DNV-RU-SHIP Part 4 Chapter 7 is the DNV class rule for rotating machinery on classified vessels — covering propulsion shafts, reduction gearboxes, flexible couplings, steering gear, and azimuth thrusters. With 118 chunks in the Leide Navigator knowledge base, Ch.7 is among the more substantive chapters in the ship rules for engineers working on OSVs, offshore construction vessels, and drillships where high-power propulsion systems and redundant steering arrangements are safety-critical.
This article provides a clause-by-clause technical commentary: what Ch.7 requires, how it divides design responsibility with the companion machinery standards (DNV-OS-D101, DNV-RU-SHIP Pt.2 Ch.2), and where the rules produce practical design constraints that offshore machinery engineers encounter during new-builds and major conversions.
1. Scope and applicability
Chapter 7 applies to the design, manufacture, and survey of rotating machinery that transmits power from prime movers to propellers, thrusters, or other driven equipment on DNV-classed vessels. The scope covers:
- Propulsion shafting — intermediate shafts, propeller shafts, stern tube arrangements, and shaft seals
- Reduction gearboxes — single-input, multi-input, and power-take-off gearboxes driving propellers or auxiliary systems
- Flexible couplings — between prime mover and gearbox, or between gearbox and shafting
- Steering gear — hydraulic or electro-hydraulic systems complying with SOLAS Chapter II-1 and the DNV class overlay
- Azimuth thrusters and tunnel thrusters — electrical or hydraulic drives with DNV type approval
- Propeller shaft bearing arrangements — white-metal or oil-lubricated stern tube bearings
Chapter 7 does not cover the prime movers (engines, motors, generators) — those are addressed in DNV-RU-SHIP Pt.4 Ch.2 (internal combustion engines) and Pt.4 Ch.8 (electrical installations). It governs the power transmission path from engine output flange to propeller hub.
2. Propulsion shafts — dimensioning requirements
Ch.7 provides explicit formulas for minimum shaft diameter at each cross-section. For intermediate shafts transmitting only torsion, the basic requirement is derived from the transmitted power P (kW) and shaft speed n (rpm):
where C is a material-dependent constant specified in the rule tables, accounting for the permissible torsional stress. For standard carbon steel (ReH ≥ 245 MPa), C takes a value of approximately 100 in the DNV formula when P is in kW and n in rpm, yielding d in mm. The constant reduces for higher-strength shafting materials, reflecting the increased permissible stress.
The formula applies to the minimum diameter at the weakest cross-section. Where shafts pass through stern tube bearings, flange connections, or keyways, the local stress concentration must be evaluated and the diameter increased if the resulting stress exceeds the permissible value.
Propeller shaft — tapered-cone connections
The propeller shaft between the stern tube bearing and the propeller hub has additional requirements. DNV specifies:
- Taper ratio of the cone — typically 1:12 to 1:15 — to achieve reliable hydraulic press-fit or key-driven connection
- Contact length between cone and hub: minimum 1.5 × propeller shaft diameter
- Pull-up force and press-fit pressure must be documented in the class-approved drawing package
- Propeller nut arrangement must prevent loss of propeller even if the press fit relaxes under transient loading
The stern tube — the structural tube through which the propeller shaft exits the hull — must provide adequate bearing support. Ch.7 specifies maximum permissible bearing loads and span-to-diameter ratios to prevent shaft whip and premature bearing wear.
3. Torsional vibration analysis
Torsional vibration (TV) analysis is a mandatory submission requirement under Ch.7 for all propulsion installations. The analysis must demonstrate that torsional resonances do not produce alternating stresses exceeding the permissible values in any component of the shafting system across the full operating speed range.
Barred speed ranges
Where resonances occur at speeds that cannot be avoided entirely, Ch.7 allows a barred speed range — a region of the speed envelope that the propulsion control system actively avoids during normal operation. Requirements:
- The barred range must be clearly marked on the tachometer or bridge display
- The propulsion control system must transit through the range without dwelling (ramp rate enforced by the automation system)
- The range must be documented in the stability and operating manual as a restriction
Class surveyors verify TV compliance during sea trials — the shaft system is instrumented and the measured alternating torque is compared to the submitted TV analysis. Significant deviation from the analysis values triggers a hold notice and requires re-analysis.
