Marine relay modules in the 2026 catalog split into three functionally distinct tiers, and choosing between them starts with the bus already wired on the vessel, not the relay's amp rating [S1][S3][S4].
Network-attached smart relays (Maretron DCR100-01, Barantech Cruzo Smart Relay CSR8/1, KeyBox KB series) cover 8-13 channels at 5-20 A per output and sit on NMEA 2000 or CAN-bus, while legacy multi-function units such as the Woodward MFR1 (LR21035 Rev J, 5448-885 Rev F) handle converting, controlling, and monitoring inside Woodward engine-control racks [S1][S2][S3][S4][S8].
NMEA 2000 Smart Relays: DCR100 and Cruzo CSR8/1 on a 10-20 A Per-Channel Grid
Maretron's DCR100 exposes six DC relays, each rated 10 A, and drops onto any NMEA 2000 backbone for remote switching from a Maretron display or compatible MFD, which makes it the default pick for retrofits where 6 x 10 A covers lighting, bilge pumps, and blowers on a 30-50 ft boat [S3][S5].
Barantech's Cruzo Smart Relay CSR8/1 is the higher-current sibling: four 10 A and four 20 A channels, opto-isolated NMEA 2000 interface, supply 7-30 VDC, continuous current measurement across every channel with ±0.5 A accuracy and 10 mA resolution, IP67 housing at 229 x 146.4 x 48.5 mm, 960 g, and an M8 battery stud torqued to 9 Nm [S2]. The CSR8/1 draws up to 600 mA and carries Load Equivalence Number (LEN) 1, which means it must be powered from the battery side, not the NMEA 2000 bus [S2]. For relay module buyers running windlasses or larger blowers, the 20 A channels are the deciding spec, not the channel count.
CAN-bus I/O Relays: KeyBox for Distributed, High-Count Loads
Orbitron's KeyBox is a 12V/24V CAN-bus relay block with 13 fused output circuits, two of which are PWM-capable for dimming or throttle, and it integrates with Blink Marine PowerKey and PowerKey Pro keypads over a shared bus [S1]. Supply is 9-16 V (12V nominal) or 18-31 V (24V nominal), standby draw stays below 30 mA per PME, load max is 5 A per output at 24V or 10 A at 12V on resistive loads, with protection by automotive mini-blade fuses and an FR-4 PCB rated 1.6 mm double-clad 2 oz copper [S1].
Three KeyBox variants cover the common builds: KB312V-10A (11 x 10 A relays + 2 x 3 A MOSFET outputs), KB312V-05A (11 x 5 A + 2 x 3 A), KB324V-05A (24V, 11 x 5 A + 2 x 3 A), with the salt-spray rating at 96 hours and ingress at IP67 plus IP69K [S1]. Operating window is -40 to +85 C, dimensions 146.5 x 140 x 48.5 mm at 405 g, which lets a yard stack several KeyBoxes on one bus for 26, 39, or more controlled circuits without adding standalone fuses [S1]. The 10 A and 20 A limits of the smart tier are easy to forget when reading the channel count; a 5 A KeyBox output will not substitute for a CSR8/1 20 A windlass channel.
Discrete and Waterproof Relays: 12V IP67 Kits and 75 A Heavy-Duty
For single-circuit jobs (trim tabs, a high-amp horn, a davit winch) the catalog still uses discrete 5-pin SPDT changeover relays, and Baywatt's 50/30A Waterproof Relay Kit (SKU RW1250) is the 12V IP67 example: 5-pin SPDT (NO/NC), 50 A on the NO contact, fused pre-wired harness with a sealed socket and waterproof inline fuse holder [S6]. On the heavy end, Egis lists a 12V, 75 A relay aimed at the same wet, vibration-heavy marine envelope, and it is a useful benchmark for the upper amp tier a smart module cannot reach without external contactors [S9].
These discrete units are not networked, so they need a separate switch leg, but they remain the right call where the load exceeds what any 6- or 8-channel smart module can switch directly, and they are commonly wired through a marine valve actuator or a small contactor coil. Phoenix Contact's PLC-RSC-230AC/21-21/SO46/HI (GTIN 4055626798479) sits on the AC side of the same problem: IP20 relay base, IP54 once installed, CE/UKCA/cULus/DNV GL/EAC approved, which is the class-society angle discrete 12V kits usually cannot offer [S7].
