Steam Thruster?

[cyberbadger]

A couple years back at a SBA Rally in the UK there was an engineering competition of building a removable steam powered "outboard".

The basic rules were they were going to take one of the members steam launches, and provide everyone with drawings for a simple stern outboard style mounting attachment and the promise of a steam hose from the launches boiler. (And assurances that the boiler pressure/state would be roughly equally provided to all contestants)

Then they tried the different submissions over a short course without the launches engine running but the outboard submissions and the one with the quickest time one.

Most of the submissions used reciprocating steam engines - but...

One of the submission was based around the idea of using eductors AKA ejectors to provide a stream of water to propell the launch. I remember that that entry did not fair well - actually I think it moved slightly backward.

I'm curious if there have been any examples of a steam thruster of any type in the past in service?

I'd imagine you could get a small amount of thrust with just an open pipe - but it might be prohibitively loud - sort of like during a blowdown.

How would one go about designing a nozzel to be provided with steam/water and used to provide a thruster?

-CB

[dampfspieler]

Hi Bart,

the simpliest possibility is a so called pop-pop-boat. I have a little toy.

A video that shows the prinziple - https://youtu.be/-9EOhISI62k
Some other Videos - "knatterboot", "pop pop boat"

An other interesting Article - modern torpedo-wapons

Best Dietrich

[fredrosse]

A thruster utilizing the kinetic energy of a steam nozzle to directly produce low velocity thrust is very inefficient. For example, consider a 23 ft steam launch, 1.5 tonn, capable of a little more than "hull speed", 4.5 knots, with a 3.1 horsepower steam engine, propeller driven with a propulsive efficiency of 60%. This little steamer has an actual propeller thrust of 130 pounds. The boiler needed to run this reciprocating engine must produce about 150 PPH (pounds per hour) steam flow, at say 150 PSIG (10 Barg) steam pressure. These numbers are typical of many steam launches in this nominal size.

So now take the entire boiler output and use it as a thruster nozzle. The thrust force is only about 4 pounds. This is so small that further evaluation of the use of a steam jet for propulsive thrust is absurd.

Actually a Pop-Pop steam boat engine is virtually a reciprocating steam engine.

[Lionel Connell]

There is a way to do it.

If you take a piece of straight tube with an automatic shutter type valve in one end ( think German Buzz Bom pulse jet engine) and you rig it with the ability to inject steam into the pipe just inside the the shutter valve. Now you mount this thing under your boat, ( think inverted floating buzz bom) the tube will completely flood. when you inject the steam into the tube the water will try to go in both directions out of the tube, but the shutter valve will close forcing the water to be blown out the other (rear) end of the tube. The tube will recoil like a gun pushing the boat forward. When the steam has filled about 20% of the tube the steam is cut off. The steam will continue to expand pushing further water out of the tube. When the steam stops expanding and starts contracting, cooled by the water and cold pipe, the water hammer effect of the water exiting the tube combined with the drop in steam pressure will open the shutter valve at the front end of the tube and draw more water into the tube, effectively reloading the gun. The cycle is repeated, a mechanical link from the shutter valve is used to operate the steam valve. The thing will run like a machine gun although very quietly as the entire device is submerged. As the boat starts gaining forward motion the efficiency of the device will improve as the pressure differential from front to rear of the tube will increase, thus assisting in reloading the tube with water.

[DetroiTug]

The steam would condense rapidly depleting energy/pressure, and produce very little thrust. I have a steam ejector on the tug as a bilge pump. The water exiting the ejector is very low velocity in relation to the steam pressure/velocity.

-Ron

[cyberbadger]

A different approach...

I said steam thruster. What if a steam pump, engine driven reciprocating pump, or steam turbine that was turning a mechanical water pump of some sort. Would that be a better approach?

What if you had something like a turbo from a car. Ran steam through one side, and the otherside was pumping lake water?

-CB

[fredrosse]

Ron, I agree that the "Buzz Bomb" type thruster would condense a very large amount of steam, and would use a tremendous amount of steam for the thrust produced. Also note that this concept is also a reciprocating piston type engine, just like the "Pop-Pop" boats, the "piston" is a slug of water being thrust out the back of the "engine"

CB: "I said steam thruster. What if a steam pump, engine driven reciprocating pump, or steam turbine that was turning a mechanical water pump of some sort. Would that be a better approach?"

That approach is exactly what is used on the rear end of almost every steamboat or oceangoing ship, an engine turning a propeller, which is in effect a big pump! Doing the math, a high speed jet flow of low mass flow, vs. a relatively low speed and large volumetric flow, propulsive efficiency very much favors low velocity and large mass flow, that is why steam launches and oceangoing ships have relatively large, slow turning propellers. Same for a bow thruster, they use a relatively large propeller to work effectively, engineers figured all this out many decades ago.

As far as driving this big pump, a reciprocating steam pump uses on the order of 10x the steam consumption of a decent steam launch engine, so that would be a poor choice. Very small steam turbines are worse, using on the order of 50x to100X the steam consumption compared to a decent steam launch engine.

An automotive turbo? They need on the order of 100,000 RPM to have any sort of decent efficiency, and the turbo compressor at that same RPM can work OK IF you are pumping a fluid of very low density, such as Air. Water pumps turning 100,000 RPM are not in the realm of anything realistic here.

[fredrosse]

Several years back this device was promoted as an effective steam powered thruster. Where has this thing gone?....nowhere. A quick review of their advertising nonsense, quoted below, is just "Mumbo-jumbo".

"The Pursuit Marine Drive produces thrust by using the energy from high-pressure steam to draw in water through an intake at the front and expel it at high speed through the rear. The steam emerges at high speed from a rearward-facing ring-shaped nozzle into a cone-shaped chamber, where it mixes with the water (see graphic). Shock waves created as the steam condenses are focused by the chamber to blast water out of the back...............' followed with several more paragraphs of statements that are laughable to anyone who has a firm knowledge of thermodynamics and fluid dynamics.

[cyberbadger]

Fred,

statements that are laughable to anyone who has a firm knowledge of thermodynamics and fluid dynamics.

They didn't really have a full understanding of thermodynamics when Watt and Fitch were around, and as far I know, this level of knowledge while maybe necessary for ASME boards and PE exams is not really required to build a steam launch or participate in a technical discussion about steamboats theoretical or otherwise.

I really do appreciate your engineering/science explanations when you give them though.

Same for a bow thruster, they use a relatively large propeller to work effectively, engineers figured all this out many decades ago.

This confuses me. In comparison to the vessels main propeller aren't bow thruster propellers usually smaller?

Thinking about indirectly steam powered thrusters ... There are many steam launches today that have electrical generating systems, or batteries onboard...
Here is a 1.5KW 12V electrical thruster example.
http://www.boatersland.com/lew591102.html
You wouldn't even need to have the steam electrical system capable of putting out 1.5KW electrical continous as long as you had a decent battery bank as a buffer. Remember, a bow thruster is often used intermittently, not continuously.

-CB

[Lopez Mike]

It's not clear to me what all of this has to do with steam launches. Why would you want a bow thruster on a one ton boat? Use a boat hook or a paddle!