Working with Imperial and FPS Units

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Overview

Many engineering domains, particularly naval and aerospace applications, use the imperial foot-pound-second (FPS) unit system. This example demonstrates seamless interoperability between imperial/FPS units and SI, using World War II battleship specifications as a practical case study.

Key Concepts

Multi-System Definitions

Real-world applications often need to work with data defined in different unit systems:

--8<-- "example/foot_pound_second.cpp:52:65"

The Ship structure naturally expresses specifications in FPS units, as they would appear in historical naval documentation.

Transparent Unit Conversion

The example shows how to display the same quantity in multiple unit systems simultaneously:

--8<-- "example/foot_pound_second.cpp:67:73"

This fmt_line helper function prints a quantity in its native units plus converted to alternative systems, making it easy to compare specifications across different engineering traditions.

Real-World Application

The example models three WWII battleships, each defined in their historically appropriate units:

--8<-- "example/foot_pound_second.cpp:97:128"

Notice:

KMS Bismarck: Defined in SI/metric units (meters, kg, km/h) as used by Germany
USS Iowa & HMS King George V: Defined in FPS units (feet, inches, pounds, knots) as used by USA and UK
All mixed types work seamlessly without manual conversions

Multi-Column Output

The print_details function demonstrates displaying quantities in original, alternative imperial, and SI units:

--8<-- "example/foot_pound_second.cpp:75:91"

Sample Output:

KMS Bismark, defined in appropriate units from the SI system
length               :              823.49 ft,           274.50 yd,            251.00 m
draft                :               30.51 ft,            10.17 yd,              9.30 m
beam                 :              118.11 ft,            39.37 yd,             36.00 m
mass                 :        110892517.88 lb,          49505.59 t,          50300.00 t
speed                :             51.04 ft/s,            30.24 kn,          56.00 km/h
power                :    2621018.31 ft pdl/s,        148.12 hp(I),           110.45 kW
main guns            :               14.96 in,            14.96 in,           380.00 mm
fire shells weighing :             1763.70 lb,              0.79 t,           800.00 kg
fire shells at       :           2690.29 ft/s,        1834.29 mi/h,        2952.00 km/h
volume underwater    :         1777123.68 ft³,         50322.54 m³,       50322538.73 L

Why This Matters

Legacy Data: Integrate historical or international data expressed in various unit systems
Industry Standards: Naval, aerospace, and construction industries often use imperial/FPS units
International Collaboration: Teams across different countries can work with their native units
No Manual Conversions: The library handles all conversions automatically with full type safety

This capability is essential for applications that must bridge different engineering traditions or work with legacy specifications while maintaining modern safety standards.

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