models.rb

### app/models/boat.rb ###
class Boat < ActiveRecord::Base
  belongs_to :captain
  has_many :boat_classifications
  has_many :classifications, through: :boat_classifications
end

### app/models/captain.rb ###
class Captain < ActiveRecord::Base
  has_many :boats
end

### app/models/boat_classification.rb ###
class BoatClassification < ActiveRecord::Base
  belongs_to :boat
  belongs_to :classification
end

### app/models/classification.rb ###
class Classification < ActiveRecord::Base
  has_many :boat_classifications
  has_many :boats, through: :boat_classifications
end