Marine engineering · naval architecture

Services · Propulsion shafting · Propeller and engine matching

Propeller and engine matching

A matching study shows where the propeller holds the engine across the vessel's operating range: whether the engine reaches its rated power at rated speed with a clean hull, how much margin is left as the hull fouls and the weather rises, and where the operating points sit on the engine's load diagram. It supports the choice of a new propeller, a repowering or derating, a change of service speed, and the investigation of an engine that runs heavy or cannot reach its power.

Discuss a matching study The complete shafting service
Schematic engine load diagram: power against speed, with the engine load limit, the speed limit and the rated point. Three propeller curves are shown: light running with a clean hull, the design curve, and a fouled hull in heavy weather. The light running margin and the service point are marked. 70 % 80 % 90 % 100 % 110 % 40 % 50 % 60 % 70 % 80 % 90 % 100 % 110 % engine speed, per cent of rated engine power, per cent of MCR speed limit engine load limit fouled hull, heavy weather design propeller curve light running: clean hull, calm sea MCR service point light running margin engine margin
The engine load diagram, in schematic form. The propeller absorbs power roughly as the cube of speed, so each hull and sea state gives its own curve. A propeller designed light running, with the clean-hull curve to the right of the rated point, keeps the engine inside its load limit as the hull fouls and the weather rises. A propeller matched too heavy pushes the engine onto its load limit and costs speed and fuel.

What the study covers

  • Resistance and propulsion factors. From model tests, sea trials, CFD or a recognised method for the hull form, with wake fraction, thrust deduction and relative rotative efficiency stated with their source.
  • Propeller open-water performance. From the propeller's own data or from systematic series, set at the operating points of interest.
  • Propeller curves on the load diagram. Clean hull in calm water, the design condition, and the fouled hull in weather, against the engine's load, speed and torque limits.
  • Margins. The light running margin, the sea margin and the engine margin, chosen for the trade and the docking interval.
  • Trial and service data. Speed trials corrected to ISO 15016, and noon-report or monitoring data used to track how the operating point has moved.
  • Shaft-line consequences. The torque and thrust the match imposes, carried into the shaft strength, the thrust bearing and the torsional calculation.

Why the shafting specialist does the matching

The propeller sets the torque the shaft transmits, the thrust the thrust bearing carries, the inertia the torsional calculation depends on and the weight the alignment hangs from the aft end. A change of propeller or of engine rating changes all of these, and a matching study that stops at the speed-power curve leaves them unchecked.

Here the match is made on the same vessel case as the shafting calculations. When a new propeller is proposed, its mass and inertia go straight into the alignment and torsional models, and the report states the effect on the barred range and the bearing loads alongside the effect on speed and fuel.

The propeller open-water calculator on this site shows the Wageningen B-series and Gawn series calculations used at the screening stage of a matching study.

Decisions it supports

Where a matching study is used

Newbuilding

Propeller design point

The light running margin and the design speed fixed against the engine's layout, so the vessel keeps its service speed through the docking interval.

Retrofit

New propeller

A propeller for a lower service speed, a derated engine or a changed trade, checked for the engine, the shaft line and the class calculations it alters.

In service

Heavy running

An engine on its load limit or short of power, traced to fouling, propeller condition, a heavy match or a change in the hull, with the remedy quantified.

Trials

Speed and power

Trial measurements corrected for wind, waves, current and displacement, and the result set against the contract curve.

Deliverables

What you receive

  • The speed-power prediction with its sources and uncertainty.
  • The load diagram with the propeller curves and operating points.
  • The margins, and the docking interval or fouling level at which each is used up.
  • For a retrofit, a comparison of the candidate propellers on speed, fuel and engine loading.
  • Shaft torque, thrust and the propeller data for the alignment and vibration calculations.

Input data

What the study needs

  • Hull particulars and lines, model test or sea trial reports where they exist.
  • Propeller drawing or particulars: diameter, pitch, blade area, number of blades.
  • The engine's load diagram and specified MCR.
  • Service data: noon reports, shaft power or fuel monitoring, and the docking history.

If the hull's 3D model has not survived, it can be rebuilt from the drawings and its resistance and wake calculated: see 3D hull, CFD and wake.

Questions from owners and yards

Frequently asked

What light running margin should we choose?

It depends on the trade, the hull coating, the docking interval and the engine type, and is typically a few per cent of speed. The study shows how far the propeller curve moves over the docking interval for this vessel, and the margin is chosen so that the engine still reaches its contract power at the end of it.

We plan to reduce service speed. Should we change the propeller?

Often it pays. A propeller designed for the lower speed and the reduced power runs at a better efficiency than the original at part load. The study compares the fuel saving with the cost, and checks the new propeller's effect on the shaft line and on the torsional barred range.

Our engine reaches its torque limit before rated speed. Why?

The propeller is running heavy: fouling, propeller damage or roughness, added resistance in weather, or a match that left too little margin. Service data is used to separate these, and the report states which applies and what restores the margin.

Can you work with controllable pitch propellers?

Yes. The combinator curve of pitch against speed is assessed on the load diagram in the same way, and an improved combinator schedule can be proposed.

Commissioning

Start with the drawings you hold

Send the shafting arrangement drawing and whatever machinery data is to hand. An initial technical response follows within two working days, then a written scope and proposal: the conditions to be calculated, the data still needed, the deliverables, the fee and the schedule. A non-disclosure agreement can be put in place before any drawing is exchanged. The calculation is then carried through to a report a yard, a superintendent or a classification society can follow and reproduce, and questions raised on it are answered by us until the review is closed.

Send an enquiry How a study is run

Related analyses

Shaft alignment analysis

Bearing reactions, slopes, lift-off and influence coefficients for the hot and cold conditions, with chock heights or offsets for installation.

Torsional vibration analysis

Natural frequencies, order crossings, shaft stress against class limits and barred speed ranges, on a mass-elastic model of the complete train.

Axial and whirling vibration

Axial modes against blade-rate excitation and lateral whirling with bearing coefficients from the film solution, in one rotor model.

Stern tube bearing analysis

Film thickness, edge loading, lift-off speed and comparative wear for water- and oil-lubricated stern tube bearings, straight or slope bored.

Propeller geometry and offsets

Blade geometry, offset tables and section drawings for the manufacturer, with mass and inertia for the shafting calculations.

3D hull, CFD and wake

The hull rebuilt from your drawings, with viscous resistance and the wake the propeller works in.