Marine Transmission Specifications & Applications of the Allison MH Series

Overview of the Allison MH Series Marine Transmission

Allison MH Series Marine Transmission

The Allison MH Series marine transmissions are hydraulically actuated gearboxes developed for use with Detroit Diesel two-stroke engines. Designed for medium- to heavy-duty commercial marine applications, these transmissions offered smoother gear engagement, higher torque capacity, and lower maintenance cycles than their mechanically shifted M Series counterparts.

Produced under the Allison Division of General Motors and later Detroit Diesel, the MH Series was widely installed in government, commercial, and industrial vessels throughout the 1970s to 1990s. These transmissions remain operational in legacy fleets due to their dependable service life, compatibility with Detroit engines, and robust rebuildability.

Key Specifications of the Allison MH Series

• Transmission Type: Hydraulically shifted marine gear

• Control System: Hydraulic shift with mechanical or solenoid control valves

• Typical Models: MH20, MH25, MH30, MH40, MH45

• Input Power Range:

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  • MH20: up to ~300 HP

  • MH25: up to ~400 HP

  • MH30: up to ~500 HP

  • MH40: up to ~650 HP

  • MH45: up to ~800 HP

• Reduction Ratios Available (varies by model):

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  • 1.00:1

  • 1.48:1

  • 2.00:1

  • 2.48:1

  • 3.00:1

• Input Housing: SAE No. 1, No. 2, or No. 3 (depending on configuration)

• Output Shaft: Straight or down-angle, flange-mounted

• Clutch Type: Oil-cooled multi-disc hydraulic clutch

• Cooling System: Oil-to-water heat exchanger standard

• Lubrication: Pressurized internal lubrication system

• Weight: Approximately 600–900 lbs (model-dependent)

The Allison MH Series offers durable hydraulic shifting that delivers reliable performance under continuous load, ideal for pushboats, twin-screw vessels, and boats requiring frequent gear reversals.

OEM Detroit Diesel Engine Pairings with Allison MH Series

The Allison MH Series was purpose-built for seamless integration with Detroit Diesel Series 53, 71, and 92 engines, across a wide range of 2-stroke configurations.

Detroit Diesel Series 53

• 6-53: Paired with MH20 or MH25 (utility and light-duty patrol vessels)

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Detroit Diesel Series 71

• 6-71: Paired with MH20 or MH25

• 6V71: Paired with MH30 (common in ferries and twin-screw workboats)

• 8V71: Paired with MH30 or MH40 (used in tugs, ferries, and pilot vessels)

• 12V71: Paired with MH40 or MH45 (heavier vessels with dual-output requirements)

Detroit Diesel Series 92

• 6V92: Paired with MH30 or MH40 (popular in higher HP vessels)

• 8V92: Paired with MH40 or MH45 (used in powerful push boats and offshore service vessels)

• 12V92 / 16V92: Paired with MH45 (for high-horsepower towing and ocean-going workboats)

These pairings offered torque-matched performance, supported by SAE housing alignment and consistent shift quality under variable loads.

Common Marine Applications of Allison MH Series Transmissions

Thanks to their high torque rating and smooth shifting capabilities, the MH Series transmissions found success across a wide range of vessel types that demanded reliability and power:

• Tugboats and Push Boats: MH40 and MH45 were widely used with Detroit 8V92 and 12V71 engines

• Commercial Ferries: Offering seamless shifting in high-traffic routes

• Patrol and Enforcement Boats: Used by coast guards and harbor police vessels

• Pilot Boats: Twin-screw configurations with MH30 and 6V71 or 8V71 power

• Crew Transfer Vessels: Where continuous forward/reverse operation required hydraulic clutching

• Utility and Construction Barges: MH40 and MH45 paired with high-torque Detroit 2-strokes

Advantages of the Allison MH Series

• Hydraulic Shifting: Smooth and efficient under load, ideal for high-duty-cycle operations

• Durable Clutch System: Oil-cooled multi-disc clutches for longevity

• Designed for Detroit Diesel Engines: Seamless integration with factory engine specifications

• Supportable Long-Term: Components remain available for rebuild and maintenance

• Versatile Output Options: Straight and down-angle configurations suit varied hull types

• Robust Construction: Operates under saltwater, debris, and high-vibration environments

Ideal Use Cases for Allison MH Series Transmissions

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