Pump-Jet Idea/Question

I was wondering if a pump-jet that converted sea water into steam and then used that as thrust would be useful at all as a method of propelling water craft, or if it makes no difference whether you're using a normal water-jet vs a steam-jet. I do not have an engineering background and do not have any clue if it would actually work or not. The idea just popped into my head.

Anyone who could answer this for me?

 

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You could do that but you would have to use oil, diesel or gas to create the heat needed to make the steam. Electric motors are much more efficient than steam driven ones so that's why we don't use steam any longer. No matter how you create the steam you are going to have to use something in order to power it. Even after you create the steam the efficiency is far less than even a gas powered engine. Not a good idea to me.

 

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I don't know if you answered my question, really. I was wondering if steam is more efficient in terms of the amount of thrust it provides, as opposed to a normal water pump-jet, i.e. can you get higher speeds from building what is essentially a steam rocket than from a water rocket?

I wasn't talking about using a steam engine to power a propeller, I was talking about using the steam itself as the thrust mechanism in a sort of underwater jet engine.

 

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Again electric motors are much more efficient and powerful than steam ever will be. All submarines use electric motors to propel them.

 

Ugh. Submarines use electric motors TO TURN PROPELLERS to propel them. Not all, either. There are several classes of submarine that use a kind of pump-jet (although it's more like a propeller with a shroud over it).

I am talking about essentially building a steam ROCKET.

 

One word, don't . It won't work.

 

Now I am asking why? Pump-jets work fine. Are you telling me that a steam-jet will not produce enough thrust to propel a watercraft to any significant speed? What is your reasoning? I am interested in and answer regarding pounds of thrust of a steam-jet compared to a normal pump-jet.

 

Such a concept already exists. Charged plates accelerate salt water into a jet and gives quite a good thrust. I've been on such a boat before.

Steam can have some thrust as well, but you cannot control it electrically and the result is a thermal-based design, which will operate with some degree of efficiency that is determined by the speed. If you can electrically extract steam from water, which is the mode of operation of some humidifiers, then you can increase efficiency to a certain extent.

 

It may work. But think of the size of such a mechanism- wont it have a detrimental effect on the thrust you so dearly yearn for. Then its also much more expensive. As the others have said, electric motors are much more efficient.

 

The overheating would be a major problem...you would need liquid nitrogen.

 

Let's take it on a molecular scale -

Mechanical designs, such as turbine or cylindrical engines, inherently lock the kinematic degrees of freedom in the molecules. That is to say, only the components that push the cylinder in the axial directions or that which pushes the turbine blade at a specified angle will be kinematically admissible. If it goes in any other direction, it's not allowed in theory (in reality it is wasted as heat).

Free propulsion designs do not utilize any constraints in the molecular degrees of freedom. That is why they tend to be less efficient than mechanical engines.