Replacing a CAT 3412 Tier 0 with a Mitsubishi S6R-Y1/Y3 and S12A2-Y1

Replacing a CAT 3412 with a Mitsubishi S6R or S12A2

Replacing a CAT 3412 with an S6R or S12A2 depends on the vessel’s operating horsepower and RPM requirements. The S6R-Y1 provides a Tier 1 option in the 590 HP range but introduces an RPM change that may require gear ratio or propeller adjustments. The S6R-Y3 at 1800 RPM is a strong fit for vessels running 600–700 HP, with matching speed and comparable heat rejection, though an additional cooling circuit is required for Tier 3 installations.

At the upper end of the 3412’s 700–720 HP range, the S12A2-Y1 becomes an attractive replacement. While the engine is larger and approximately seven inches longer, it can be dialed back to 1800 RPM for a near drop-in fit, and fuel burn around 700 HP is nearly identical to the original 3412. Cooling capacity has proven sufficient in recent repower surveys, with existing coolers acceptable in comparable installations.

Across all options, physical fit is generally favorable due to the S6R’s smaller and narrower footprint, while the S12A2’s additional length requires attention to forward clearances. Gearbox compatibility is typically straightforward at matching RPMs, and overall scope is driven by cooling additions on Tier 3 engines, RPM alignment, and any propeller changes needed to match the vessel’s true operating horsepower.

3412C- Core Engine Specs

Attribute	Value
Tier	0
Configuration	V12, 4-Stroke
Displacement	27 L
Cooling	Single Circuit
Dimensions	71.1 x 53.0 x 51.7 (LxWxH)

Continuous Duty Ratings (A/B)

Model	Rating	HP @ RPM	Fuel Burn (GPH)	Mitsubishi Replacement
3412C	A	503 @ 1800	26.6	S6R-Y1
3412C	A	540 @ 1800	28.6	S6R-Y1
3412C	A	600 @ 1800	32.0	S6R-Y1
3412C	A	650 @ 1800	33.7	S6R-Y1 or S6R-Y3
3412C	B	720 @ 1800	37.4	S6R-Y3 or S12A2-Y1

Comparison to the S6R and S12A2

Specification	3412C	S6R-Y1	S6R-Y3	S12A2-Y1
Configuration	V12	Inline-6,	Inline-6	V12
HP @ RPM	See above	590 HP @ 1600 RPM 650 HP @ 1650 RPM	630 HP @ 1600 RPM 684 HP @ 1800 RPM	850 HP @ 1940 RPM (can be dialed back to 1800 for ~700 HP applications)
Displacement	27 L	24.5 L	24.5 L	33.9 L
Cooling	Single Circuit	Single Circuit	Dual Circuit Required	Single Circuit
Dimensions (L × W × H)	71.1" × 53.0" × 51.7"	71" × 44" × 64"	71" × 44" × 64"	90.2" × 56.6" × 64"

Dimension Delta Comparisons

Spec	CAT 3412C	S6R Delta	S12A2 Delta
Length	71.1 in	–.1 in	+19.1 in
Width	71.0 in	–9.0 in	+3.6 in
Height	90.2 in	+12.3 in	+12.3 in

Common Repower Scenarios

These engines are widely used in push boats, especially in fleets with larger horsepower requirements than the 3408 class.
The 500–720 HP range is common among workboats running 3412s, with 720 HP being the most frequently encountered field rating.
Many vessels in this segment have aged 3412s with multiple overhauls, often four or five rebuilds, leading operators to consider repower instead of continued rebuild cycles.
Operators often have uprated 3412s, meaning the installed power may not match the original data plate, which influences whether the S6R or S12A2 is the best fit.
Boats using 3412s typically have more engine room space than 3408 vessels, but installations must still check forward clearance when considering the longer S12A2.
The 3412 is a very popular and well-liked engine in the industry, so many operators continue rebuilding them until parts lead times or reliability concerns push them toward a repower.
Fleet operators such as Marquette and others with high counts of 3412 boats are prime candidates for multi-vessel repowers when engines become worn or support issues increase.

Common Modifications to Plan For

Gear Ratio Changes for S6R-Y1

The S6R-Y1 operates at 1600–1650 RPM rather than 1800 RPM, which typically requires a new gear ratio to align the vessel’s propulsion system with the lower engine speed.

