DNV DNV-RP-0675 PDF

General

The technical ship performance represented by the VTI is related to the additional power needed given a specific ship draft and speed relative to the reference case of a clean hull and polished propeller. This implies that added resistance from waves, wind, current, and temperature is removed.

The main scope is related to which quantities to measure, calculation of the VTI, verification of the VTI, and evaluation of the associated uncertainties and data quality.

The document is split into six main sections and appendices which reflect the scope:

— general description of this RP

— technical ship performance framework

— measurements

— calculation procedure of technical ship performance

— uncertainties in measurements, analyses and models

— how to document and verify technical ship performance.

Appendices are included to cover more details.

Reservations

While the current RP is addressing hull and propeller fouling, there are a number of effects that are implicit or disregarded in the current formulation and may be included in later revisions of this RP when more sensors and input data are available:

— Hull and propeller fouling may be separated into two different effects.

— Main machinery is not considered and may not operate under optimal settings and conditions.

— Auxiliary is not considered and may not operate under optimal settings and conditions.

— Heavy rudder use may cause additional drag. This is implicit in the assessment of the hull and propeller performance but is not included in the reference values from model tests.

— Boiler is not considered and may contribute to significant energy and fuel consumption.

— Wind assistant propulsion systems may provide forward thrust that, in certain conditions, may overcome the reduced performance due to propeller and hull fouling. This may result in an apparent performance that is occasionally better than the ideal case. It may, however also imply increased rudder use and more rudder drag and also drag when the sail is not folded down in head wind and sea conditions, so overall, it may also introduce more variation and uncertainty to the answer on propeller and hull performance. The best way at the moment is to assess the performance due to propeller and hull fouling when the sail is out of operation and tilted down or the wind speed is small, so isolated periods. Later the thrust and performance of the sail system may be included by additional sensors to separate out the performance of the sail system. This is regarded useful as the performance of the sail system itself needs to be considered over a longer time than spot checks during sea trial, and this belongs to the topic of technical ship performance.

— Slender vessels and high speed vessels are currently disregarded as some formulations for those vessels need to be adjusted.

— The effect of variable pitch propellers is disregarded.

— The effect of trim without changing the displacement may reduce or increase the resistance depending on the displacement and speed. This may be calculated by CFD analysis and trim is especially relevant for slender vessels, but may also affect the performance of blunt vessels. Extreme cases of trim like 'bow up' may also be prevented by design rules.

Neglecting certain effects does not remove the value for the intended application of the current version of this RP.

Note:

Other data sources like DCS data may be useful to confirm fuel consumption over a longer time period, but DCS data is considered less useful to explain technical ship performance. Additional available data may be used to enhance the value of DCS data, but this RP represents an improved level of accuracy related to fuel consumption in calm water.

This RP goes beyond the ISO 19030 standard addressing hull and propeller fouling:

— by making corrections for the added resistance in waves

— adopting some recent formulations on added resistance due to the environment

— using different methods for filtering noise and evaluating steady state conditions

— having a different definition of the index

— being able to compare ships

— by having a focus on continuous monitoring of the index, including its uncertainty.