Yanmar 3YM20 Specifications
| Engine Model: Yanmar 3YM20 |
| Engine Type: 4-stroke inline 3-cylinder water-cooled diesel |
| Total Displacement: 52.1 cu.in (0.85 L) |
| Rated Engine Power: 21 Hp (15.3 kW) at 3600 rpm |
| Fuel System Type: Indirect injection |
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The Yanmar 3YM20 is a modern, 3-cylinder,
naturally aspirated marine diesel engine designed to be compact and
lightweight for small craft applications. Introduced as the successor to
the older GM series, this engine is commonly installed in sailboats up
to 30 feet in length and small powerboats requiring reliable auxiliary
propulsion. The 3YM20 incorporates an indirect injection swirl chamber
combustion system, freshwater cooling with a heat exchanger, and a 12V
electric starting system. Its compact dimensions and low dry weight of
110 kg make it suitable for vessels where engine room space is limited.
The Yanmar 3YM20 represents a clean-sheet marine diesel design
prioritizing low weight, compact packaging, and smooth operation. Its
three-cylinder configuration delivers balanced power pulses suitable for
displacement hull propulsion at cruising speeds. The indirect injection
swirl chamber combustion system provides quiet operation and reduced
combustion noise compared to direct injection diesels, a benefit for
sailboat auxiliary applications where engine noise is a concern.
The freshwater cooling system with a heat exchanger isolates the engine
block from seawater contact, reducing internal corrosion. The closed
forced lubrication system includes a spin-on full-flow oil filter for
simplified maintenance. The 1.2 kW electric starter and 60A alternator
provide reliable starting and battery charging for marine electrical
systems. The counter-clockwise rotation viewed from the flywheel end
matches standard marine propeller configurations.
Engine Type: Vertical inline 3-cylinder, 4-cycle, water-cooled,
naturally aspirated diesel.
Family: Yanmar YM Series.
Displacement: 0.854 liters (854 cc / 52.1 cu.in).
Bore x Stroke: 70.0 mm x 74.0 mm (2.76 in x 2.91 in).
Compression Ratio: 23:1 (approximate, indirect injection design).
Combustion System: Indirect injection with swirl chamber.
Firing Order: 1-2-3.
Maximum Output (Crankshaft): 15.3 kW (21 hp) @ 3,600 rpm.
Continuous Rating (Crankshaft): 13.5 kW (18.1 hp) @ 3,489 rpm.
Maximum Torque: 48 Nm (35 lb-ft) @ 2,400-2,600 rpm.
Cruising RPM Range: 3,000-3,200 rpm.
Rotation Direction: Counter-clockwise when viewed from the flywheel
(stern) end.
Valvetrain: OHV, 6 valves (2 valves per cylinder).
Mechanical pushrods and rocker arms.
Indirect injection with swirl chamber.
The cylinder block is constructed from cast iron with a compact
architecture designed for marine installation space constraints. The
block utilizes a parent-bore design with cylinder bores machined
directly into the cast iron casting. The three cylinder bores are evenly
spaced with a bore pitch optimized for the 0.85-liter displacement.
Three main bearings support the crankshaft. The water jacket surrounds
each cylinder with coolant passages cast into the block. The camshaft is
located within the cylinder block, driven by a gear train from the
crankshaft. The engine mounting feet are integrated into the lower block
casting, accommodating flexible marine engine mounts for vibration
isolation from the hull structure.
The cylinder head is manufactured from cast iron, selected for thermal
stability under marine diesel operating conditions. It employs an OHV
configuration with the camshaft located within the cylinder block. The
six valves are arranged with two valves per cylinder in a vertical
orientation. Valve actuation is achieved via mechanical pushrods and
rocker arms, with screw-and-locknut adjusters for valve lash setting.
The combustion system utilizes indirect injection with a swirl chamber
(pre-combustion chamber) for each cylinder. This design promotes
complete combustion across the speed range and reduces combustion noise,
a benefit for marine applications. The cylinder head incorporates
separate intake and exhaust ports on opposite sides, with the intake
manifold positioned for fresh air supply.