4. Gearboxes and gear tooth loads
Reduction gearboxes on DNV-classed vessels must be designed to a service factor that accounts for the nature of the driven load. Ch.7 specifies service factors KA for different drive types:
| Drive application | Application factor KA | Notes |
|---|---|---|
| Fixed-pitch propeller (uniform torque) | 1.25 – 1.50 | Lower end for diesel-electric drives, upper for direct diesel |
| Controllable-pitch propeller (CPP) | 1.50 – 1.75 | Higher due to blade pitch change transients |
| Thruster drive | 1.50 – 2.00 | Bollard pull loads and wave-induced torque variations |
| PTO gearbox (shaft generator) | 1.25 – 1.50 | Depends on generator loading transients |
Gear tooth loads are evaluated per ISO 6336 (Hertzian contact stress, bending root stress) with the application factor applied to the nominal transmitted torque. DNV accepts ISO 6336 as the gear calculation method when referenced explicitly in the class submission. The calculated contact stress σH must not exceed the permissible contact stress σHP = σHlim × ZNT / (SH,min × ZW), accounting for material quality grade (MQ, ME), lubrication, and gear accuracy grade.
Gear housing and mounting
The gearbox housing must be rigid enough to maintain gear alignment under rated torque and thrust loads. Ch.7 requires:
- Housing deflection calculations or FEA if the housing is non-standard geometry
- Documentation of thermal expansion allowances at the mounting feet
- Thrust bearing arrangement to absorb propeller thrust reaction — either internal to the gearbox or in a separate thrust block
- Lube oil system with pressure switches, temperature sensors, and a standby pump for main propulsion gearboxes
5. Flexible couplings and alignment
Flexible couplings serve two purposes in a propulsion train: transmitting torque while accommodating angular and radial misalignment, and attenuating torsional vibration transmission between engine and shafting. Ch.7 requirements for flexible couplings include:
- Torque rating — continuous rated torque must exceed peak output torque at full load including the application factor; coupling must withstand transient torques up to two-pole short-circuit torque for electrical drives
- Torsional stiffness — must be declared to the TV analyst; stiffness variation between fresh and worn coupling must be documented as it affects the TV natural frequencies
- Fail-safe mode — loss of the flexible element (rubber pack, elastomeric spider) must not cause loss of propulsion; many installations use a backup metal-to-metal interface that transmits reduced torque after failure
- DNV type approval — flexible couplings for main propulsion are subject to type approval testing before installation on a classed vessel
Shaft alignment — the static deflection curve of the shafting system under gravity, propeller weight, and thermal expansion — must be submitted for approval. The aligned bearing reactions must remain positive (no bearing lift-off) under all combinations of load and temperature, including dry-dock condition (propeller shaft supported without the stern tube bearing).
6. Steering gear — redundancy and sizing
Steering gear on classed vessels is a SOLAS/flag-state safety system, and Ch.7 provides the class rules that layer on top of SOLAS Regulation II-1/29. The key class requirements beyond SOLAS are:
Sizing: torque and response time
The hydraulic system must move the rudder from 35° one side to 30° the other side in not more than 28 seconds at maximum service speed — the SOLAS baseline. Ch.7 adds that the steering gear must also satisfy this requirement at 70% of maximum speed (the maneuvering condition). The actuator sizing must account for the maximum rudder torque, including the correction factor for asymmetric loading and ice-class vessels.
