On any marine engineering drawing, NBR (acrylonitrile butadiene rubber) is the right call for the oil, fuel, and hydraulic side of the vessel, and the wrong call for anything bathed in seawater or weather.
The reason is polarity. Acrylonitrile groups in the polymer chain repel non-polar hydrocarbons, which is exactly what makes nitrile the workhorse seal for petroleum oils, diesel, grease, and hydraulic fluid [S1][S4]. But the same polarity that blocks oil also limits its resistance to ozone, UV, and prolonged saltwater exposure, so the marine spec has to be drawn around the fluid being sealed, not the boat around it.
What NBR actually is and what governs the grade
Commercial nitrile sheet is a copolymer of butadiene and acrylonitrile (ACN), and the ACN percentage is the single most important variable in the drawing, not the durometer [S3]. Standard commercial grade nitrile runs 28–34% ACN, with 33% ACN the most common industrial specification for oil-contact service in the UK and US stock lines [S1].
Higher ACN content (e.g. 38–50%) gives better oil and fuel swell resistance but pushes the low-temperature flexibility limit up, and lower ACN (18–28%) improves cold flexibility at the cost of oil resistance [S3][S4]. A drawing that says "NBR, 70 Shore A" without an ACN percentage leaves grade selection to the compounder, and that is a known source of warranty disputes [S3].
Property bands that drive the marine selection
For marine hydraulic and fuel system service, NBR typically operates in a -30°C to +120°C continuous window (with some grades rated -40°C), against a 1,500–3,000 psi tensile strength band, hardness 40–90 Shore A, and good abrasion resistance [S4][S1]. Compared to neoprene and EPDM on the same parameters, NBR wins on oil and fuel, loses on weather and ozone, and ties roughly on the temperature envelope [S1].
That trade-off is the entire reason the question of "which rubber for marine" is really a question of "which fluid". The rule of thumb is straightforward: petroleum oil, fuel, or hydraulic fluid on the wetted side, specify NBR; seawater, steam, or outdoor exposure on the wetted side, specify EPDM rubber or neoprene, and accept the cost [S1][S2].
ACN and Shore A decision matrix for marine parts
For marine engine rooms, the practical spec for a fuel or hydraulic gasket is NBR at 33% ACN, 60–70 Shore A, 1.5–3.0 mm sheet thickness. This is the most widely stocked grade and seals adequately under standard bolt loads on a flange face [S1].
For a piston or rod seal in a hydraulic cylinder exposed to cold North Sea or Baltic water, the spec shifts toward a lower-ACN grade (around 28–30%) to retain flexibility at sub-zero start-up, and a 70–80 Shore A durometer to resist extrusion under pressure [S3][S4]. For higher-temperature hydraulic service above 110°C, or where extended oil life is required, HNBR is the standard upgrade path, and FKM (Viton class) is the next step above that [S4].
Where NBR fits on a ship, and where it does not
On a vessel, NBR is correctly applied to fuel-line O-rings, diesel return gaskets, hydraulic cylinder rod and piston seals, lubricating-oil sump gaskets, and oil-resistant hose liners, all of which match NBR's polarity profile and continuous temperature ceiling [S4][S5][S2]. The O-ring format in particular is dominated by NBR across the marine hydraulics market because of the combination of petroleum resistance and low cost [S5][S6].
Where NBR is the wrong call: any seal or gasket with seawater, brine, or raw cooling water on the wetted side, any dynamic seal exposed to direct sunlight or salt-air ozone, and any service above roughly 120°C continuous [S1][S4]. For those positions, EPDM rubber is the typical marine-side replacement, and HNBR or FKM is used where both heat and oil are present [S4].
Comparing NBR, HNBR, and EPDM on the four criteria that matter at sea
On oil and fuel resistance, NBR and HNBR are excellent, while EPDM is poor and should not be used for petroleum service. On seawater and weather resistance, EPDM is excellent, HNBR is good to excellent, and NBR is fair to poor, which is the flip side of the same coin. On continuous upper temperature, EPDM and NBR both plateau around 120°C, while HNBR extends to roughly 150°C. On cost, NBR is the cheapest by a meaningful margin, HNBR runs roughly 1.5–2.5× NBR, and FKM is the most expensive again by a wide step [S4][S1][S6].
That gives the marine engineer a clean three-way decision: oil side and budget, NBR; oil side and heat or longer service life, HNBR; seawater and weather side, EPDM. The mistake to avoid is mixing these by fluid, for example, putting an EPDM O-ring in a fuel line because the boat is "marine" [S1][S5].
Standards, callouts, and sourcing notes
Under ASTM D2000, the standard classification system for rubber materials, nitrile compounds are classified by type, class, and hardness, and that callout is the cleanest way to put NBR on a controlled drawing [S1]. For O-rings, the standard sizing is governed by AS568, and the most widely used O-ring material globally for petroleum oils and hydraulic fluids remains NBR [S5][S6].
For specification, marine buyers should treat ACN content, Shore A hardness, and tensile range as the three non-negotiable fields, with temperature envelope and fluid compatibility as the two qualifiers, and they should reject any drawing that calls out NBR without an ACN value, because that effectively hands the grade decision to the supplier [S3].
Cross-reference: NBR in adjacent service bands
The same NBR selection logic, ACN content, hardness, service temperature, fluid side, drives spec across adjacent industries, and the band-by-band reasoning is portable. The aerospace variant sharpens ACN and hardness controls around fuel and pressure cycling, as laid out in Nitrile Rubber (NBR) Selection for Aerospace: ACN, Hardness, Standards. [S4]
For construction-side applications where the fluid is less aggressive but the abrasion and weathering load is higher, the band shifts again, and the comparison against industrial rubber and construction-grade alternatives is mapped in NBR Selection for Construction: ACN, Hardness, and Service Bands. Marine engineering sits between those two, more chemical exposure than construction, less thermal and pressure cycling than aerospace.
Trackable signals for the next spec cycle: the marine hydraulics aftermarket remains the largest single pull on NBR O-rings, and HNBR substitution is most aggressive in deepwater hydraulic systems where sustained oil temperature pushes past 110°C; on the seawater side, EPDM continues to displace NBR in any exposed or sun-side seal position [S4][S5][S1].
For component-level specifications, see nitrile rubber.