Propellers

[OceanObsessed]

Just a bit of trivia... cavitation is noisy and it is one of the reasons that submarines dive deep. The deeper you go... the colder the water and the higher the pressure which means less cavitation so the sub becomes more silent and undetectable by the enemy.

 

[cool_hardware52]

How do you know this?

Converting liquid water to water vapor does not create Oxygen and Hydrogen, it's a phase change, not a chemical change.

Tobin

 

[bobbates]

It is called cavitation. You know that water will vaporize (boil) at 212 degrees Farenheight at atmospheric pressure. The forward (bow) side of the propeller is experiencing extremely low pressure… low enough pressure to make the water vaporize at the current surrounding water temperature.

That is almost correct. In other words, the gas dissolved within the liquid (h20) is "boiled" out at ambient temperature because of the lowered pressure created by the prop turning torque. This is an example of the Integrated Gas Laws...the inverse relationship between pressure, density, volume and temperature. The bubbles will pit and damage cheap props...prop blades that flex over much will make this worse (cheapo aluminum props for ex)...the physical properties of water are fascinating, eh?

 

[Gilldiver]

Hehe, you know you spent too much time in engineering when cavitation and mixed-phase flows seem like normal, everyday conversation.

I know what you mean, you want to add prop slip and efficancy to really confuse them?

I do Helicopters and we could start talking about advancing and retreating blades!

Here is one for you. The first propellors were iron plates welded onto a shaft. If you look up HMS Warrior they have some photos of her prop from the 1850's. Some early ships also had 2 bladed props that could be retracted into the hull above them so that they would not create drag under sail (1840'/1850's).

Then when bronze props came into use the cavatation would eat away at them so much they would cary a spare prop, or they would have a propellor hub that the blades were bolted onto and they had one or two spare blades on deck. I even came across an account where a ship lost a prop in mid Atlantic. The captain flooded down his bow compartments to bring the hub above water and put on his spare. Doing that in mid ocean - They be real Sailor Men!

Pete

 

[Seadeuce]

On the Umbria wreck in the Red Sea you can dive past two spare prop blades, lashed upright against a bulkhead.

Probably a common occurrence, but I don't remember seeing spares on any other wreck.

Hmmm ... the above can aptly be described as thinking out loud


Seadeuce

 

[Tigerman]

So, to summarize;

The propeller will create a preassure difference on the two sides of the propeller, and the rapid change in preassure is high enoug to make the water "boil" and create gas pockets just as in a pot of boiling water on the stove?

(Simplified non-scientific version)

 

[bobbates]

So, to summarize;

The propeller will create a preassure difference on the two sides of the propeller, and the rapid change in preassure is high enoug to make the water "boil" and create gas pockets just as in a pot of boiling water on the stove?

(Simplified non-scientific version)

Well said.

 

[1_T_Submariner]

That is not why they dive deep. To control cavitation the Throttle controlman is in charge of monitoring the Screw (Submarine Props are called screws) for cavitation when changing speeds (accelerating).

They may change depth to take advantage of the temperature and other conditions of the water.

Just a bit of trivia... cavitation is noisy and it is one of the reasons that submarines dive deep. The deeper you go... the colder the water and the higher the pressure which means less cavitation so the sub becomes more silent and undetectable by the enemy.

 

[OceanObsessed]

That is not why they dive deep. To control cavitation the Throttle controlman is in charge of monitoring the Screw (Submarine Props are called screws) for cavitation when changing speeds (accelerating).

They may change depth to take advantage of the temperature and other conditions of the water.

I can't remember if I got that from my college underwater acoustics teacher or from The Hunt for Red October but since it could be the latter I certainly would not argue with a guy named 1_T_Submariner .

 

[PanamaMike]

That is almost correct. In other words, the gas dissolved within the liquid (h20) is "boiled" out at ambient temperature because of the lowered pressure created by the prop turning torque. This is an example of the Integrated Gas Laws...the inverse relationship between pressure, density, volume and temperature. The bubbles will pit and damage cheap props...prop blades that flex over much will make this worse (cheapo aluminum props for ex)...the physical properties of water are fascinating, eh?

I really don't think so. Cavitation behind a propeller
isn't due to the water boiling, but voids, i.e., a vacuum
because the ambient water pressure can't fill in behind the
propeller as fast it it can throw water out of the way.

If the void was due to the water boiling, it would be be
at a (slightly) higher pressure than the ambient water
and wouldn't cause a shock wave as it collapses.

I think the vapor pressure of water is irrelevant in
the process.


Regards,

--Mike