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The Yanmar 3YM20 21HP inboard diesel engine is a compact three-cylinder marine diesel designed for smaller sailboats and pleasure craft. Yanmar specifies 15.3 kW / 21 mhp at 3,600 rpm from a 0.854-liter, three-cylinder engine with natural aspiration and indirect injection. The engine uses mechanical controls, fresh-water cooling with a heat exchanger and a 12V 125A alternator, providing a practical configuration for marine propulsion installations where compact dimensions and a mechanical engine system are required.
Performance Engines offers this Yanmar 3YM20 as a marine propulsion engine. Before ordering, confirm the required marine gear or saildrive, mounting arrangement, propeller system, engine-bay dimensions, exhaust configuration and control requirements for the vessel.
The current Yanmar specification lists the 3YM20 at 15.3 kW / 21 mhp at 3,600 rpm. Yanmar’s operation documentation separately identifies a continuous crankshaft output of 13.9 kW / 18.9 hp metric at 3,489 rpm and a fuel-stop output of 15.3 kW / 20.8 hp metric at 3,600 rpm.
These ratings are manufacturer test values and should not be treated as a guaranteed boat speed. Actual propulsion performance depends on hull design, vessel displacement, propeller selection, marine gear ratio, loading and operating conditions.
The 3YM20 uses a 0.854-liter three-cylinder diesel configuration with a 70 mm bore and 74 mm stroke. It is a vertical, four-stroke, water-cooled marine diesel with natural aspiration.
Yanmar identifies the combustion system as indirect injection with an in-line pump. The mechanical architecture places the 3YM20 within Yanmar’s YM series of mechanical marine engines, which the manufacturer describes as being designed for smaller sailboats and pleasure craft.
The 3YM20 is naturally aspirated, so it does not use a turbocharger or intercooler. Fuel delivery uses an indirect-injection system with an in-line injection pump.
This specification is important when comparing the 3YM20 with newer common-rail marine engines. Buyers should use the actual engine configuration and installation documentation when planning a repower rather than assuming that accessories or controls from a different Yanmar engine will transfer directly.
Unlike a direct-seawater-cooled small diesel, the 3YM20 uses fresh-water cooling with a heat exchanger. The engine’s standard package includes an expansion tank for the fresh-water cooling system.
The cooling arrangement should be considered together with the seawater intake, seawater pump, heat exchanger, hoses and exhaust mixing components during installation. Correct cooling-water routing is essential to reliable marine engine operation, so installation should follow the applicable Yanmar documentation.
The 3YM20 uses a 12V electrical system and Yanmar’s current specification lists a 12V 125A alternator. The technical datasheet also identifies electric starting at 12V with a 1.4 kW starter.
The relatively substantial alternator output can be relevant to boats with navigation electronics, refrigeration, lighting, communications equipment and other onboard electrical loads. Battery capacity, cable sizing, grounding and charging arrangements should nevertheless be designed for the complete vessel rather than selected from alternator output alone.
| Specification | Yanmar 3YM20 |
|---|---|
| Engine type | 4-stroke, vertical, water-cooled marine diesel |
| Rated output | 15.3 kW / 21 mhp |
| Rated engine speed | 3,600 rpm |
| Continuous crankshaft output | 13.9 kW / 18.9 hp metric at 3,489 rpm |
| Fuel-stop crankshaft output | 15.3 kW / 20.8 hp metric at 3,600 rpm |
| Displacement | 0.854 L / 52.1 cu. in. |
| Cylinders | 3, in-line |
| Bore × stroke | 70 × 74 mm / 2.76 × 2.91 in. |
| Combustion system | Indirect injection, in-line pump |
| Aspiration | Natural |
| Starting system | 12V electric starting, 1.4 kW |
| Cooling | Fresh-water cooling with heat exchanger |
| Alternator | 12V – 125A |
| Controls | Mechanical |
| Dry weight without gear | 110 kg / 243 lb |
| Current published dimensions | 694 × 463 × 600 mm |
| Flywheel housing | SAE #6 |
| Certifications | RCD 2, EPA Tier 3, EPA Tier 3C, BSO II, EMC |
Yanmar’s current 3YM20 information identifies the KM2P-1 marine gear and SD25 saildrive among the compatible propulsion arrangements. The manufacturer’s published dimensions need to be interpreted according to the selected configuration.
Yanmar notes that the current published engine length includes the KM2P-1 marine gear and the height includes a U-type exhaust elbow. Dimensions can therefore vary when another gear or exhaust-elbow configuration is selected. Confirm the exact drawing for the engine package being ordered.
