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Replacing a KTA38 1000HP @1800 with a Mitsubishi S12R-Y3

Understand the physical, mechanical, and cooling considerations when replacing a Cummins KTA38 Tier 1 or Tier 2 engine with a Mitsubishi S12R-Y3 1100 HP @ 1800/1600 RPM.

Replacing a Cummins KTA38 with a Mitsubishi S12R

Replacing a KTA38 with an S12R requires changes to exhaust piping, air intake, cooling circuits, and bellhousing due to differences in heat rejection and engine dimensions.
While the S12R provides higher horsepower at the same RPM and a heavier-duty design, operators must account for additional cooler circuits, larger exhaust piping, and bellhousing adapter requirements.

In many cases, existing jacket water cooling capacity is close to S12R needs, but the aftercooler circuit will require significant upgrades.
Across multiple vessel surveys, the length increase of up to 10-12 inches has not been a limiting factor on typical 2000 HP push boats.

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Power & Configuration Differences

Metric KTA38 Tier 1 KTA38 Tier 2 Mitsubishi S12R-Y3
HP Rating 1000HP @1800 RPM 1000 HP @ 1800 RPM

1100 HP @ 1800 RPM
1100 HP @ 1600RPM
Configuration V-12 V-12 V-12
Displacement  38L 38 L 49.4 L 
Weight 9300 lbs 9300 lbs 11729 lbs
Fuel System     Mechanical
Bell Housing Typically 0 Typically 0 Double 00

Dimensional Delta: KTA38 vs Mitsubishi S12R-Y3

Dimension KTA38 Mitsubishi S12R Delta
Length 84 in 94.13 in +10.13 in
Width 58 in 59.54 in +1.54 in
Height 82 in 68.78 in -13.22 in

Why this matters:

  • Length delta (+9.54 in):
    The S12R is about 10" longer than the KTA38. Check to confirm ample space in front of engine. In most surveyed 2000 HP vessels, forward clearance has not presented a major obstruction, but racor filters may need relocation. 

Cooling Differences 

Metric KTA38 Tier 1 KTA38 Tier 2 Mitsubishi S12R-Y3
Jacket Water Heat Rejection 27314 BTU/min 26733 BTU/min 23,375 BTU/min
Cooling Circuit Configuration Varies: 1 or 2 coolers Varies: 1 or 2 coolers Requires dual circuits
Aftercooler BTU Requirement   6301 20,778 BTU/min
Fuel Burn (GPH) 48.9 52.8 55.0

Key Notes

  • Although the S12R shows lower jacket water BTU, flow rate differences must be verified to determine whether existing coolers can be reused.

  • Many Tier 2 KTA38 vessels use a single combined cooling circuit, producing 32,000–33,000 BTU/min total.

  • Some boats have dual coolers, matching the S12R configuration, but single-cooler boats will require an added circuit.

  • Aftercooler BTU load on the S12R is significant and often requires new or larger cooling hardware.

 

Common Repower Scenarios

  • Tier 1 and Tier 2 KTA38 engines used widely in 2000 HP push boats.

  • Many Tier 2 units use a single combined cooler, while others use dual circuits similar to the S12R.

  • Engines with single cooler systems will require additional cooler circuit for S12R aftercooler.

Common Modifications to Plan For

Cooling System

  • KTA38 Tier 1 units commonly use one combined cooler (JW + AC).

  • KTA38 Tier 2 units vary — some single-cooler, some dual-cooler setups.

  • The S12R requires dual circuits:

    • Jacket Water: ~23,375 BTU/min

    • Aftercooler: ~20,778 BTU/min

  • Minimum required modification:

    • Add a second cooling circuit for the aftercooler

    • Existing JW cooler may be reusable depending on flow rate

Each vessel is different — channel iron coolers cannot be verified by model alone. Flow rate and physical dimensions must be measured.

Exhaust & Air Inlet Piping

  • KTA38 commonly installed with:

    • 10″ muffler and 10″ exhaust piping

    • 7″ air intake

  • S12R requires:

    • 12″ muffler recommended

    • 8–10″ air intake piping

  • Larger exhaust flow on S12R requires up-sizing from 10″ to 12″ for performance.

Bellhousing & Gear

  • KTA38 = Single “0” bellhousing

  • S12R = Double “00” bellhousing

  • An adapter is required
    (adds ~2 inches to total length)

  • RPM match is identical:

    • S12R: 1800 RPM

    • KTA38: 1800 RPM
      → Gear ratio is typically acceptable

Propeller

  • The 1600 RPM S12R may require modifications to fully capture its available power, such as a propeller adjustment and/or gear ratio change.

  • Because the propeller determines how much power is actually absorbed, a propeller expert should review the vessel and updated performance data to ensure the engine’s full output can be utilized.

  • S12R adds ~100 HP over KTA38

  • To achieve full performance, propeller pitch and/or diameter adjustments are likely required.

