user: Question: What is the length when the propulsion is Controllable pitch propeller?
Columns: ['Vessel', 'Built', 'Max Speed', 'Length', 'Breadth', 'Flag', 'Propulsion']
Types: ['text', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Gallion', 2010.0, '30 knots', '20 m', '6.5 m', 'United Kingdom', 'Prop']
SQL:

assistant: SELECT "Length" FROM "Table" WHERE "Propulsion" = 'Controllable pitch propeller';