The engine is equipped with a mechanical indirect injection fuel system.
An in-line injection pump, mounted to the side of the cylinder block, is
driven by a gear from the camshaft. The injection pump incorporates a
mechanical governor, providing speed control and load response for
marine propeller law operation. Each cylinder receives fuel from a
nozzle-type injector mounted in the cylinder head, directed into the
swirl chamber.
The indirect injection system operates at lower pressures than direct
injection systems, with injector opening pressure calibrated to 120-130
bar (1,740-1,885 psi). A fuel supply pump draws fuel through a primary
filter and water separator assembly.
The lubrication system is an enclosed, forced lubrication configuration
designed for marine duty cycles. A gear-type oil pump, driven by the
camshaft or crankshaft, circulates engine oil through a full-flow
disposable spin-on oil filter.
Oil galleries within the block and cylinder head supply pressurized oil
to the main bearings, connecting rod bearings, camshaft bearings, and
rocker arm assemblies. A pressure relief valve maintains system pressure
within specified limits. The oil sump is constructed from pressed steel
with a shallow profile to accommodate low engine room clearances. Total
oil capacity at the effective level is 2.7 liters (2.85 qt).
The cooling system is a freshwater (indirect) cooling design with a heat
exchanger, isolating the engine block from seawater contact. A
belt-driven centrifugal freshwater pump circulates coolant through the
cylinder block, cylinder head, and the heat exchanger. The heat
exchanger transfers heat from the freshwater circuit to the seawater
circuit, which is drawn from the surrounding water body and discharged
overboard.
A wax-type thermostat regulates freshwater flow to maintain optimal
operating temperatures. A seawater pump, separate from the freshwater
circuit, supplies cooling water to the heat exchanger and the exhaust
injection elbow. Total coolant capacity for the engine and heat
exchanger is 3.0 liters (3.2 qt).
Glow Plug System: The engine is equipped with glow plugs for cold-start
assistance. One glow plug per cylinder is installed in the swirl
chamber, controlled by a key-switch circuit with a pre-heat indicator
lamp.
Timing System: The camshaft and injection pump are driven by a gear
train housed at the front of the engine. The crankshaft gear drives an
idler gear, which in turn drives the camshaft gear and injection pump
gear. This gear-driven arrangement eliminates timing belt or chain
replacement requirements.
Governor System: The mechanical governor integrated into the injection
pump is calibrated for marine propeller law characteristics, maintaining
engine speed under varying load conditions.
Exhaust System: A water-injected exhaust elbow mixes seawater with
exhaust gases, cooling the exhaust system and reducing surface
temperatures in the engine compartment.
Electrical System: The charging system consists of a 12V/60A alternator
as standard equipment, with a 12V/80A alternator available as an option.
The starter motor is a 12V, 1.2 kW (1.6 hp) unit.