Operational Notes From Repower Surveys

Based on an example survey of Kirby M/V Lafitte with KTA38 Tier 2 engines:

Findings:

  • 48″ rail spacing supports S12R mounting (required for S12R: 44" from bolt hole to bolt hole on the mounting feet)

  • 44″ clearance below crank centerline → acceptable oil pan depth (required for S12R: 25.8")

  • 32″ frame below crankshaft → Although it's not an obstruction, it's good to note that any findings beneath the engine.  Could potentially limit the ability to drop the new engines oil pan for major service in the future, because you're only going to have seven inches of clearance from the bottom of our oil pan to that frame.

  • Ventilation ducting near inboard side may need relocation for height clearance

  • 10″ mufflers present and room for 12″ upgrade

  • Air intake requires up-sizing from 7″ flex pipe

  • Fuel return line required upgrade from ½″ to ¾″

In summary:
To summarize everything: based on the repower survey and engine specifications, the main changes required when moving from a KTA38 to an S12R are increasing the exhaust and air intake piping diameters, verifying the cooling system, potentially reusing parts of the existing cooling, and adding an additional circuit to meet Tier 3 requirements for Tier 1 engines.

Target Markets

The S12R is a viable repower option anywhere a KTA38 is currently working. These engines are widespread across multiple sectors, and ongoing issues with parts availability, long lead times, and declining reliability make Mitsubishi a strong alternative. If the vessel floats, it is essentially a target market.

Key segments include:

  • Inland river push boats operating high annual hours and needing dependable mechanical power.

  • Harbor and coastal tug boats looking for stronger durability and easier long-term maintenance.

  • Offshore tugs and supply vessels where uptime and service access are critical.

  • Ferries and workboats that benefit from consistent torque delivery and reduced electronic failure points.

IRR Companies with KTA 38 Boats (As of February 2022)

Company Quantity
Kirby Inland Marine LP 248
Blessey Marine Services, Inc. 106
LeBeouf Bros. Towing LLC 52
Enterprise Marine Services LLC 35
Canal Barge Co., Inc. 28
Marquette Transportation Co. Gulf-Inland LLC 20
American River Transportation Co. LLC 20
Turn Services LLC 16
Buffalo Marine Service, Inc. 14
Ingram Barge Co. 14
D&S Marine Service LLC 12
American Commercial Barge Line LLC 12
Centerline Logistics Corp. 12
Zito Companies 11
Devall Towing & Boat Service 10
Carline Companies 8
Wepfer Marine, Inc. 8
Marquette Transportation Co. Gulf-Offshore LLC 8
Amherst Madison, Inc. 6
Superior Marine, Inc. 6

Why Operators Choose Mitsubishi Even With Upfront Work

Repowers from Cummins or CAT to Mitsubishi often require changes to piping, cooling, mounts, and sometimes gears. Even so, many operators are choosing Mitsubishi because the long-term savings are far greater than the installation effort.

A major driver is engine life. Many fleets are frustrated with engines that only last 30,000–40,000 hours before needing full replacement. After one or two engine swings, operators have already spent what a single Mitsubishi installation would have cost—without gaining the longevity Mitsubishi provides.

Operators are also reporting 30–35% better fuel economy in real-world use. Even when published fuel burn numbers appear higher on paper, performance at matched horsepower often shows Mitsubishi running more efficiently on-vessel. This becomes a major advantage for operators managing tight fuel budgets or negotiating fuel-sensitive contracts.

In short, despite the upfront repower work, Mitsubishi engines deliver longer life, lower operating cost, and better real-world efficiency, making the investment well worth it for many fleets.

 

Service Challenges with Cummins KTA Engines

Operators are increasingly frustrated with long service delays on Cummins equipment. What once took a day or two is now commonly 7–10 days before a technician can get to the vessel. With downtime costing $3,000–$5,000 per day, a simple issue like a failed sensor can quickly turn into $30,000–$50,000 in lost revenue.

These delays don’t even account for parts shortages, which can push downtime even further. After experiencing a few of these events, many operators realize they have spent enough in lost operation to justify a complete repower. This growing pain point is a major reason fleets are moving away from the KTA platform and toward more service-friendly mechanical alternatives.

Repower Planning Summary

Primary Required Changes

  • Increase exhaust diameter

  • Increase air intake diameter

  • Add or expand cooling circuits

  • New aftercooler cooling hardware

  • New mounting pedestals and bellhousing adapter

  • Propeller rework to utilize full power output

  • Verify overhead and under-engine clearances

  • Possible relocation of ventilation ductwork

General Observations

  • Most KTA38 vessels surveyed have enough length and width to accommodate the S12R.

  • Cooling variations between boats make each repower unique—no two surveys will be identical.

  • Many vessels will reuse jacket water cooling but require new aftercooler systems.