wrong call in marine service
Three failure modes end a nylon conversation on a marine print. First, strong acids and oxidizers: HDPE is the default chemical container for a reason, while PA 6 is rated only moderate on acid resistance and is attacked by strong mineral acids and oxidizers [S3]. Second, sustained sub-freezing exposure on a dry part: PA 6 is prone to embrittlement when dry and cold, while HDPE retains impact strength down to about minus 40 degrees C [S3]. Third, any application where dimensional stability under water absorption is non-negotiable and the part cannot be re-machined after conditioning: in that case, the answer is usually PA 12 or a metal housing with a nylon wear insert, not unfilled PA 6/6 [S2][S4].
Cast PA 6 and oil-filled cast PA 6 specifically are not the answer for fuel-handling components where the elastomer or fluoropolymer compatibility has not been checked, and they are not the answer for food-contact marine hardware unless the grade is rated for it [S5]. For higher-temperature engine-room duty, glass-filled PA 6/6 is usually the right ceiling before the spec climbs to PEEK or PPS, both of which sit at a different cost point [S4].
Selection flow for a marine nylon part
The decision order on a real spec: (1) is the part continuously wet or only splash-exposed? If continuously wet, default to PA 12 or PA 11 unless cost forces a compromise; (2) what is the continuous service temperature? Above ~120 degrees C unfilled, move to PA 6/6; above ~150 degrees C, move to glass-filled PA 6/6; (3) is it a sliding wear part? If yes, specify cast PA 6 or an internally lubricated cast PA 6 grade; (4) does it need static dissipation or extra stiffness? Move to carbon-filled or glass-filled PA 6/6; (5) is tight dimensional tolerance required in service? If yes, design at conditioned dimensions, not dry-as-molded, and prefer long-chain grades [S2][S4][S6].
Two trackable signals for the next selection cycle: ASTM and ISO updates to marine-grade polyamide test protocols (tensile, moisture-conditioned, salt-spray) typically land in the second half of a calendar year, and resin suppliers tend to publish refreshed PA 11/PA 12 datasheets in Q1; both are useful checkpoints when a print has a 12-month spec validity. The broader marine engineering context, including where marine HVAC housings and instrumentation brackets interact with the same resin choices, is covered in the marine equipment reference set.
Background reading: PTFE Grade Selection for Electronics: Dielectric, Thermal, and Purity Trade-offs.