on the stocks

Currently undergoing construction, preparation, or repair. Said of vehicles, especially seafaring vessels. The huge passenger ferry that sank over the weekend was still on the stocks just a day before it set sail on its doomed first voyage.

on the stocks

in construction or preparation.

During construction, a ship is supported on a frame or scaffolding known as the stocks .

on the ˈstocks

in the process of being made, built or prepared: Our new model is already on the stocks and will be available in the spring.

Stocks refers to the frame that supports a ship or boat when it is being built.