WyleECoyoteNZ:

 

frankv:

 

Rikkitic:

 

If you mount a windmill on a ship, and use it to drive a screw propeller, you can sail directly into the wind and otherwise make use of the wind regardless of which way it is blowing. All you have to do is rotate the blades of the windmill to keep them facing into the wind. It doesn’t matter what the wind direction is.

 

 

A vertical axis wind turbine is even better... no rotating of the windmill to face into the wind. But I think there are issues of stability with large surface area objects on the tops of ships.

 

 

 

 

Might(?) work on like grain\crude oil\gas ships where the cargo is below decks.

 

Can't see how it would work on a container or cruise ship, and what would be the impact of a rotating blade on the stability of the vessel??

 

What's the point of having a windmill driving a generator to drive a motor to turn a propeller to push a boat through the water, when you could just use sails to push the boat along?

Actually there is a point, without a keel effect then you can't sail efficiently (make way) into the wind, and even with a keel effect you can't sail directly into the wind - but with a windmill powering a prop, in theory you could. 

It seems to be a very complex answer to a simple problem perhaps answered that by just using sails where at random odds you'd be going downwind half the time, but if you plan your route effectively, you can improve those odds substantially.  I guess flight planners for aircraft use that concept to maximise push from the jetstream, minimise losses when it's blowing the wrong way.