A Reversing Screw Propeller is a marine screw propeller that produces astern thrust without reversing the engine — either by rotating its blades around their spindle axes (controllable pitch) or by mechanically flipping the rotation direction at the shaft. It solves the problem of stopping and reversing a heavy vessel quickly when the prime mover cannot itself run backwards, such as a gas turbine or a single-direction diesel. The Reversing Propeller swings blade pitch from positive through zero to negative, reversing the thrust vector while shaft RPM stays steady. The outcome is fast crash-stop response — a 4,000 t ferry can reach full astern thrust in 8-12 seconds.
Reversing Screw Propeller Interactive Calculator
Vary blade pitch limits, servo stroke, and crosshead travel to see the resulting propeller pitch and astern/ahead command.
Equation Used
The calculator maps servo crosshead travel across the full controllable-pitch range. At zero travel the blade is at the ahead pitch angle; at full stroke it reaches the astern pitch angle. The feather position is where the interpolated pitch equals 0 deg.
- Crosshead travel is approximated as linearly proportional to blade pitch angle.
- Shaft RPM remains constant while blade pitch changes.
- Positive pitch indicates ahead thrust; negative pitch indicates astern thrust.
How the Reversing Screw Propeller Works
The Reversing Screw Propeller, also called the Reversing Propeller in commercial marine catalogues, works by changing the angle of attack of each blade relative to the oncoming water. On a controllable pitch hub, every blade sits on its own spindle inside the boss, and a hydraulic servo piston inside the hub pushes a crosshead fore and aft. That crosshead drives a Scotch yoke or crank pin on each blade root, swinging the pitch from roughly +30° ahead through 0° (feathered, no thrust) to about -20° astern. Shaft RPM stays constant. Thrust direction reverses because the blade's lifting face swaps sides relative to the flow.
Why build it this way? A direct-reversing diesel takes 15-25 seconds to stop, reverse air-start, and rebuild RPM. A Reversing Propeller does it in under 10 seconds because the prime mover never stops. That matters for ferries on tight schedules, naval vessels needing crash-stop, and tugs that need to swap from push to pull without dropping line tension. The cost is mechanical complexity inside the hub — typically 6-10 moving parts running in oil at 8-12 bar, with shaft seals that must hold against full sea pressure plus servo pressure.
Tolerances matter. Blade pitch repeatability across the four blades must hold within ±0.25° or you get differential thrust between blades, which shows up as a 1-2 Hz hull vibration at cruise. If the hub seal leaks, oil discharges into the sea (an environmental violation) and air ingests into the servo, which softens pitch response. The classic failure mode is crosshead bushing wear — once radial clearance exceeds 0.15 mm, blade pitch hunts under load and you'll see the bridge pitch indicator wandering 1-2° at steady throttle. The other common failure is salt-water ingress past the inboard oil distribution box, which emulsifies the hub oil and seizes the spindle bearings inside 200 operating hours.
Key Components
- Propeller Hub (Boss): The pressure vessel housing the pitch-change mechanism. Typically nickel-aluminium-bronze cast, 600-1200 mm diameter on a medium ferry, holding hub oil at 8-12 bar. Wall thickness rarely drops below 35 mm because the boss carries blade root bending moments up to 80 kN·m on a 4 m diameter prop.
- Blade Root and Spindle Bearing: Each blade bolts to a flange on a vertical spindle running in two journal bearings inside the hub. Spindle diameter is sized for blade-spinning torque — typically 15-25% of shaft torque — and bearing radial clearance must stay under 0.10 mm or pitch repeatability suffers.
- Servo Piston and Crosshead: Hydraulic actuator inside the hub that converts oil pressure into axial motion. Stroke is typically 200-400 mm corresponding to the full +30° to -20° pitch range. Working pressure 60-100 bar on the servo side, fed through a rotating oil distribution box on the inboard end of the shaft.
- Oil Distribution Box (OD Box): Stationary-to-rotating transfer that pipes servo oil into the hollow propeller shaft. Carbon-face seals run against a hardened steel runner at shaft RPM. Seal life is typically 8,000-15,000 hours and a leak here is the single most common reason a CPP system goes offline.
- Pitch Feedback Rod: A concentric rod inside the hollow shaft transmits actual pitch position back to the bridge controller. Position sensor resolution is typically 0.1° and the control loop closes on this signal, not on commanded servo pressure.
- Blade Crank Pin (Scotch Yoke or Slipper): Converts axial crosshead motion into blade rotation. The slipper block rides in a slot on the crosshead while pinned eccentrically to the spindle. Bushing wear here is the primary cause of pitch hunting under load.
Real-World Applications of the Reversing Screw Propeller
The Reversing Screw Propeller earns its complexity wherever shaft RPM cannot or should not change quickly. That includes gas-turbine warships, ice-class ferries, dynamic positioning vessels, and any boat where the prime mover drives PTO loads (shaft generators, fire pumps) at fixed speed. You also see Reversing Propellers on tugs and trawlers that swap thrust direction dozens of times per shift — a direct-reversing diesel would burn out its starting air system inside a week of harbour work.