Redundancy: two-circuit hydraulic systems
For vessels above 10,000 GT (and all offshore vessels with Class notation DYNPOS-2/3), Ch.7 mandates a two-circuit hydraulic steering arrangement. Requirements:
- Each circuit must independently deliver the full rated steering torque
- Circuits must be isolated by stop valves — failure of one circuit must not impair the other
- Each circuit requires its own hydraulic power unit (pump + motor + reservoir)
- Automatic switchover or manual crossover within 60 seconds of circuit failure
- Power unit located in the steering gear room — not combined with other hydraulic consumers
7. Azimuth thrusters and tunnel thrusters
Azimuth thrusters — podded drives or Z-drives — are increasingly dominant on modern OSVs. Ch.7 governs both the structural arrangement and the propulsion system integration. Key requirements:
Azimuth thrusters
- Sealing and hull penetration — the slewing ring seal must maintain watertight integrity in all operating orientations; class drawings must show the seal arrangement with material specifications
- Slewing mechanism — the slewing ring gear and drive must achieve the required rotation rate (typically 360°/min or faster); bearing loads from propeller thrust and torque reaction must be verified
- Redundancy for DP — two or more azimuth thrusters are typically required for DP-2; they must be powered from separate bus sections to satisfy the single-failure criterion
- Type approval — the complete thruster unit (including electric motor, frequency converter, and gearbox) requires DNV type approval; installation drawings are verified against the approved drawings
Tunnel thrusters
Bow and stern tunnel thrusters provide lateral thrust for DP and harbor maneuvering. Ch.7 requirements for tunnel thrusters include:
- Tunnel diameter sized to limit inflow velocity and avoid cavitation at rated thrust
- Tunnel structure integral with the hull framing — not a separate tube simply penetrating the hull
- Retractable thrusters must have a fail-safe retraction system (loss of hydraulic power must retract the unit, not leave it deployed)
- Watertight integrity maintained when thruster is in the raised position — a deployable gate or hydrodynamic fairing is required
8. Materials, fabrication, and heat treatment
Ch.7 specifies material grades for each rotating component class. The requirements feed into both the design calculations (yield strength, fatigue endurance) and the survey requirements (material certificates, NDE):
| Component | Minimum material grade | Heat treatment / Notes |
|---|---|---|
| Intermediate shafts (solid) | Carbon steel: σU ≥ 400 MPa, ReH ≥ 200 MPa | Normalized or quenched & tempered; ultrasonic tested per NV class material certificate |
| Propeller shaft | Alloy or carbon steel: σU ≥ 590 MPa for high-torque applications | Q&T; shaft must be free from internal defects — UT to ASTM A388 or equivalent |
| Gear wheels (pinion and wheel) | Alloy steel: case-hardened or through-hardened as specified by the gear calculation | Core hardness and case depth verified by coupon testing from the same heat; magnetic particle inspection of tooth roots |
| Gear housing (cast iron or fabricated steel) | GG-25 (EN-GJL-250) minimum for cast; S275 or S355 for fabricated | Pressure-tested after machining; weld procedures qualified to EN 15614 for fabricated housings |
| Rudder stock | Forged steel: σU ≥ 500 MPa | Forging quality certificate; rudder stock is safety-critical — no repair welding permitted without class approval |
9. Type testing and commissioning trials
Ch.7 distinguishes between type testing (performed once on a representative production unit to establish DNV type approval) and factory acceptance testing (FAT, performed on each production unit before delivery). The requirements for rotating machinery are:
Type testing
- Gearboxes: full-load endurance test at rated power for a minimum number of hours (specified in the type approval programme)
- Flexible couplings: torque-overload test to 150–200% of rated torque, followed by inspection of elastomeric elements
- Azimuth thrusters: thrust and efficiency measurement at design power; slewing ring load testing
- Steering gear actuators: pressure test to 1.5× design pressure; stroke and force verification
Factory acceptance testing
- Each production gearbox: no-load run, full-load run at rated torque and speed, temperature stabilization check on lube oil
- Each propulsion shaft (forging): dimensional inspection, ultrasonic testing, hardness testing at witness inspection
- Each steering gear: hydraulic pressure test, full-stroke timing test, alarm function test
FAT records are part of the class-certified documentation package — without them, the surveyor cannot grant a Form E survey certificate for the machinery installation.
