Hydraulics: why not use water?

[chabrenas]

Frosty: I agree with you from the point of view of normal sailing folks. I was just using the bleeding edge of non-stop round-the-world racing as a trigger to investigate whether we got where we are by continuously honing an established technology, or whether there was long-term potential for a different approach.

Many changes come about because the enabling technology matures to a level where it is reasonably well understood. For instance, my own lifetime covers a long period where GRP couldn't compete with wood for serious boatbuilding, particularly in racing dinghies.

[El Sea]

I don't think you can compress H2O....

[mydauphin]

Things are they way they are for a reason.
Physics, chemistry, metallurgy have something to do with it.
Hydraulics is the multiplication of forces by increasing and decreasing pressure upward of 1000 psi to 4000psi.
Oil is more stable than water, does not expand or compress as easily, also it has a wider temperature range. It is also a barrier against corrosion as compare to water or even glycol. I have small diesel Deutz that is oil cooled and it is virtually bullet proof.
Also a properly setup hydraulic system has very little oil usage, a quart of oil might last years.
Now you want something to change the world, a biodegradable oil at a reasonable price, that dissolves into water not leaving a slick.
Biodiesel works good but eats some of the seals and retains water...

later

[Splint]

Originally Posted by El Sea

I don't think you can compress H2O....

I was going to answer your question and say that all gases can be compressed and all liquids cannot be compressed as this is what I have been led to beleive. I though I might just do a quick search before making that claim and found this interesting article on a Physics web site.

Question

Can you compress a liquid (water)?

Answer

The answer is yes, You can compress water, or almost any material. However, it requires a great deal of pressure to accomplish a little compression. For that reason, liquids and solids are sometimes referred to as being incompressible.

To understand what happens, remember that all matter is composed of a collection of atoms. Even though matter seems to be very solid, in actuality, the atoms are relative far apart, and matter is mostly empty space. However, due to the forces between the molecules, they strongly resist being pressed closer together, but they can be. You probably have experienced compressing something as hard as steel. Have you ever bounced a steel ball bearing off a sidewalk? When you do that, the 'bounce' is due to compressing the steel ball, just a tiny little spot that comes into contact with the sidewalk. It compresses and then springs back, causing the bounce.

The water at the bottom of the ocean is compressed by the weight of the water above it all the way to the surface, and is more dense than the water at the surface.

A consequence of compressing a fluid is that the viscosity, that is the resistance of the fluid to flow, also increases as the density increases. This is because the atoms are forced closer together, and thus cannot slip by each other as easily as they can when the fluid is at atmospheric pressure.

That should clarify the situation.
Cheers
Splint

[robherc]

Yes, but for most practical applications you're not going to compress water very far at all beyond it's state at 40*F/7*C (highest naturally-occurring density at +1atm). The magnetic structure of water keeps it from being compressible to solid form, except maybe under EXTREME pressures (millions of PSI).
I THINK that at some point you could compress oil into a solid at room temperature, but it would probably take unreasonable amounts of pressure, too.

[chabrenas]

It hadn't occurred to me that viscosity would go up under pressure (I'm sometimes a bit slow...). I guess that is one reason why the fluid gets so hot.

Do hydraulic fluid developers try to minimise viscosity increase in order to reduce heat generation?

[Poida]

There seems to be a lot of people that think that it is uncommon to use water for hydraulics. It may be more common than you think.

I have worked on water hydraulics, the last time about three years ago.

Powerboats work on water hydraulics. It's the compression of the water on the prop blade that propells the boat, hence hydraulic.

There are reasons for using water hydraulics but I can't think of any application on a boat where it would be an advantage over oil.

And in a pleasure craft I can't see any advantage in hydraulics over electric.

Incidently the heat from hydraulics comes from friction.

There is no good or bad in any system, you use whatever is suitable for that application.

[Frosty]

Thats a point ever felt the bounce of a hammer on an anvil?

[ancient kayaker]

When I was working in a high pressure project I noted that water compressed about 30% at 50,000 psi. In case anyone wanted to know! Cylinders, pumps etc for water-based hydraulics are available but more expensive, but they don't use pure water, it's a glycol mix usually. I was in a research program that was looking at use of pure water for a situation where contamination could not be allowed but I don't remember it going anywhere, lots of technical problems.

[Fanie]

ever felt the bounce of a hammer on an anvil?

Most haven't It makes you envy the strength of iron though.

[Fanie]

I noted that water compressed about 30% at 50,000 psi

Good grief, that much ? I know the air in the water allows some compression but I didn't know it would compress 30%.

[chabrenas]

Kayaker:

I noted that water compressed about 30% at 50,000 psi

I'd have guessed around 17%, if Wikipedia is right in quoting 5.1×10 to power (-5) per bar at O°C. Or have I messed up my arithmetic...

Fanie: 50,000 psi (3,333 bar) is a hell of a pressure. Don't stick your finger over a hole in the pipe.

[ancient kayaker]

I think your math is correct: I was quoting something I was told more than 20 years ago from memory. I amazed myself by finding an ancient document on the system, the pressure was actually 55,000 psi and the compression would therefore have been about 20%. That's still quite a bit of compression though.

It was a waterjet cutting machine and yes, it would easily have cut off a fingertip. Rumour has it you didn't notice for a few seconds but I never met anyone who could speak from experience!

Are we off the topic yet?

[Poida]

Hi Terry

You hit the magic word "expense"
The subject of hydraulics in boats has been bantered around by many that obviously do not know the expense of setting up a hydraulic system.

Last week I got an invoice for making up and connecting 8 hoses about 1 metre long. $2,150.00. That's just hoses. Put in a pump and valves? It is really only an option when absolutely nothing else will do the same job.

[Frosty]

Ide guess those hoses where 6 inch diam ,--about 100 quid each?

It all depends on pressure . if your using a steering pump on a boat well,--its just compression fitting stuff.

There is serious hydraulics and there isnt, same as electricity.