- Naval surface combatants: The Royal Navy Type 23 frigate uses CPP propellers driven by Spey gas turbines and Paxman diesels through cross-connect gearboxes — gas turbines cannot reverse, so the propeller must.
- Ro-Pax ferries: Stena Line's Stena Hollandica operates Wärtsilä CPP units that allow the ship to dock at Hoek van Holland with shaft generators still running at synchronous speed, keeping bow thrusters live during the manoeuvre.
- Harbour and escort tugs: Damen ASD 2810 tugs use twin Rolls-Royce/Kongsberg CPP azimuth units to swing from 80 t bollard pull ahead to full astern in roughly 6 seconds without RPM change.
- Ice-class supply vessels: Aker Arctic-designed PSVs working the Sakhalin shelf run CPP propellers because feathering blades at zero pitch reduces ice-impact damage when the vessel is stationkeeping in pack ice.
- Stern trawlers and seiners: Norwegian factory trawlers like the Havfisk Stamsund run CPP to deliver high pull at low boat speed during net haul, then transition to free-running pitch on the steam home — same shaft RPM, different load.
- Dynamic positioning offshore vessels: DP2 platform supply vessels use CPP main propellers because the DP controller can command thrust through zero faster than any RPM-based control loop, holding station within ±2 m in 30-knot wind.
The Formula Behind the Reversing Screw Propeller
The thrust output of a Reversing Screw Propeller is set by blade pitch, shaft RPM, propeller diameter, and water density. The formula below is the dimensional thrust equation used to size pitch-change response. At the low end of the pitch range — say +5° — you're producing only 10-15% of design thrust, useful for slow harbour creep but generating heavy slip. At nominal design pitch (often around +18° to +22° on a working tug), thrust efficiency peaks at 65-72%. Push beyond +28° and blade stall begins, thrust plateaus, and torque demand spikes — that's why pitch-change schedules cap at the design point rather than chasing geometric maximum.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| T | Propeller thrust (positive ahead, negative astern) | N | lbf |
| KT | Thrust coefficient — function of pitch ratio P/D and advance ratio J | dimensionless | dimensionless |
| ρ | Water density (sea water ≈ 1025 kg/m³) | kg/m³ | slug/ft³ |
| n | Shaft rotational speed | rev/s | rev/s |
| D | Propeller diameter | m | ft |
Worked Example: Reversing Screw Propeller in a 32 m harbour tug repitch calculation
Your harbour tug operating company in Rotterdam is sizing the astern bollard pull for a Damen ASD 2810 with twin 2.6 m CPP propellers turning at 250 RPM constant shaft speed. You need to know what thrust per shaft you'll see at three operating pitch settings: -5° (creep astern), -15° (nominal full astern), and -20° (commanded crash-stop maximum). Sea water density is 1025 kg/m³. From the manufacturer's open-water diagram, KT at the relevant pitch ratios reads 0.08, 0.21, and 0.26 respectively at zero advance (bollard condition).
Given
- D = 2.6 m
- N = 250 RPM
- ρ = 1025 kg/m³
- KT at -5° = 0.08 —
- KT at -15° = 0.21 —
- KT at -20° = 0.26 —
Solution
Step 1 — convert shaft speed to rev/s and pre-compute the rotational and geometric terms that don't change with pitch:
Step 2 — compute nominal astern thrust per shaft at -15° pitch (KT = 0.21):
Across both shafts that's roughly 342 kN, or about 35 t of astern bollard pull — the working sweet spot. The boat will pull off a barge with authority but the engine governor isn't fighting peak torque.
Step 3 — at the low end of the operating range, -5° creep pitch (KT = 0.08):
That's roughly 13 t of astern pull across both shafts — barely enough to overcome the tug's own hydrodynamic drag at 2 knots, which is exactly the regime you want for nudging a barge into a lock without crushing fenders.
Step 4 — at the commanded crash-stop maximum, -20° (KT = 0.26):
In theory that's 43 t per shaft. In practice, blade stall above -18° means the real measured thrust plateaus around 195-200 kN and torque demand spikes 25% above design — the engine governor will pull RPM down 8-12 RPM and you'll hear it in the exhaust note before the bridge ever sees the load alarm.
Result
At nominal -15° pitch the tug produces 171 kN of astern thrust per shaft, or about 35 t of astern bollard pull across both propellers. That's the operational sweet spot — the tug responds crisply to bridge orders without the prime mover labouring. The low-end creep pitch (-5°) gives you a controllable 65 kN for delicate harbour work, while the high-end -20° crash-stop setting theoretically reaches 212 kN but in practice plateaus near 200 kN as blade stall sets in and the engine governor drops RPM. If you measure thrust 15-20% below predicted at the nominal setting, check three things in this order: (1) the OD box pitch feedback calibration — a 2° feedback offset is the most common cause and shows as steady-state thrust shortfall across all settings, (2) hub oil emulsification from a degraded inboard shaft seal, which softens servo response and limits actual blade angle achieved under load, and (3) blade surface fouling — a 1 mm calcareous growth layer on a 2.6 m prop drops KT by roughly 8% and is invisible from the deck.