Propeller Adjustments for RPM Differences

Replacing a 3412 with an S6R-Y1 may require propeller pitch changes or resizing to properly load the engine at its lower operating RPM.

Additional Cooling Circuit for S6R-Y3

Tier 3 S6R-Y3 engines require an added cooling circuit for the aftercooler. This modification applies only to Tier 3 installations.

Clearance Verification for S12A2

The S12A2 is longer than the 3412, so forward engine room clearance must be checked to ensure there are no obstructions such as generators, bulkheads, or piping.

Cooling Capacity Checks for 700–720 HP Replacements

When replacing 700–720 HP 3412s with the S12A2 at 1800 RPM, cooler capacity should be verified. In some vessels, existing coolers may already be sufficient.

Target Markets

Push Boats
Tug Boats
Crew Boats
Fishing Vessels

3412 Operators IRR as of April 2022

Operator
Marquette Transportation Co. Gulf-Inland LLC	30
Crosby Marine Transportation LLC	10
Stone, John W., Oil Distributor LLC	9
Corps of Engineers	8
Bellaire Harbor Service LLC	8
Terral RiverService, Inc.	8
Turn Services LLC	8
Louisiana Dept. of Transportation	6
United States Coast Guard	6
Illinois Marine Towing, Inc.	4
Massman Construction Co.	4
Magnolia Fleet LLC	4
Cook Coal Terminal	4

Third-Party Stories

Central Boat Rentals — Columbia Repower (Completed)

Replaced a 700 HP CAT 3412 with Mitsubishi S6R-Y3 engines (1800 RPM).
Provided a strong horsepower match with same RPM
Added the required Tier 3 cooling circuit.
Additional cooler installation was part of the repower scope.
Performed propeller modifications.
Adjustments were needed to properly load the new engines.
Previous CAT engines had five overhauls and were worn out.
The decision to repower was driven by age, fatigue, and diminishing reliability.
Customer reports strong satisfaction with the Mitsubishi package.
The vessel has performed well, and Central is now considering a second Mitsubishi-powered build (As of April 2022)

Marquette Transportation — Fleet Repower Evaluation (In Progress as of April 2022)

Initial request explored moving a vessel up to 1600 HP.
Marquette first asked about the S6R2-Y3, but it required extensive modifications.
S6R2-Y3 was not a viable fit.
Gear changes and major cooling upgrades would have been required when replacing an 1800 RPM, 700 HP 3412.
Discussion shifted to the S12A2-Y1 as a better solution.
Matching the horsepower curve made the S12A2 a strong candidate for 700–720 HP 3412 replacements.
Existing coolers may already be sufficient.
East Park confirmed cooler capacity appeared adequate for the S12A2-Y1 during a repower survey.
Marquette has renewed interest in the project.
Ongoing conversations indicate the potential for a future sale. (As of April 2022)

Sales Strategy

Overhaul costs are extremely high.
A 3412 overhaul is around $100,000, often on engines that have already been rebuilt four to six times and may have over 100,000 hours.
New Mitsubishi engines offer far better value.
Customers can purchase brand-new S6R-Y1, S6R-Y3, or even the S12A2 for similar cost to a major overhaul, giving them zero hours and a fresh factory engine.
Aging blocks make overhaul a risky investment.
Many 3412 blocks are worn out after multiple rebuilds, making repower a smarter long-term decision.
Parts availability is getting worse.
Since CAT no longer produces these engines, operators are seeing increasing lead times and inconsistent global parts supply.
Service support gaps create operator frustration.
CAT support issues open the door for Mitsubishi advantages in responsiveness, reliability, and customer care.
Repower messaging is simple and compelling.
For nearly the same price as a rebuild, operators can install a new engine with minimal additional vessel cost and keep their asset running reliably.

General Observations

The S6R series provides strong coverage across the lower and mid-range 3412 ratings, with the S6R-Y3 offering the cleanest fit due to matching 1800 RPM.
The S12A2 becomes the most suitable replacement for higher 3412 ratings (700–720 HP), especially when de-rated to 1800 RPM, where fuel burn and performance closely match the original engine.
Dimensional fit is generally favorable on the S6R models, which are shorter and narrower than the 3412; the S12A2 requires forward-clearance checks due to increased length.
Cooling and gear modifications depend on model selection: Tier 3 S6R engines require an added cooling circuit, while RPM differences on the S6R-Y1 may require gear or propeller changes.