A controllable-pitch propeller reverses the sign of blade pitch without reversing shaft rotation. In a force-rod design, axial crosshead travel moves an eccentric crank pin through a transverse slot and turns the blade spindle. Use the calculator to compare blade angle and crosshead travel between ahead and astern limits.
Propeller Pitch-Control Linkage Calculator
Enter ahead and astern angles and full crosshead stroke, then move Crosshead Travel to change the diagram and results. Play and Cycle position do not move this particular diagram.
Equation Used
- The crosshead slot is straight and transverse to axial travel.
- The crank pin is offset from the blade spindle and stays within the slot.
- The two endpoint angles and full stroke together set the implied crank radius.
- Hydrodynamics, thrust, actuator pressure, loads, strength, multi-blade equalization and ratings are excluded.
The companion figure repeats the same crosshead, pin and angle indicator. It shows one linkage, not the complete three-dimensional propeller hub.
How the crosshead changes blade pitch
A force rod moves along the propeller-shaft axis and carries a crosshead. A sliding block in a transverse slot connects that crosshead to a crank pin offset from the blade spindle. Moving the crosshead rotates the crank and blade spindle.
The calculation treats the slot as straight and the pin radius as fixed. Real propeller hubs connect the control motion to several blade roots. The cited patents show examples of these crossheads, sliding blocks and crank rings; their complete hub and hydraulic systems require more dimensions than this tool uses.
Comparing pitch-control travel
Set the ahead and astern angles, the full crosshead stroke and the travel percentage. The results give blade angle, distance from the ahead limit to zero angle, and the local crosshead travel needed for one degree of blade rotation.
For fixed angle limits, increasing stroke increases the implied crank radius and travel per degree. It does not change the angle at a given travel percentage. Hydrodynamic thrust and actuator force need blade, flow and loading data that are not inputs here.
Crank-pin and slot relationship
Let θa be the ahead angle, θb the astern angle, S the full crosshead stroke, and u the entered travel percentage divided by 100. Travel x is measured from the ahead limit toward the astern limit, so x = uS.
With positive crank radius R = S/(sinθa − sinθb), the straight-slot geometry gives x = R(sinθa − sinθ). Therefore sinθ = sinθa + u(sinθb − sinθa), and θ is the inverse sine of that value. The permitted angles stay between −35° and +45°, where this angle solution is single-valued.
Zero-angle travel is x₀ = S sinθa/(sinθa − sinθb). Local travel per degree is the magnitude |dx/dθ| = R cosθ × π/180, in mm/degree. It varies with angle; it is not the average stroke divided by the total angle change.
Worked example
With +30° ahead, −20° astern, 200 mm full stroke and 50% travel, blade angle is about +4.53° at 100 mm crosshead travel. The implied crank radius is about 237.52 mm, and local travel is about 4.13 mm per degree. Zero angle occurs at about 118.76 mm from the ahead limit.
Zero angle is the chosen geometric datum. It does not establish zero thrust in flowing water, and feathering requires a separately specified blade angle and mechanism range.
Reading the diagram and results
The pin and angle indicator respond to Crosshead Travel. Play and Cycle position currently advance a timer without changing this diagram; use the travel input to examine its motion. The small ZERO PITCH status band covers angles within ±0.25°; the numeric angle remains the calculated value.
The illustration uses a fixed screen radius and symbolic hub outline, so changing full stroke changes the millimeter results without resizing the drawing. The blade shape is an angle indicator, not a three-dimensional blade section, and the shaft and force-rod lines do not show the full physical connection.
A complete hub design needs blade-root geometry, control linkages, bearings and actuator loads. Pitch angle alone cannot predict thrust, efficiency, cavitation or a safe operating setting.
Pitch-control questions
Is zero pitch the same as feathering?
No. Zero pitch is the blade-angle datum used here. Feathering turns blades toward a low-drag alignment and requires its own specified blade angle and mechanism range.
Does astern pitch require reversing the shaft?
No. A controllable-pitch propeller can change blade pitch for astern operation while the shaft continues turning in the same direction.
Does pitch percentage equal thrust percentage?
No. Thrust also depends on blade geometry, shaft speed, vessel advance and water flow. The travel percentage here is crosshead travel, not thrust.
Why does Play not move the crosshead?
This diagram uses the Crosshead Travel input only. Play and Cycle position advance a separate timer that is not connected to its pose. Change Crosshead Travel to move the pin and blade-angle indicator; the numeric results update with it.
Does this calculate the whole propeller hub?
No. It calculates one crank pin in a straight transverse slot. It does not size the hydraulic actuator, coordinate multiple blades or predict hydrodynamic performance.
Primary references
- US5145318A, Flange-mounted controllable pitch marine propeller — force rod, axial crosshead, slideways and eccentric blade crank pins; identifies neutral as zero pitch.
- US4907992A, Oil distribution box for a marine controllable pitch propeller — crosshead slots, sliding blocks and crank pins rotating blade crank rings.
- US4028004A, Feathering controllable pitch propeller — explicitly distinguishes full astern, zero pitch, full ahead and feathered positions.
- US2925131A, Controllable pitch propeller — crossheads and crank pins for angular blade adjustment.
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