10. Cross-standard relationships
| Design area | Governing standard(s) | How Ch.7 interacts |
|---|---|---|
| Structural steel for machinery foundations | DNV-RU-SHIP Pt.2 Ch.2 | NV-grade plate and profiles for engine/gearbox seatings; Ch.7 refers to Pt.2 for material requirements |
| Electrical drive systems | DNV-RU-SHIP Pt.4 Ch.8 | Frequency converters, electric motors, and switchboards for thruster drives governed by Ch.8; Ch.7 covers the mechanical shaft/gearbox side |
| Marine machinery systems (piping, lube oil) | DNV-OS-D101 | Lube oil systems, cooling water circuits, and shaft sealing water are within OS-D101 scope; Ch.7 sets machinery-specific temperature and pressure limits |
| Gear tooth load calculations | ISO 6336 (parts 1–3) | DNV accepts ISO 6336 as the gear calculation method; the application factor KA from Ch.7 is applied to the nominal torque before the ISO calculation |
| Torsional vibration analysis | Internal DNV rules (Ch.7 §7) + engine manufacturer TV data | Analysis method is prescribed in Ch.7; requires damping values and cylinder pressure diagrams from the prime mover manufacturer |
| SOLAS steering gear (flag state) | SOLAS Ch. II-1 Reg. 29/30 | Ch.7 adds class-specific requirements on top of SOLAS minimums — both must be satisfied |
| Hull structural design | DNV-RU-SHIP Pt.2 Ch.1 | Sterntube, thruster tunnels, and rudder trunk are hull structural items; loads from Ch.7 machinery are transferred to hull structure governed by Pt.2 |
11. Common engineering errors
The following recurring issues arise in DNV-RU-SHIP Pt.4 Ch.7 compliance reviews and class submissions:
- TV analysis excluded from the class submission — Ch.7 is explicit that TV analysis is required for all main propulsion installations. Omitting it — even for "standard" diesel-direct drives — results in a class comment that blocks the Final Construction Survey.
- Shaft diameter calculated only at the midspan — the minimum diameter formula applies to the critical cross-section, which may be at the keyway, flange fillet, or bearing journal. Calculating only the free-bore diameter and assuming all other sections are adequate is a systematic error.
- Service factor applied to the wrong torque — the application factor KA should be applied to the rated input torque, not the allowable output torque. Inverting the application produces an unconservative result that surveyors have learned to check.
- Steering gear sized to SOLAS minimum only — vessels with Class notation DYNPOS-2 or heavier must meet Ch.7's redundancy requirements, which are stricter than the SOLAS baseline. Using SOLAS-only sizing documentation will not satisfy the class survey.
- No FAT records for shaft forgings — class forgings require a 3.2 material certificate with witness inspection. If the forging was procured without DNV survey attendance at the forge, a retroactive survey is not always possible, and replacement may be required.
- Flexible coupling stiffness not passed to the TV analyst — when a coupling is changed (e.g., upgraded for higher torque), the TV analysis must be re-run with the new stiffness values. Many vessels have had barred speed ranges imposed retroactively after a coupling upgrade changed the natural frequencies.
12. Using Leide Navigator for Ch.7 queries
DNV-RU-SHIP Pt.4 Ch.7 is fully ingested into the Leide Navigator knowledge base — 118 chunks covering shaft dimensioning, torsional vibration requirements, gearbox service factors, steering gear redundancy, and type-testing requirements. The corpus also includes the related standards (DNV-OS-D101, DNV-RU-SHIP Pt.2 Ch.1, DNV-RU-SHIP Pt.2 Ch.2) that Ch.7 cross-references for materials and structural design.
Sample queries that Navigator can answer from the Ch.7 corpus:
- "What is the minimum propulsion shaft diameter formula in DNV-RU-SHIP Pt.4 Ch.7?"
- "What are the torsional vibration submission requirements for a new main propulsion installation?"
- "What application factor applies to a controllable-pitch propeller gearbox?"
- "What are the steering gear redundancy requirements for a DP-2 vessel under DNV rules?"
- "When is a barred speed range required for torsional vibration compliance?"
Ask Leide Navigator about DNV-RU-SHIP machinery rules
DNV-RU-SHIP Pt.4 Ch.7 (118 chunks), DNV-OS-D101, DNV-RU-SHIP Pt.2 Ch.1 and Ch.2 are all in the knowledge base. Ask specific clause questions about shaft dimensioning, gearbox sizing, torsional vibration, or steering gear redundancy.