Legacy Engine-Control Relays: Woodward MFR1 LR21035 and 5448-885
The Woodward MFR1 series covers converting, controlling, and monitoring inside Woodward marine control architectures and is identified by part number plus revision, since revision mismatches are the dominant service issue [S4][S8]. Two part numbers dominate the 2026 aftermarket: LR21035 Rev J (MFR1345B/GP configuration) and 5448-885 Rev F (MFR1110B/G59), both described as multi-function relays used in engine control, governor systems, power management, and switchboard automation on cargo, tanker, offshore, and ferry installations [S4][S8].
Because Woodward does not publish a generic voltage or response-time sheet for the MFR1 in the aftermarket listings, the safe procurement gate is nameplate photo, full part number with revision letter, associated system model/software version, and the vessel IMO number, which is the exact data set Woodward integration support asks for [S4]. For new builds the MFR1 is rarely a free choice; it is specified by the engine-control platform, and a mismatched revision breaks the control logic, which is why the catalog treats it as a direct replacement part, not a generic relay [S4][S8].
Selection Criteria: Bus, Channel Count, Current, IP, Class Approval
A working decision tree for a 2026 spec starts with the bus already in the boat: NMEA 2000 backbone in place, choose DCR100 for 6 x 10 A or CSR8/1 for 4 x 10 A + 4 x 20 A; CAN-bus keypad architecture, choose KeyBox; Woodward engine-control retrofit, choose MFR1 with exact revision match; single high-amp load, choose a discrete IP67 50-75 A kit or a Phoenix Contact PLC-RSC on the AC side [S1][S2][S3][S4][S6][S7]. Channel count, per-channel amps, IP rating, supply window, and class-society approval (DNV GL on the Phoenix Contact part is the only clearly listed one) are the five filters that sort the candidates in practice [S7].
A second pass against four hard criteria sharpens the choice: per-channel current (5 A KeyBox, 10 A DCR100, 10/20 A CSR8/1, 50 A Baywatt, 75 A Egis, plus Woodward MFR1 sized to its system), ingress (IP67-IP69K on KeyBox, IP67 on CSR8/1, IP67 on the Baywatt kit, IP20/IP54 on PLC-RSC), supply window (7-30 VDC CSR8/1, 9-31 V KeyBox, NMEA 2000 bus-powered DCR100), and feedback (per-channel current measurement on the CSR8/1 only, bus telemetry on the DCR100 and KeyBox, none on the discrete kits) [S1][S2][S3][S6][S9]. For builds where auxiliary fans, dampers, or a small chiller need to interlock with the relay, a wireless module or a linear module stage may sit upstream of the relay, but those belong to a separate spec path.
Failure Modes and Sourcing Constraints to Track
The recurring failure mode on marine relay modules is not the relay itself but revision drift and bus-power assumptions: a CSR8/1 attached to an NMEA 2000 backbone expecting bus power will not run, since the unit draws up to 600 mA and is rated LEN 1 with a note that it cannot be powered from the NMEA bus [S2]. A KeyBox will run from CAN, but only with the Blink Marine PowerKey/PowerKey Pro keypad protocol on the bus, and a bare CAN-bus connection to a non-Blink MFD will not address it [S1].
On the legacy side, MFR1 revisions (LR21035 Rev J, 5448-885 Rev F) are not interchangeable, and the aftermarket catalog treats them as direct replacement parts, so procurement must lock the revision letter before ordering [S4][S8]. Phoenix Contact's PLC-RSC-230AC/21-21/SO46/HI carries DNV GL among its CE/UKCA/cULus/EAC list, which is the clearest class-society signal in the relay-module set and worth tracking if the yard needs class-approved switching rather than a sealed discrete kit [S7]. DCR100 inventory on distributor sites such as Sprinter Parts Depot sits at the $458.17 price point for part DCR100-01, which is the current 2026 benchmark for the 6 x 10 A NMEA 2000 tier [S3]. Two trackable signals: confirmed Phoenix Contact DNV GL coverage for the PLC-RSC-230AC family and the next Maretron firmware note covering DCR100 channel-current telemetry.
Background reading: Sand blasting machine picks for farm machinery: 2026 selection map.