Friction

[Matt]

Despite the title, this is not a reference to captain's self inflicted injuries. Pretty amazing how much friction there is on a boat then!

<img src='http://www.mattyorke.com/articles/friction/friction.jpg'></img>

 

[jw.]

How is dynamic pressure defined? What forces are we actually talking about?

 

[Matt]

I didn't say I understood it.

 

[jw.]

I didn't say I understood it.

JF'll tell us.

 

[Jon Fuller]

I would imagine they mean stagnation pressure, ie, the pressure that builds up directly in front of an object as it's pushed through a medium, there's a moment when the medium (water) is stationary in that zone, or it 'stagnates' there.

this is the principle used for pitot speedos used in aircraft and boats.

I happen to know that at 70mph, the stagnation pressure of water is 70 psi, but of course the lines are crossing over on a chart, so they don't rise equaly together (speed and pressure)

Jeff, there's quite an intersting graph showing all this on that Bill Maloney paper I sent you.

 

[Captain Chaos]

No...........he's full of **** and only understands backstreet garage mechanics anyway.............I.e bodyfiller & tyre kickin.

 

[jw.]

Jeff, there's quite an intersting graph showing all this on that Bill Maloney paper I sent you.

It's sitting at the side of me. I'll need to swat it up again.

Off to me kip just now.

Nite ol.

 

[BluFin]

Have a read of Mechanics of Flight it will explain all
along with calc formulas.
Pitot is a dynamic pressure
Static is the stagnation one.
Normally you would read a dynamic then build in the static for temp pressure corrections a perfect example would me a Mach meter as the speed of sound changes greatly with altitude and temperature as they are inter-related bit like rednecks.
Standard lapes rates.................. get a bit....yawn!

Ref the Pic Think that was Brian Grimshaw (Barracuda Boats) up at windermere

 

[Jon Fuller]

Pitot is a dynamic pressure

Static is the stagnation one.

Cool!

John, could you go into a little more detail for us about the diiference between dynamic and stagnation pressure please.

Very interesting indeed.

jf

 

[BluFin]

Cool!

John, could you go into a little more detail for us about the diiference between dynamic and stagnation pressure please.

Very interesting indeed.

jf

You Bar*turd do I have too, I' try and do something over the weekend, Oh and this thing on Pressures

See Link for typical Aircraft Application
http://www.boeing-727.com/Data/systems/infopitotstatic.html

See Link For Standard Altitude
http://www.boeing-727.com/Data/fly odds/standard alt.html

This is an intersting one if you like playing with ya PC
http://www.boeing-727.com/Data/fly odds/distance.html
This will show you how to work out courses and stuff from lat longs if you have excel s'sheets just type in the positions.


If I dont manage it for a few days I wont forget give me a shove. christ I just realised I'm an ANORAK...........................

 

[BluFin]

Here are two good links on the Dynamic topic.

http://www.grc.nasa.gov/WWW/K-12/airplane/dynpress.html

http://www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html This is Ok though I dont know of anybody who is performance minded would try to push a flat plate through the air, so the stagnation point is a bit of an ass, but it will exist it's just at what level. Increasing profile drag is not good thats why we streamline for an increas in the laminar flow, less drag.

Genrally speaking for boats all are constat with the change being the velocity.
To cut the nut cotton it's quite easy to calculate the desired lift you need to generate from the cats tunnel by trasposing the formulas about.

It's an intresting topic as water and air react in the say way and observe the same rules.

Christ, I sound a really nerd.................

 

[BluFin]

I fekn hate this loaner keyboard. excuse spelling

 

[BluFin]

Ref the above on pitot tube, compressabilty does become an issue above 60Kts possible less in water, thats why the old GPS speeds are such a fav.

Gets even worse when you build in changing temps, density.

Moral here "stay at sea level far more fun"