- Identity Check: What This Cheat Sheet Covers
- The Module Catalog at a Glance
- Route and Scope: Which Modules Apply to You
- Q620 Motor Plants: Must-Know Topics
- Q621 Steam Plants: Must-Know Topics
- Q622 Gas Turbine Plants: Must-Know Topics
- Q623 Safety and Environmental Protection
- Q624 General Subjects: The Widest Net
- Q625 Electricity, Electronics and Control Engineering
- Sequencing Your Review by Module
- Frequently Asked Questions
- DDE here means National Designated Duty Engineer Steam/Motor/GT, administered through the U.S. Coast Guard National Maritime Center (NMC).
- The catalog lists six module groups, Q620 through Q625, but not every applicant tests on all six.
- The published guide describes the DDE01 Motor-Gas Turbine combination as Q620, Q622, Q623, Q624 and Q625.
- Q621 Steam Plants exists in the current NMC catalog but sits outside that DDE01 combination.
Identity Check: What This Cheat Sheet Covers
Several unrelated credentials share the "DDE" acronym. This page is about one thing only: the National Designated Duty Engineer Steam/Motor/GT credential, a U.S. Coast Guard marine engineering endorsement tested through the National Maritime Center. If you need background on the acronym itself, see What Is DDE? and What Does DDE Stand For?.
Treat this article as a one-page mental map rather than a substitute for the official sources. The facts below come from the NMC examination guide (regulations effective 2014-03-24) and the published NMC module-topic catalog for DDE Unlimited. It is not a new 2026 universal exam specification, and it does not publish official percentage weights, a universal exam duration or a retirement schedule for any module. Always confirm your own module list against your individual NMC approval to test.
The Module Catalog at a Glance
The NMC organizes DDE Unlimited content into module codes. Here is the whole catalog on one screen.
| Module | Name | Core Subject Area |
|---|---|---|
| Q620 | Motor Plants | Diesel/motor propulsion, auxiliary engines, fuel and injection, plus auxiliary boilers |
| Q621 | Steam Plants | Main boilers, steam theory, main and auxiliary turbines, boiler water chemistry |
| Q622 | Gas Turbine Plants | Gas turbine fundamentals, start, fuel, bleed air, controls, troubleshooting |
| Q623 | Engineering Safety & Environmental Protection | Fire, flooding, damage control, pollution prevention, regulations |
| Q624 | General Subjects | Pumps, piping, hydraulics, refrigeration, steering, ship construction, administration |
| Q625 | Electricity, Electronics & Control Engineering | Generators, motors, distribution, high voltage, networks, troubleshooting |
For a deeper domain-by-domain walkthrough, the companion piece DDE Exam Domains 2026: Complete Guide to All 6 Content Areas expands each group.
Route and Scope: Which Modules Apply to You
This is the single most misunderstood point about the DDE, so it earns its own section. The catalog and your personal exam list are different things.
| Route | Modules | Note |
|---|---|---|
| DDE01 Motor-Gas Turbine (original or increase in scope) | Q620, Q622, Q623, Q624, Q625 | Per examination guide section 3.2.3 |
| DDE02 increase in scope | Q620 | Motor plants |
| DDE03 increase in scope | Q622 | Gas turbine plants |
| DDE1000/4000HP (DDE04) | Q630-Q634 | Separate catalog, not interchangeable with Unlimited |
| Q621 Steam Plants | Current NMC catalog | Present in the catalog; not part of the DDE01 combination above |
Eligibility questions (sea service, medical, prerequisites) are covered in DDE Requirements 2026, and timing questions in DDE Exam Dates 2026.
Q620 Motor Plants: Must-Know Topics
Q620 Motor Plants
The catalog opens with Motor Propulsion fundamentals, then moves through the full engine system chain, and ends with steam generators and auxiliary turbines that support a motor plant.
- Engines: main engines, auxiliary engines, starting systems
- Fuel path: fuel, fuel systems, fuel injection systems
- Air and gas path: intake-exhaust systems, air-charging systems
- Support systems: lubrication, cooling, drive systems, governors
- Operations: safety, casualty control, control and automation systems
- Steam side: auxiliary boilers, feedwater systems, condensate systems, auxiliary turbines
Think in systems and failure chains. A governor question, for example, may really be testing whether you understand fuel injection response. Because Q620 is part of the DDE01 route and also the DDE02 increase in scope, it is the one module nearly every motor-focused candidate must master cold.
Q621 Steam Plants: Must-Know Topics
Q621 Steam Plants
If steam is part of your approved test list, this module is a full course in boiler and turbine plant operation.
- Generation: steam theory, main boilers
- Water and condensate loop: feedwater system, condensate system, recovery system
- Fuel and chemistry: fuel, main fuel system, boiler water chemistry
- Prime movers: steam engines, main turbines, auxiliary turbines, governor system
- Plant support: lubrication systems, drive systems, auxiliary diesels
- Operations: control systems, automation systems, safety, casualty control
Boiler water chemistry is the topic candidates most often underestimate, because it feels like a lab subject rather than an engine-room one. Trace the full loop in your head: feedwater in, steam out, condensate back, treatment throughout.
