Replacing a CAT 3508B-TA with a Mitsubishi S12A2-Y1 and S12R-Y3

Replacing a CAT 3508 with a Mitsubishi S12A2-Y1 or S12R-Y3

The information in this comparison is based on field experience from a territory where many CAT 3508 installations happen to be in the fishing industry. The 3508 has a wide range of published horsepower ratings, but for this evaluation we are focusing on the two ratings most commonly seen: 905 HP at 1600 RPM and 960 HP at 1800 RPM. Replacing a CAT 3508 with a Mitsubishi S12A2-Y1 or S12R-Y3 requires careful evaluation of space, cooling capacity, and RPM compatibility. The S12A2-Y1 is the closest physical and horsepower match, offering a smaller footprint, lower fuel burn, and an easier fit in vessels with horsepower limits or permit requirements. The S12R-Y3 provides more displacement and higher output, but it is roughly 23 inches longer than the 3508, which can create significant interference with generators, bulkheads, or any forward engine-room obstructions. Both Mitsubishi engines operate at 1600 RPM, so vessels originally configured for higher CAT RPM ratings may require a gear ratio change or propeller pitch adjustment. Cooling capacity must also be confirmed through a vessel survey, because CAT does not publish jacket-water BTU data for the 3508 and existing cooler boxes may not meet Mitsubishi flow requirements. For operators willing to address these dimensional and RPM-driven changes, the S12A2-Y1 and S12R-Y3 offer mechanically reliable, durable, and well-supported alternatives to aging 3508 installations.

Jump to:

• Power & Configuration Differences
• Common Modifications to Plan For
• Challenges to Overcome
• Repower Planning Summary

Power & Configuration Differences

Gear ratio adjustments

• The 3508 commonly operates at 1600 or 1800 RPM.

• Many 3500-series boats use older Caterpillar gears that are no longer supported, which often pushes operators to replace the gear entirely at repower.

• In cases where operators want to retain the existing CAT gear, the gear’s horsepower and RPM limits must be verified.

Cooling system review

• The S12A2 and S12R have different heat-rejection profiles than the 3508.

• Because CAT does not publish BTU discharge numbers for many 3508 ratings, cooler compatibility must be confirmed directly with a cooler manufacturer.

• Some vessels may require expanded or modified cooler boxes.

Exhaust and air system adjustments

• Differences in displacement and airflow may require resizing of air inlet piping or exhaust components.

• Backpressure limits and routing around existing structures need to be evaluated.

Bellhousing and mounting considerations

• Both Mitsubishi engines use a double-00 bellhousing, while many 3508s were paired with Caterpillar gears using different configurations.

• New mounting pedestals or a bellhousing adapter may be required depending on gear selection.

Propeller & driveline considerations

• For vessels running 3508s at 1800 RPM, switching to a 1600 RPM Mitsubishi rating may require propeller pitch adjustments or a new gear ratio to maintain performance.

• This becomes especially relevant for vessels that tow nets or need specific bollard pull characteristics

Challenges to Overcome

• Fishermen strongly favor the CAT 3500 series because of its wide range of published horsepower ratings. These ratings make it easy for them to match strict fishing-permit horsepower limits, which creates deep loyalty to the 3508 and makes replacement conversations more challenging.

• Strong operator loyalty to the CAT 3500 series. Many customers have run these engines for decades and trust their durability, wide horsepower rating options, and long history in the field. This loyalty makes it difficult to convince operators to consider a replacement, even when parts support and long-term costs have become challenging.

• Hull access limitations in fishing vessels
  Most fishing vessels cannot remove engines from above due to deckhouse structures. Repowers often require cutting a large opening in the side of the hull, sometimes through bulkhead fuel tanks. This adds cost, labor, and shipyard time compared to pushboat installations.
• One-time access drives gear replacement decisions
  Because cutting the hull is such an extensive process, operators typically prefer to replace the gear and the engine together, rather than reopening the vessel later. This influences both quoting strategy and the recommended scope of work.

Repower Planning Summary

Primary Required Changes

• Gear ratio changes

• Possible gear replacement

• Bellhousing compatibility check

• Cooling system review

• Exhaust piping updates

• Air inlet piping updates

• Hull access requirements

• Propeller pitch adjustments

General Observations

• The CAT 3508 is a well-liked, long-lived platform, which makes operator loyalty a real factor in any repower discussion.

• Both Mitsubishi options provide simpler mechanical systems with lower lifecycle costs despite horsepower and RPM differences that must be managed.

• Space, gear compatibility, and vessel-specific installation constraints ultimately determine whether the S12A2 or S12R is the better fit.