Reversing Screw Propeller vs Alternatives
A Reversing Screw Propeller is one of three ways to get astern thrust on a powered vessel. The other two are a fixed pitch propeller with a reversing reduction gear, and a fixed pitch propeller on a direct-reversing engine. Each has a clear application window — the choice comes down to how often you reverse, what drives the shaft, and how much hub complexity your crew can support.
| Property | Reversing Screw Propeller (CPP) | Fixed Pitch + Reversing Gearbox | Fixed Pitch + Direct-Reversing Engine |
|---|---|---|---|
| Time to full astern thrust | 6-10 s | 10-15 s | 15-25 s |
| Mechanical complexity (moving parts in wet end) | High — 6-10 parts in hub | Low — solid prop | Lowest — solid prop, no clutch |
| Capital cost vs fixed pitch baseline | 2.5-3.5× baseline | 1.3-1.6× baseline | 1.0× baseline |
| Hub oil seal service interval | 8,000-15,000 hours | N/A | N/A |
| Compatible with gas turbine prime mover | Yes | Yes (with reverse gear) | No |
| Compatible with shaft generator at constant RPM | Yes — primary advantage | No — RPM drops in reverse | No |
| Reversing cycles per day before fatigue concern | Unlimited | 200-400 | 30-60 (starting air limit) |
| Typical installed efficiency at design point | 65-72% | 68-74% | 68-74% |
| Best application fit | Tugs, ferries, frigates, DP vessels | Cargo ships, fishing vessels | Bulk carriers, tankers |
Frequently Asked Questions About Reversing Screw Propeller
This is almost always a feedback calibration drift, not a hydraulic problem. The bridge indicator reads the position of the feedback rod at the OD box end of the shaft, but if the rod's mechanical zero has shifted — typically from a loose locknut on the inboard coupling — the indicator can read -20° while actual blade angle is only -8°.
Diagnostic check: at zero pitch commanded, look over the stern at the prop in clear water. The blade chord lines should be visibly aligned with the shaft axis. If they're already cocked when the indicator reads zero, your feedback zero is off and every commanded pitch downstream is wrong by that offset.
At 6 reversals per day, a reversing reduction gearbox is the better economic choice. CPP earns its premium when reversal frequency exceeds roughly 30-50 cycles per day, when shaft generators must stay synchronised, or when the prime mover physically cannot reverse (gas turbines, some medium-speed diesels with PTO loads).
For 6 reversals on a coastal cargo run, the gearbox solution gives you 95% of the operational capability at 40-50% of the lifetime cost. The CPP hub seal alone will cost you €15,000-25,000 every 10,000 hours, which the gearbox simply doesn't have.
Trapped air. When you drain and refill a CPP hub, the upper galleries inside the boss hold air pockets that the servo has to compress before it can move the crosshead. Until that air migrates to the OD box and bleeds out, you'll see 0.5-1.5 second lag on pitch commands.
The fix is to cycle the propeller from full ahead to full astern 8-10 times at the dock with the engine at idle. Each cycle pushes air toward the inboard end where the bleed point lives. If sluggishness persists past 20 cycles, suspect a sticky pilot valve in the OD box rather than air.
Differential blade pitch. If one blade sits 0.4° or more off the others, you get one revolution per cycle of unbalanced thrust — at 90 RPM cruise that's 1.5 Hz, exactly in the reported band. The hull is essentially being kicked once per shaft revolution.
The cause is usually one worn crank pin slipper or a loose blade-root bolt allowing the blade to lag under load. Pull the boat and check each blade's pitch independently against a fixed reference at the hub flange — they should agree within ±0.25°. Anything beyond that and you've found your vibration source.
A well-tuned CPP holds zero thrust to within ±2-3% of rated, which is good enough for DP2 stationkeeping in moderate weather but not for DP3 deepwater work. The limit is the resolution of the pitch feedback rod and the deadband of the servo pilot valve — typically 0.3-0.5° of pitch, which translates to small but measurable thrust at constant RPM.
For DP3 or precision survey work, azimuth thrusters with RPM-modulated control beat CPP because they don't have the pitch deadband. CPP wins on a budget and on vessels that need the main propulsion shaft for transit speed.
Two reasons stack up. First, the hub boss is physically larger on a CPP — typically 28-32% of propeller diameter versus 18-22% on a fixed-pitch unit — and that extra blockage interferes with the inflow to the blade roots, costing 2-4% open-water efficiency. Second, the constant-RPM operating mode means at part load you're running off the optimum advance ratio, where a fixed-pitch shaft would just slow down and stay near peak efficiency.
On a long ocean passage at constant load, fixed pitch wins on fuel every time. CPP pays back only when manoeuvring frequency or PTO requirements dominate the duty cycle.
References & Further Reading
- Wikipedia contributors. Controllable pitch propeller. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
