This is MERELY a Mental Workout

[mgriffin]

I have always wondered if you could somehow make a propulsion system based on an underwater "sail" system. I know the idea sounds stupid, but the sails are not going to be made of cloth, but you probably figured that out. The boat that can use these sails will not be shallow draft. I just made sketch of what it might look like when not on the hull of the boat. I got this idea thinking that the ocean has currents, and you could just stick to the currents that you want to follow. I posted a picture of ocean currents so you could see what I'm getting at.
REMEMBER: THIS IS JUST A MENTAL WORKOUT!

[thudpucker]

Mental workouts are strange territory for me.

I read a book way back in the 50's where they used somehting like your idea.
It was an adventure novel so I never gave it another thought.

However, if you were the kind to move a large vessel, a long ways, cheaply, over the open waters where you didnt have to worry about manuevering or underwater obstacles, and had no time constraints, that idea looks workable to me.

YOu could use a normal propulsion system for manuvering in close, but once you hit the open water, drop the sails and use the existing currents.

Some folks might think you are net fishing though.

[mydauphin]

Benjamin Franklin invent this idea, discovered Gulf Stream. Therefore submarine named after him, that in July 1969, same day that Apollo 11 went to Moon travel the Gulf Stream using it for propulsion. Also every mariner that cross oceans knows these currents and uses them, at least surface ones.

[thudpucker]

It had some real possibilites for ol' Franklin. He crossed that ocean many times.
Imagine a thoughtful and inventive guy stuck on that Ocean for a month with nothing to do but look over the side.

I bet there were many times he looked down into the water and noticed the ship was moving thorugh the water faster at times, and slower other times.
Eventually the boat is moving east, and the water under the boat is moving east as well, but the water is going faster than the boat.
You can tell. There is always something in the water, dirt, foam, seaweed etc.

Nobody's mentioned yet about the perils of really bad water, typhoons etc.
Steering is gonna be a problem too.
When you reach the limit of an east bound current, and want to go north, your gonna have to put up the sails or fire up the diesels to change from one current to another.

Also, how you gonna know where the currents you want are?
How close to the surface do these currents run?

I'm a dull witted sailor. I'd have to put down a Sea Anchor on a long tether. Then wait and see which way the tether wanted to go.

[mgriffin]

Well, the good thing about this idea is that I will leave it to someone else to test it

[mgriffin]

And what I mean by mental workout is mental exercise.
Btw, what is a "thudpucker"?

[thudpucker]

A thudpucker is a dandy ol' dude that's enjoying life as an old retired guy.

Its' a long distorted story.

[Beau Brown]

I can see where you're coming from with the idea, but I don't think it would be all to wise to have it be flat on both sides like a typical sail. The water from the current pressing on the flat side of the sail would create some force, however I can imagine that having another flat side would only result in an opposite force that would work against the underwater sail. However, I bet that if the front side of this underwater sail was to be hydrodynamic (planed or something), then there would be a chance that an underwater sail such as this one would work. I've worked out a drawing of what I think would make this idea somewhat practical. It might also be able to act as a keel of sorts if it's heavy enough?



And here's an idea that just popped into my head while typing this post; say we follow the same principle of having one side be hydrodynamic, but have just a bunch of these small "walls" around the bottom of the boat set up in a way that water would hit all of them at one point: I've done another drawing to demonstrate this:



So that's what I have so far... This is a fun thread, as I like to think of things such as these. Good luck!!

Regards,
-Beau Brown

[daiquiri]

@ Mgriffin and Beau Brown:

Nice mental exercise, but those flat plates don't need to have the configurations you have sketched. A common airfoil/hydrofoil would do the job much better.
Think a bit about how a sailboat works:

You have to fluids. One is stationary (water, assume no waves) and the other is moving (air).
- The sail is exposed to the moving fluid and generates the aerodynamic forces which, initially are not balanced by the keel.
- The boat accellerates and the keel starts working in it's own moving (relative to the boat) fluid, which is water.
- So the keel now also produces a hydrodrodynamic force which, when an equilibrium is reached, is equal and opposite to the sail aerodynamic force.
- The boat is now moving on a steady course, with a steady velocity.

Now think of yourself standing on the boat's deck. From your point of view:
- below, you can see the water moving nearly along the boat's centreline.
- above, you see (or feel) air moving in some other (whatever) direction.
- behind, you see a train of waves and a white wake behind the transom.
- the boat itself appears to be in equilibrium, heeled at some angle due to the forces acting on sails and on the keel.
- if you could somehow visualize the air behind the sails (with a smoke?), you would have noticed a turbulent wake. If you could measure the average speed in that wake, you would have discovered that it is smaller than the speed of the undisturbed airflow in front of the boat.

So, let's come to the point:
- the water in front of the boat is dead flat.
- the water behind the boat is foamy (indicating a turbulent motion) and there are waves chasing the boat.
So, the water has been energized. An energy has been somehow provided which has put the water in motion.

- the air in front of sails is moving with some speed
- the air in the sail's wake is moving with a smaller speed, it has been slowed down.
So, the kinetic energy has somehow been taken from the air.

The magical object which has taken the kinetic energy from the air and has transferred it to the water, creating waves and a wake, is - the sailboat.

During that process, the boat has moved from one place to another, relative to a fixed observer on the ground. By rotating the hull (thanks to the rudder) relative to the two flows, the sailboat can even choose the path along which it will move.

This energy transfer is possible each time there is a difference in speed (and hence in kinetic energy) between two fluids. And each time it can be used to move an object (a boat) during the process.
It is not even necessary that the two fluids are different, like air and water. It can also be two currents in the same fluid. The only thing that would change would be the sail and keel areas.

It is then perfectly conceivable (at least as a mental experiment) to have an "inverted" sailboat, where sails are immersed in some underwater current, and the keel is moving through the air (either still or moving in some different direction and/or with a different speed).
Surely, the sail and keel dimensions would be completely different from what we are used to.

But the very existance of two currents or fluids with different speeds and/or flow directions makes it physically possible to realize a machine which can use it for a propulsion. Sailboats are the proof of the concept. Now, the real feasibilty (intended as cost and technical difficulty) of this "inverted" sailboat is another story...

[whoosh]

guess you would use the same thermal imaging fishers use, the fish have no chance now Before the days of sat nav I was coming home from Vanuatu to NZ, the Three Kings Currant swept me so far east that it took a whole day to find NZ again, silly really we had the USA chart book of sailing directions onboard , these show current, and wind for each month

[Ad Hoc]

Hi D

Yes, those thermal currents too, in different layers and different temp gradients will all play a part on the ability to sail and in one direction. The lower end of the sail may be forced into another by the upper end, owing to different currents.

It is not unlike that of hotair ballooning. Once cannot just say...oh, i want to steer this way. One needs to find the right thermals and air streams in the intended direction.

[mark775]

If all you are doing is utilizing surface currents, you might as well just have a heavily ballasted boat. Where your plan gets interesting is when you consider an Ekman spiral and start traveling where you want by ocean currents. Fly your kite at different depths...

[daiquiri]

Originally Posted by mark775

Where your plan gets interesting is when you consider an Ekman spiral and start traveling where you want by ocean currents. Fly your kite at different depths...

That's what I was talking about. Either kite or sail.

[apex1]

It is impressive to see what a 13 year old boy has ransomed here!

[daiquiri]

It is impressive but not unexpected to me. Kids have much more imagination then we (adults) do.
If we could have their fancy and creativity, plus our knowledge, I'm sure that some really revolutionary and marvellous stuff would come out of that mix.