Section 3 Units
Clasification Society 2024 - Version 9.40
Clasifications Register Guidance Information - Guidance Notes for General Overview of Ship Structural Vibration Problems, September 2021 - Chapter 1 General Overview of Ship Structural Vibration Problems, September 2021 - Section 3 Units

Section 3 Units

Parent topic: Chapter 1 General Overview of Ship Structural Vibration Problems, September 2021

3.1 Units introduction

3.1.1 A section dedicated to the subject of units is included in this document because it is critically important to use a consistent set of units for dynamic analysis. This is one of the most common errors that will be found when reviewing the initial results of a dynamic analysis.

3.1.2 Whereas the distinction between mass and weight (force due to gravity acting upon mass) is not important for stress analysis, it is important for dynamic analysis. In fact, for stress analysis, mass is not required to be specified at all unless gravity is to be applied; otherwise all loads including weight can be described as static forces. For dynamic analysis, static forces are not directly recognised, and mass should be specified within a consistent system of units.

3.1.3 Five consistent sets of units are shown in Table 1.3.1 Five examples of consistent sets of units. Using these will ensure the avoidance of errors in a dynamic analysis emanating from units. The first set is, of course, the SI system, which is the one that is most commonly employed.

Table 1.3.1 Five examples of consistent sets of units

  Force Mass Length Time Modulus Density
1 Newton kilogram metre second Newton/ metre2 kilogram/ metre3
2 Newton tonne millimetre second Newton/ millimetre2 tonne/ millimetre3
3 MegaNewton kilotonne metre second MegaNewton/ metre2 kilotonne/ metre3
4 kilogram kilogram/ g = 9,81 metre second kilogram/ metre2 kilogram/ g-metre3
5 kilogram kilogram/ g = 981 centimetre second kilogram/ centimetre2 kilogram/ g-centimetre3
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