The units used in Aqwa are decided by the input values for the water density and gravitational acceleration. For example, if metre, Newton are to be used as the units for the length and force, users should use 1025 for the water density and 9.806 for the gravitational acceleration.
In the output, the unit for the rotational motions is in degrees (although they are originally calculated in radians), while the rotational terms in the stiffness and damping matrices are output in radians.
The user is free to choose any system of units for the data, with the proviso that the system must be consistent. This means that the unit of mass must be consistent with the units of length and force already selected.
Examples of consistent sets of units are:
SI units: force in Newtons, length in meters, mass in kilograms, time in seconds, acceleration in meters/sec2
Imperial units: force in poundals, length in feet, mass in pounds, time in seconds, acceleration in feet/sec2, or force in pounds, length in feet, mass in slugs, time in seconds, acceleration in feet/sec2
For any other set of units, the consistent unit of mass will be a multiple of the basic unit of mass because it is a derived unit.
The consistent unit of mass is obtained by dividing the unit of force by the acceleration due to gravity, which itself has units of length divided by time squared. A change in the unit of length, for example, from feet to inches or metres to millimeters, requires a corresponding change in the unit of mass used for calculating the density. A list of sets of consistent units is given below.
| Force | Length | Value of E for steel | Acceleration due to gravity | Unit of mass | Density (mass/volume) | |
|---|---|---|---|---|---|---|
| Steel | Seawater | |||||
| Newton | Meter | 2.1 x 1011 | 9.81 | 1.0 kg | 7850 | 1025 |
| Newton | Centimeter | 2.1 x 107 | 981 | 100 kg | 7.85 x 10-5 | 1.025 x 10-5 |
| Newton | Millimeter | 2.1 x 105 | 9810 | 1000 kg | 7.85 x 10-9 | 1.025 x 10-9 |
| Kilopond | Meter | 2.14 x 1010 | 9.81 | 9.81 kg | 800 | 104.5 |
| Kilopond | Centimeter | 2.14 x 106 | 981 | 981 kg | 8.00 x 10-6 | 1.045 x 10-6 |
| Kilopond | Millimeter | 2.14 x 104 | 9810 | 9810 kg | 8.00 x 10-10 | 1.045 x 10-10 |
| Kilonewton | Meter | 2.1 x 108 | 9.81 | 1000 kg | 7.85 | 1.025 |
| Kilonewton | Centimeter | 2.1 x 104 | 981 | 1.0 x 105 kg | 7.85 x 10-8 | 1.025 x 10-8 |
| Kilonewton | Millimeter | 2.1 x 102 | 9810 | 1.0 x 106 kg | 7.85 x 10-12 | 1.025 x 10-12 |
| Tonne | Meter | 2.14 x 107 | 9.81 | 9.81 x 103 kg | 0.800 | 0.1045 |
| Tonne | Centimeter | 2.14 x 103 | 981 | 9.81 x 105 kg | 8.0 x 10-9 | 1.045 x 10-9 |
| Tonne | Millimeter | 2.14 x 101 | 9810 | 9.81 x 106 kg | 8.0 x 10-13 | 1.045 x 10-13 |
| Poundal | Foot | 1.39 x 1011 | 32.2 | 1.0 lb | 491 | 64.1 |
| Poundal | Inch | 9.66 x 108 | 386 | 12 lb | 2.37 x 10-2 | 3.095 x 10-3 |
| Pound | Foot | 4.32 x 109 | 32.2 | 32.2 lb | 15.2 | 1.985 |
| Pound | Inch | 3.0 x 107 | 386 | 386 lb | 7.35 x 10-4 | 9.597 x 10-5 |
| Kip | Foot | 4.32 x 106 | 32.2 | 3.22 x 104 | 1.52 x 10-2 | 1.985 x 10-3 |
| Kip | Inch | 3.0 x 104 | 386 | 3.86 x 105 | 7.35 x 10-7 | 9.597 x 10-8 |
| Ton | Foot | 1.93 x 106 | 32.2 | 7.21 x 104 | 6.81 x 10-3 | 8.892 x 10-4 |
| Ton | Inch | 1.34 x 104 | 386 | 8.66 x 105 | 3.28 x 10-7 | 4.283 x 10-8 |
Note:
| 1 kip = 1000 pounds force |
| 1 kilopond = 1 kilogram force |
| All times are in seconds |
| Specific gravity of steel = 7.85 |
| Specific gravity of sea water = 1.025 |