How do propellers propell a boat.

I'd say its just the difference in 2 pressures

 

So they don't use suction caps to stop spacewalkers floating away from the space station ?

 

Dude, it's not a Tardis
Works the way sir Isaac say -
With a little twist.

 

The engine spins the prop, the boat moves on. So simple, what's the big deal?

 

Its a bit like gravity

 

Tough to be lower than ambient pressure when ambient pressure is zero absolute.

 

Technically the space station is not at zero absolute pressure.
That happens further out in interstellar or intergalactic space.

Wow. Isn't that good? We are actually having a technical discussion about propellers.
Sort of.

Turns out I was wrong in post 2.
We are not having a big argument.

 

I'm not sure about many theories.

There a lot of different models (i.e. calculation techniques) that are more or less detailed, but they are all some level of simplification of reality (with various empirical factors) to make calculations feasible. They all basically converge on the same physics.

Current viscid CFD codes are pretty close in that they include RANS stuff, cavitation and so on using either a sort of hybrid FEA/poential flow or smoothed particle hydrodynamics.

Oceanic (in Newfoundland) has a well-regarded CFD propeller code and there are many others. MARIN probably does too, I don't recall off-hand. NSWCCD has a suite of codes, but they don't make them public. (If you are interested, there are a number of SNAME papers covering this type of thing by NSWCCD folks, including Karafaith, et al; "Hydrodynamic Efficiency Improvements ..." which is in the 2001 Transactions. Some of them are in the various small craft CDs.)

There are even some (sort of) open source codes (COSMIC used to have one back in the day that was mainly for aircraft, but could be used for ships).

As in most questions hydrodynamic, start out with Saunders.

 

But the guy that bangs the prop with a hammer on the tow hitch ends up faster than the guy with the computer every time

 

Lets beat up the suction issue a bit more but more importantly clarify what most pressure gauges are reading.

Almost all pressure gauges read in pounds per square inch (or equivalent metric) with 14.7 Psia being the zero point. Sea Level at a specific temperature which I think is 68 degrees F

So when you would read one atmosphere of pressure off a typical gauge, the correct way to state this would be 0 (zero) psig,,, the "g" standing for gauge.


If you were working with the gas laws, Charles, Boyle, the Ideal gas law, the calculations are taken as "absolute" pressure and hence you would write zero gauge
as 14.7 psia, a for absolute.

I would venture that all the contributors would never work off of psia in any work that they are doing when they are calculating pressures as the gauges are all start out at zero psig.

Even if you were trying for extremely accurate pressure readings like an inclined manometer, the pressure is taken from 1 standard atmosphere or 14.7 psia

To clarify

Say you take a standard pressure gauge and it is reading the number 10 psi
To be correct for normal design work, you would say 10psig
To be correct for doing calculations regarding the gas laws, you would say 24.7 psia

As we live at one atmosphere at 14.7 psig, any thing less can be called suction.

 

"the engine spins the prop, the prop hits a rock", the boat does not move on

A Tom Kane's pivotal drive spins the prop, the prop rides over the rock, the boat moves on

 

Capt. Schettino's dream!

 

Well, they would probably be the same people who build the boats by "eye" sans plans, as outlined in a recent thread.

 

It always happens..when some can not cope with the subject at hand they have to turn to comedy..shows up every time,but the comments are great.
Did anyone bother to Google the suggestions I made? The web links I put in the post would not work.

 

Well, in the OP you said:

So may I reiterate this question: