If hours are not considered a derived unit of the SI second and thus a part of
SI, then why does the USMA and SI promote using kilometers/HOUR (km/hr) in
place of miles/hour?
Because the SI official documents (you really should read them!) state that hours and minutes are not SI units (because they are not coherent with the rest of SI). However, those documents also state that there are some non-SI units that are permitted for use with SI, and that hours and minutes are among them.
The official document of the SI metric system is titled "The International System of Units (SI)". It is published by BIPM, the Bureau International des Poids et Mesures (The International Bureau of Weights and Measures). That Bureau was established by the 1875 Treaty of the Metre. The USA is a signatory to that treaty along with at least 47 nations. I am using the official English translation of the 7th edition (1998) of that document, plus the year 2000 Supplement. These documents are available on the internet at:
http://www1.bipm.org/en/si/si_brochure/
Section 4 of that document is titled "Units outside the SI". In the introductory paragraphs (page 104) to that section is the statement: "The inclusion of tables of non-SI units in this text does not imply that the use of non-SI units is to be encouraged. With a few exceptions discussed below, SI units are always to be preferred non-SI units."
On page 106 is a Table 6 titled "Non-SI units accepted for use with the International System". Among others, this table includes:
the minute (60 s),
the hour (3600 s) and
the day (86 400 s).
You may not agree with the distinction SI makes between units of the SI system and units permitted with the SI system, but it IS the official classification. There are even several other tables of non-SI units given, but only the ones in table 6 are permitted without condition.
Therefore, hours are not SI units (neither are minutes or days) but are (perhaps reluctantly) allowed for use with SI. That use is acceptable but does not make them part of the SI system.
Regards, Bill Hooper Fernandina Beach, Florida, USA
PS Some other units that you may be surprised to learn are NOT units of the SI system are:
the litre
the tonne (metric ton)
the hectare
the bar (and, by inference, the millibar)
the calorie and
the micron.
