[rnd90]

You can, but it's not always that simple :)

Some ships have a direct drive. So the moment the engines start, the prop shaft is turning. These ships will control thrust via Controlled Pitch Propellers. Basically the pitch of the propeller blades are controlled hydraulically and the steeper the pitch, the more the blades cut through the water and the more thrust generated. CPP systems allow ships to use shaft generators for electrical power at sea. The engine's revs are constant (to keep the electrical frequency the same) and power is changed via setting the propellers pitch.

Some ships have drive trains which let you clutch engines in and out, but you wouldn't run them for long periods of time de-clutched.

The engines do not like being run at an idle load for long periods of time (whether direct drive or clutched), you get loads of crud building up as they don't properly warm up. Also you are burning fuel and increasing their hours (planned maintenance is usually based around running hours). So you wouldn't generally drift and idle. You would shut the engines down and keep them on immediate notice.

A lot of modern ships are diesel electric. So the props are driven by large electric motors. So in this case, you can stop and start propulsion instantly (as long as you have enough generators running and connected to the switchboard)

[jacquesm]

I think 'screw in neutral' adequately addresses variable pitch screws.

The proper term in windmills and airplanes is 'feathered', I don't know what to call that state in a boat.

[fiter]

Is feathered the same? It seems like a feathered propeller is pitched with the blades flat surfaces oriented parallel to the forward direction, but on a CPP you'd set the flat surfaces of the blades oriented perpendicular to the forward direction to reduce the load on the spinning engine.

[jacquesm]

In a plane (where you are minimizing drag when an engine is out) you would put the blades such that the prop presents the minimum surface to the airstream, in a windmill you have a choice, you can put the blade in 'neutral' by picking one of two 90 degree offset orientations, one parallel to the apparent wind, one perpendicular to it. In either case the blades won't turn.

For safety purposes 'coarse' is not always the best position because if the wind is heavy enough it will have a lot of torque to work with, 'flat' is much easier to hold down with a brake.