Q622 Gas Turbine Plants: Must-Know Topics
Q622 Gas Turbine Plants
This module appears in the DDE01 combination and stands alone as the DDE03 increase in scope.
- Fundamentals: configurations, thermo, construction, operating parameters
- Systems: start systems, lubrication systems, fuel systems, drive systems, bleed air systems, intake and exhaust systems
- Controls: instrumentation and controls
- Care of the machine: maintenance, inspection, troubleshooting, casualty control
- Heat recovery side: auxiliary boilers, feedwater, condensate and recovery systems
Gas turbines reward candidates who can connect operating parameters to symptoms. Know what a healthy start sequence looks like, then reason backward from abnormal readings. The catalog's explicit mention of inspection and troubleshooting signals that diagnostic reasoning matters here.
Q623 Safety and Environmental Protection
Q623 Engineering Safety & Environmental Protection
A compact but high-stakes module that blends practical shipboard emergency knowledge with regulatory compliance.
- Fire: fire theory, fire prevention, fire fighting
- Water: flooding, dewatering, stability and trim, damage control
- Equipment and hazards: emergency equipment and lifesaving, hazardous materials, general safety
- Environment: pollution prevention
- Compliance: inspections and surveys, U.S. rules and regulations
This module tends to feel more memorization-driven than the plant modules. Regulations are best learned alongside the scenarios they govern, such as linking pollution prevention rules to oily water handling you will also meet in Q624.
Q624 General Subjects: The Widest Net
General Subjects has the longest topic list in the catalog, which is exactly why it deserves a strategy rather than a skim. Group the topics before you study them:
Q624 General Subjects
Cluster the catalog into families so you can study by relationship rather than alphabetically.
- Fluid handling: pumps, compressors, pipes, fittings and valves, hydraulics, bilge systems, oily water separators
- Thermal and climate: heat exchangers, refrigeration, air conditioning, ventilation, distilling systems, thermo
- Machinery and hull: bearings, propellers and shafting, steering systems, deck machinery, ship construction and repair, machine shop, hand tools
- Shipboard services: freshwater systems, sanitary-sewage systems, lubricants
- Information and rules: prints and tables, instruments and gauges, international rules and regulations, administration
Notice the overlaps: governors, cooling systems, lubrication, automation and casualty control all reappear from other modules. Studying them once at depth pays off twice.
Q625 Electricity, Electronics and Control Engineering
Q625 Electricity, Electronics & Control Engineering
The module that candidates from purely mechanical backgrounds most often find humbling.
- Foundations: theory, general maintenance, batteries
- Machines: generators, motors, motor controllers
- Power systems: propulsion systems, distribution systems, high voltage systems
- Electronics and data: electronic systems, communications, computers and networks
- Practice: safety, casualty control, troubleshooting
High voltage systems and computers and networks reflect how modern engine rooms actually run. Do not leave them for the last day.
Key Takeaway
Build your review sheet from your own approved module list, not from the full six-module catalog. Then use the shared topics (governors, cooling, lubrication, automation, casualty control, safety) as your highest-leverage overlap.
Sequencing Your Review by Module
Here is one sensible way to order a review of the DDE01 combination. Adjust it to your own module list and weaknesses. For a fuller plan, see the DDE Study Guide 2026.
Q624 General Subjects
- Start here because its fluid, thermal and machinery topics feed every plant module
- Cluster by family, not alphabetically
Q620 Motor Plants and Q622 Gas Turbine Plants
- Study plant types back to back to compare start, fuel and control systems
- Add Q621 Steam Plants here only if it is on your approved list
Q625 Electrical and Control Engineering
- Give it dedicated time rather than folding it into plant study
- Prioritize distribution, high voltage and troubleshooting
Q623 Safety and Environmental Protection, then mixed review
- Tie regulations to the scenarios from earlier blocks
- Finish with timed practice across all assigned modules
Timed practice helps most once you know your module list. Use the DDE practice tests to find weak areas, then return to the catalog topics above to close them. Questions on the practice test site are built around these same plant and safety topics.
Frequently Asked Questions
No. The six groups describe the catalog across plant types, not six compulsory modules for every applicant. Your required modules depend on the endorsement, horsepower, plant type and NMC approval to test. For DDE01 Motor-Gas Turbine, the published guide lists Q620, Q622, Q623, Q624 and Q625.
Q621 is in the current NMC catalog, but it is not part of the DDE01 Motor-Gas Turbine combination described in section 3.2.3 of the examination guide. Confirm with the NMC whether it applies to your specific case rather than assuming.
No. DDE1000/4000HP is a separate DDE04 catalog using modules Q630 through Q634. It is not interchangeable with the Unlimited modules described in this article.
The sources used here do not publish percentage weights, so none should be assumed. Treat the module groups as unweighted preparation categories and spend your time according to your approved list and personal gaps. See How Hard Is the DDE Exam? for difficulty context.
Pricing is broken down in DDE Certification Cost 2026, scoring in DDE Passing Score 2026, and results data in DDE Pass Rate 2026. Confirm every figure against current NMC guidance before you rely on it.