Something that is practical is (1) of or relating to practice, (2) capable of being put to good use, (3) concerned with ordinary, tangible things, and (4) being such for all useful purposes.
Practicable is more narrowly defined. It means capable of being put into practice.
Confusion occurs between practical‘s second definition and the main definition of practicable. Think of practical as a synonym of useful, and practicable as a synonym of doable and feasible. Another important distinction is that practical can apply to people (per definition three) and skills (definition two), whereas practicable typically applies to plans or actions.
The corresponding noun of practicable is practicability. Practical makes practicality. Practicalness is listed in some dictionaries, but it is superfluous.
Examples
Fortunately an astonishing cross-section of our music community has begun to think about how it can be of practical assistance. [Vancouver Sun]
And unlike other pollutants, no effective, commercially practicable control technology exists. [NPR]
The continued lack of significant rain is furrowing the brows of clerks of the courses countrywide – particularly at tracks where watering is not a practical proposition. [Telegraph]
Mr Fridd said strong action needed to be taken now and the council needed to change the rules as soon as practicable to demand inspections. [Stuff.co.nz]