Vessel-performance methodology

Vessel Power, Fuel and Motion Model

A route is evaluated in a clear physical sequence: first determine how fast the vessel moves through the surrounding water, then estimate the forces opposing that motion, determine the propulsion power needed to overcome those forces, convert that power into fuel consumption, and finally check vessel motion and operational feasibility.

The complete calculation flow

1Find STWSeparate vessel motion from ocean-current motion.
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2Estimate resistanceFind the forces trying to slow the vessel.
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3Calculate powerDetermine the shaft power needed to overcome resistance.
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4Estimate fuelConvert power and operating duration into fuel used.
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5Check motionEstimate roll, pitch, heave and acceleration.
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6Decide feasibilityAccept or reject the route segment against limits.
Why the order matters: fuel use cannot be understood from vessel speed alone. Current changes the speed through water, weather changes resistance, resistance changes required power, and required power changes fuel consumption.

1. Speed Through Water (STW)

What is the difference between SOG and STW?

Speed Over Ground (SOG) is the vessel's progress across the earth. Speed Through Water (STW) is the speed of the hull relative to the surrounding water. A favourable current can make SOG higher than STW; an opposing current can make SOG lower than STW.

STW is the important value for resistance and propulsion because the hull and propeller interact with water, not with the seabed.

STW and through-water heading = F(SOG, course over ground, current speed, current TO direction)

Earth-referenced directions

N / 000°E / 090°S / 180°W / 270° Vessel heading: 000° true Wind FROM 055° Wind wave FROM 125° Swell FROM 235° Current TO 125° Encounter angle

Wind, waves and swell use a FROM direction: the direction they arrive from. Current uses a TO direction: the direction the water flows toward.

SOG, current and STW vector relationship

NorthEast Ground motion: SOG and course Current motion Water-relative motion: STW Simple interpretationVessel motion through water= ground motion minusthe motion of the current

The resulting vector provides both STW and the direction in which the vessel moves through the water. Cross-current is therefore handled correctly, rather than being treated as a simple speed addition or subtraction.

2. Resistance — what tries to slow the vessel

Resistance is the total opposing force the propulsion system must overcome to maintain a chosen speed. Think of it as the marine equivalent of drag on a road vehicle, but with several additional contributors from water, wind, waves and hull condition.

Calm-water resistance

The basic resistance created as the hull pushes water aside and water flows along the hull.

Hull-condition effect

Fouling and surface roughness increase friction, so more effort is needed at the same STW.

Wind resistance

Wind acting on the exposed hull and superstructure can oppose or assist the vessel.

Wind-wave resistance

Locally generated waves create added resistance and vessel motion.

Swell resistance

Longer-period waves from distant weather systems can affect resistance and motion differently from local wind waves.

Water resistanceWindWind waves and swellMotion-related resistanceTotal resistance = F(STW, hull condition, vessel geometry, wind, wind waves, swell and encounter directions)

3. Power required to move the vessel

Resistance is a force. The propulsion system must supply enough shaft power to overcome that force at the selected STW. Higher resistance or higher speed generally requires more power.

Required shaft power = F(calm-water demand, hull condition, wind contribution, wind-wave contribution, swell contribution, propulsion efficiency, operational margin)
Clean calm-water powerThe vessel-specific baseline power at the selected STW.
+
Hull-condition allowanceAdditional power associated with fouling or surface condition.
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Weather-added powerPower needed because of wind, wind waves and swell.
+
Operational marginAn optional planning allowance applied to the combined demand.
=
Required shaft powerThe power the propulsion system must deliver.
A following wind or favourable sea condition can reduce an environmental contribution, but the final power is still checked against vessel and engine limits.

4. From power to fuel consumption

Once required shaft power is known, the model determines how heavily the engine is operating. The engine's fuel-efficiency reference indicates how much fuel is needed to produce each unit of propulsion energy at that load.

Required powerPower needed for the route segment.
Engine loadRequired power compared with rated power.
Fuel efficiencyFuel required per unit of generated energy at that load.
Operating durationHow long the vessel remains at this condition.
Fuel consumedTotal tonnes used over the segment.
Engine load = F(required shaft power, maximum continuous rating)
Fuel efficiency = F(engine load, vessel engine reference)
Fuel consumed = F(required shaft power, fuel efficiency, segment duration)

RPM is estimated from the vessel's operating reference at the selected STW so the reported speed, power, RPM and fuel values describe a consistent operating point.

5. Roll, pitch, heave and vertical acceleration

Weather does more than increase fuel use. Waves also move the vessel. The model estimates the main motions that can affect safety, cargo, equipment and comfort.

Understanding vessel motion

Roll: side-to-side rotationPitch: bow-up and bow-down rotationHeave: vertical movement

What controls the response?

Encounter directionHead seas tend to increase pitch; beam seas tend to increase roll.
Wave heightLarger waves generally produce larger forces and motions.
Wave periodThe timing between waves can approach the vessel's natural motion periods.
Vessel geometry and loadingLength, beam, draft, displacement and stability influence the response.
Vessel speedSpeed changes how frequently the vessel encounters successive waves.
Roll, pitch, heave and acceleration = F(STW, through-water heading, wave height, wave period, wave FROM direction, vessel geometry, loading and stability)

6. Operational feasibility

A route segment is accepted only when the propulsion demand and vessel motion remain within the submitted operational limits.

Power check

Can the engine provide the required shaft power without exceeding the permitted load?

RPM check

Does the operating point remain within the permitted RPM range?

Motion check

Are roll, pitch, vertical movement and acceleration within the selected limits?

Risk check

Are synchronous-roll or parametric-roll relationships absent or permitted?

Route-segment feasibility = F(power status, RPM status, motion status, risk flags and operational constraints)

Weather inputs and direction conventions

Weather valueUnitDirection conventionHow it is used
Wind speed and directionkn and degrees trueFROMApparent wind and aerodynamic contribution.
Significant wave height and directionm and degrees trueFROMOverall sea-state reporting and screening.
Current speed and directionkn and degrees trueTOSOG-to-STW vector resolution.
Swell height, direction and periodm, degrees true and sFROMSwell resistance and motion response.
Wind-wave height, direction and periodm, degrees true and sFROMLocal-wave resistance and motion response.
Avoid double counting: significant wave height describes the combined sea state. Wind-wave and swell partitions are evaluated separately for detailed resistance and motion effects; significant wave height is not added as a third independent wave system.

How to read the returned results

Navigation

SOG, STW, course, through-water heading, segment distance and duration.

Power

Calm-water baseline, hull-condition contribution, wind contribution, wave and swell contribution, and required shaft power.

Engine and fuel

RPM, engine load, fuel efficiency and segment fuel consumption.

Motion

Roll, pitch, vertical movement, vertical acceleration and risk indicators.

Feasibility

Power, motion and overall route-segment feasibility flags.

Proprietary-use disclaimer. The vessel model, methodology, diagrams, response structure and explanatory material are proprietary to Blue Green Intelligence and may not be reproduced or used to recreate a competing implementation without written authorization.