Dry Weight (Engine Only): 110 kg (243 lb)
Dry Weight (with KM2P-1 Reverse Gear): 125 kg (276 lb)
Dry Weight (with SD20 Saildrive): 130 kg (287 lb)
Overall Length (Engine Only): 570 mm (22.4 in)
Overall Length (with Gearbox): 725 mm (28.5 in)
Overall Width: 440 mm (17.3 in)
Overall Height: 550 mm (21.7 in)
Bore Pitch: 85 mm (3.35 in)
Engine Oil Capacity (Effective Level): 2.7 liters (2.85 qt)
Engine Oil Specification: SAE 15W-40 or 10W-30 meeting API CD, CE, CF,
or higher
Engine Oil Pressure (at idle, warm): Minimum 1.0 bar (15 psi)
Engine Oil Pressure (at 3,600 rpm, warm): 3.0-4.0 bar (44-58 psi)
Coolant Capacity (Freshwater Circuit): 3.0 liters (3.2 qt) including
heat exchanger
Coolant Specification: Ethylene glycol-based antifreeze with corrosion
inhibitors
Valve Clearance (Cold): Intake: 0.20 mm (0.008 in). Exhaust: 0.20 mm
(0.008 in)
Valve Clearance Inspection Interval: Every 500 operating hours
Injector Opening Pressure: 120-130 bar (1,740-1,885 psi)
Oil Change Interval: 250 operating hours or annually, whichever occurs
first
Fuel Filter Replacement Interval: 250 operating hours
Coolant Change Interval: Every 1,000 operating hours or 2 years
Idle Speed (In Gear, Engine Warm): 750-850 rpm
High Idle No-Load Speed (Maximum): 3,600 rpm
Cruising RPM Range (Continuous Operation): 3,000-3,200 rpm
Compression Pressure: Standard: 26-30 bar (377-435 psi) at cranking
speed
Cylinder Head Bolts, Stage 1: 30 Nm (22 lb-ft)
Cylinder Head Bolts, Stage 2: 60 Nm (44 lb-ft)
Cylinder Head Bolts, Stage 3: 80 Nm (59 lb-ft)
Main Bearing Cap Bolts: 50-55 Nm (37-41 lb-ft)
Connecting Rod Cap Bolts: 30-35 Nm (22-26 lb-ft)
Camshaft Bearing Cap Bolts: 15-20 Nm (11-15 lb-ft)
Flywheel Bolts: 45-50 Nm (33-37 lb-ft)
Crankshaft Pulley Bolt: 100-120 Nm (74-89 lb-ft)
Intake Manifold Nuts: 15-20 Nm (11-15 lb-ft)
Exhaust Manifold Nuts: 20-25 Nm (15-18 lb-ft)
Injector Hold-Down Clamp Bolts: 15-18 Nm (11-13 lb-ft)
Glow Plugs: 12-15 Nm (9-11 lb-ft)
Rocker Arm Shaft Bracket Bolts: 20-25 Nm (15-18 lb-ft)
Valve Cover Nuts: 8-10 Nm (6-7 lb-ft)
Oil Pan Bolts: 10-12 Nm (7-9 lb-ft)
Oil Pump Mounting Bolts: 15-20 Nm (11-15 lb-ft)
Water Pump (Freshwater) Mounting Bolts: 12-15 Nm (9-11 lb-ft)
Seawater Pump Mounting Bolts: 12-15 Nm (9-11 lb-ft)
Injection Pump Mounting Nuts: 20-25 Nm (15-18 lb-ft)
Engine Mount (Bracket to Block): 30-35 Nm (22-26 lb-ft)
Heat Exchanger Mounting Bolts: 15-20 Nm (11-15 lb-ft)
The Yanmar 3YM20 engine is utilized across a range of marine vessel
applications, including sailboats, small powerboats, and commercial
craft.
Sailboats (25-30 Feet): The 3YM20 is installed as auxiliary propulsion
in cruising sailboats from manufacturers including Beneteau, Jeanneau,
Bavaria, Hunter, Catalina, and Hallberg-Rassy. The engine is commonly
paired with the SD20 saildrive unit for reduced vibration and noise
transfer to the hull.
Small Powerboats (20-25 Feet): The engine serves in small displacement
powerboats, motor cruisers, and daysailers requiring reliable diesel
propulsion for coastal and inland waterway cruising.
KM2P-1 Reverse Gear Configuration: The engine is offered with the KM2P-1
mechanical reverse gear for conventional propeller shaft installations,
providing forward, neutral, and reverse operation with a reduction ratio
of 2.62:1 or 3.02:1 depending on propeller requirements.
SD20 Saildrive Configuration: The SD20 saildrive unit is a popular
option for sailboat installations, offering a 2.18:1 or 2.62:1 reduction
ratio with the propeller mounted directly below the engine for improved
thrust alignment and reduced shaft vibration.
Commercial Applications: The 3YM20 serves in small commercial craft
including harbor launches, pilot boats, fishing vessels, and workboats
up to 8 meters in length, where the engine's low weight and compact
dimensions are advantageous.
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