The current Yanmar product page lists dimensions of 694 × 463 × 600 mm and a dry weight of 110 kg / 243 lb without gear. A separate Yanmar technical drawing lists a 3YM20 with KM2P-1 at 613 × 463 × 600 mm, illustrating why the exact propulsion configuration and dimensional reference should be confirmed before installation.
When planning a repower, measure the available engine-bay length, width and height and check the crankshaft centerline, engine mounts, shaft alignment, exhaust route, cooling connections and access for servicing. Do not rely on a generic engine dimension when modifying a boat.
Yanmar’s current standard package includes an L-type exhaust mixing elbow, 12V 125A alternator, B20 panel, flexible mounts, fresh-water expansion tank, electric stop solenoid and belt safety cover.
The supplied equipment should be confirmed against the exact engine package and propulsion arrangement before purchase. Some installation components are vessel-specific and may need to be selected separately.
Yanmar lists optional equipment for the 3YM20 including a high-riser mixing elbow, 4 m or 6 m extension wire harness, dual harness for a second station, TFX sidemount control head, hot-water kit, 50 mm extension saildrive leg, SD25 propeller cone nut kit and onboard spare-parts kit.
The correct accessory selection depends on the boat layout and selected propulsion system. Confirm compatibility before ordering additional controls, harnesses, exhaust components or saildrive accessories.
Yanmar places the 3YM20 within its YM series of mechanical marine engines, a range designed for smaller sailboats and pleasure craft. The 3YM20 occupies the 21 mhp position in the current YM range between the 14 mhp 2YM15 and 29.1 mhp 3YM30AE.
Whether it is suitable for a particular vessel depends on the boat’s displacement, hull design, required propulsion power, propeller, transmission or saildrive and applicable vessel requirements. The 21 mhp rating should therefore be compared with the boat manufacturer’s recommended engine range.
If you are replacing an existing marine diesel, compare the 3YM20 with the complete installation rather than horsepower alone. Important points include engine-bed dimensions, mount locations, shaft centerline, transmission ratio, propeller diameter and pitch, exhaust routing, seawater intake, freshwater cooling connections, fuel lines and control cables.
The selected marine gear or saildrive can change the final installation dimensions and propulsion characteristics. Confirm the exact Yanmar installation drawing, gear ratio and drive arrangement before making permanent modifications to the vessel.
The 3YM20 combines 21 mhp of marine diesel output with a three-cylinder naturally aspirated configuration, indirect injection, mechanical controls, fresh-water cooling and a 12V 125A alternator. It is a compact mechanical engine option for suitable sailboats and pleasure craft.
For a repower or new installation, the most important purchasing checks are propulsion sizing, physical fit, marine gear or saildrive compatibility, shaft or drive alignment, cooling and exhaust installation, electrical requirements and control configuration.
Yanmar currently lists the 3YM20 at 15.3 kW / 21 mhp at 3,600 rpm. Its operation documentation separately identifies continuous and fuel-stop ratings.
The Yanmar 3YM20 has three in-line cylinders and 0.854 liters of displacement.
No. The 3YM20 is naturally aspirated, with no turbocharger or intercooler listed in Yanmar’s specifications.
The engine uses indirect injection with an in-line pump. Yanmar’s operation documentation describes the combustion system as a swirl pre-combustion chamber.
Yes. The 3YM20 uses fresh-water cooling with a heat exchanger, with an expansion tank included in the standard engine package.
The current Yanmar specification lists a dry weight of 110 kg / 243 lb without gear. The complete installed propulsion system will weigh more after the marine gear or saildrive and associated equipment are included.
Yanmar’s current product specification lists a 12V 125A alternator. The vessel’s complete charging system should still be evaluated based on its battery bank and electrical loads.
Yanmar identifies the KM2P-1 marine gear and SD25 saildrive among compatible propulsion arrangements. Confirm the exact configuration and installation drawing before ordering.
If you are looking for a compact 21HP-class marine diesel for a suitable sailboat or pleasure-craft installation, the Yanmar 3YM20 provides a three-cylinder naturally aspirated configuration with mechanical controls, fresh-water cooling and a 12V 125A alternator.
For buyers comparing Yanmar marine diesel engines at different power levels, Performance Engines also offers the Yanmar 1GM10 9HP inboard diesel engine, the Yanmar 2YM15 14HP inboard diesel engine, the Yanmar 3YM30AE 29.1HP inboard diesel engine, and the Yanmar 4JH110 110HP inboard diesel engine.
Explore more inboard engines at Performance Engines.
