universal M-25 diesel on sailboat Reality Check

Interesting history of the Universal Motor company at Catalina30.com/Techlib/Engines/History

Reality Check's diesel engine is the Universal M25, manufactured by Universal Motors and based on the Kubota tractor engine model D-850 (The M25XP is based on the D-950). Transforming a diesel engine into a marine environment is called marinizing and some interesting components have to be added to replace traditional functions.

Universal M25 diesel photos

Cooling a car engine - the radiator

Land based vehicle engines are cooled by means of a radiator. The radiator is a heat exchanger - the engine coolant (antifreeze + water) continuously circulates thru a radiator and the air rushing thru the coils of the radiator cools the coolant to keep the engine from overheating.

Boats don't have radiators

In the marine environment you can't use a radiator to cool your engine. The sea water replaces the cooling function of the air passing thru a radiator and a heat exchanger system replaces the radiator.

The diesel engine is cooled indirectly - by seawater!

Sea water is continuously moved through the heat exchanger via the "raw" water pump (aka saltwater pump, or sea water pump - the salt water pump is another "add on" to marinize the engine.) The fresh water coolant (antifreeze + water that cools the engine block) is cooled by flowing through a separate part of the heat exchanger that the cool sea water is passing through.

V-belt / Fan belt

The fresh water circulating pump is driven by the engine by means of the fan belt - there isn't a fan in the marine environment so it's sometimes called a v-belt - just look at the front view pic below and you see why.

The fresh water coolant is moved by the circulating pump through the heat exchanger, through the engine block, circulating pump and thermostat, back through the exhaust section of the manifold.

Exhaust system

The exhaust system is another interesting section - the exhaust from the diesel goes through a pipe called the exhaust riser or the exhaust mixing elbow. Why is is called a "mixing" elbow? Because the salt water from the heat exchanger is injected into the exhaust at this point, mixing the water with the exhaust to cool that hot exhaust down to eject out of the boat.

Universal M25 Diesel photo view from the front
Universal M-25 Diesel engine front view
Front view photo shows:
  • Air breather
  • Primary fuel filter / water separator assembly
  • Fuel lines
  • Injectors
  • Injector pump fuel bleed plug
  • Fuel return connector and hose
  • Coolant fill cap on top of manifold
  • Thermostat housing
  • V-belt
  • Fresh water circulating pump (top of V-belt)
  • Salt water pump - Oberdorfer 202 M-15 (4 screws)
  • Alternator
  • Oil filter
  • Oil drain hose

Universal M25 Diesel photo view from the top
Universal M-25 Diesel engine top view
Top view photo shows:
  • 3" heat exchanger
  • Fuel return
  • Decompression lever
  • Oil filler plug
  • Air breather
  • breather elbow and hose on head cover
  • Coolant fill cap on top of manifold
  • Thermostat housing
  • V-belt
  • Fresh water circulating pump (top of V-belt)
  • Salt water pump (oberdorfer)
  • Alternator

Universal M-25 manifold drain petcock
Universal M-25 block coolant drain petcock
Universal M-25 Diesel photo view from starboard side of sailboat
Universal M-25 starboard side - click to enlarge
Starboard side view photo shows:
  • Injectors
  • Fuel lines
  • Fuel return connector and hose
  • Air breather, filter
  • Injector Pump
  • Injector Pump Fuel bleed plug
  • Throttle
  • Stop lever
  • Dip Stick
  • Block coolant drain
  • Engine mounted fuel filter (air bleed on top of filter)
  • oil drain hose
  • sea water pump inlet hose
  • sea water pump outlet hose (to heat exchanger)
  • flex mounts

Universal M-25 Diesel photo view from port side of sailboat
Universal M-25 port side - click to enlarge
Universal M-25 Diesel photo view from port side of sailboat
Universal M-25 port side
Port side view photo shows:
  • Oil filler cap
  • Coolant fill cap
  • Coolant air bleed
  • Temperature Sender
  • Thermostat Housing
  • Manifold
  • Overheat alarm on manifold
  • Manifold drain
  • Alternator
  • 2 inch Heat Exchanger
  • Panel Plug
  • Starter Solenoid
  • Starter
  • Transmission
  • Transmission drain plug
  • Regulator
  • Wiring harness

Universal M25 Diesel Photos

Universal M-25 Diesel breather hose
Universal M-25 breather hose redesign
Lots of photos of the Universal M25 diesel on Reality Check and some of my neighbor's M25XP on his Catalina 34. His is the pretty white one.
Universal M-25 Diesel, exhaust riser and hoses
Exhaust riser and hoses

the diesel manual...

Universal M25, M25XP Service Bulletins...


Universal M25 Diesel

Heat Exchanger

Engine Compartment Insulation / Soundproofing

Anchor Locker





Bimini for the Boat



Edson Pedestal Controls

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alt : 1984 Catalina 36 technical specs