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Old 30 November 2005, 08:59   #51
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Hi Peeps,

I recall doing some assignment on this 20+ years ago at collage. I think that the Bernoulli equation is the answer.
Bernoulli states that if two or more pressurised bodies (volumes) are connected the sum of the pressure x volume of all the bodies will be equal, it donít mater what the size of each individual volume or pressure.
Therefore: Presure1xVolume1 = P2xV2 =P3xV3 etc etc.

So if the last pressurised volume has a small orifice where the pressure (in this case water) escapes it will exit at a large velocity accompanied with a lowering pressure. Iím pretty certain that the Bernoulli equation has something to do with water jets

By the way my understanding is, that it is possible to pressurise water but impossible to compress it like a gas.
More Bernoullli info at:
http://www.princeton.edu/~asmits/Bic...Bernoulli.html
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Old 30 November 2005, 09:00   #52
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Quote:
Originally Posted by The Jackeens
Nice bit of tug video Alex. Were they trying to spring that tug onto the dock or just testing its strength? (Reason I ask: I've done a few approaches like that myself)

(Given loadsa and a choice of boats, a deluxe fitted-out tug -none of your North Sea Boot Camp style outfits RW - would be a strong choice for me Or an ice breaker )
missus
That afternoon after I'd moved a few boats around the dock to make room for visiting trawlers, we had to check the hook was working on the back. One end of the line goes ashore, and the other attaches to the hook.... go to 80% power ahead (incase anyone wonders why, when towing, you're not supposed to goto 100% thrust on the voith, only when 'free-running'). When the thing is stable, pull a lever in the wheelhouse and the hook lets the tow go - it's quite critical if you're being pulled by a ship and something goes wrong.

I'd like to go on one of the new Rotor tugs - 3 pods, 2 in the bow and one at the stern where the skeg would normally go... it allows the tug to drive itself at speeds of upto 7 knots sideways

Ours is the only Voith tug on the haven - there used to be another just like ours at the RMAS depot, but that's been gone for years now.

Here's another video - http://videos.ribseadrive.co.uk/512k...02_10_05_small
berthing practice back in october.... also down into the engine room (nice turbo whistle). The engine is just over 300hp (but slightly bigger than the yanmar equiv ), the vid shows the flywheel and clutch as well.

-Alex
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Old 30 November 2005, 09:02   #53
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Yoyo, unless I'm mistaken, I think your equation is "Boyles Law" which states that for a perfect gas pressure x volume is constant.
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Old 30 November 2005, 09:24   #54
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Quote:
Originally Posted by Mollulnan
Some top notch "Googling" guys, Codders would be proud. The amazing thing is how the hell you've managed to make a water jet sound so bloody complicated?!!
I was a mine of useless info LONG before Google came out!!! I only use Google to CHECK my facts before I post!!!
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Old 30 November 2005, 09:27   #55
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Quote:
Originally Posted by Richard B
Yoyo, unless I'm mistaken, I think your equation is "Boyles Law" which states that for a perfect gas pressure x volume is constant.

???????????????
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Old 30 November 2005, 09:30   #56
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Well spotted - yes he IS right that Bernoulli's Equation is the one to use but for some reason he has posted Boyle's law instead - a slip of the old mouse perhaps???????
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Old 30 November 2005, 09:40   #57
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Quote:
Originally Posted by Hugh Jardon
...causing a fast stream of water to exit the hole giving thrust
You reckon? Undoubtedly there is a fast stream of water leaving the nozzle but what does it thrust against? There has to be a force pushing on the inside of the jet unit to cause it to move forward. This has to be either a pressure difference between the nozzle end and the forward end of the jet or a reactive force against the water jetstream but me brain's having difficulty with the latter concept. Garfie's link suggests the latter but I've a feeling it's a bit layman speak for what it really going on. There's another bit niggling me; when a fluid is accelerated, the pressure within it falls. hmmm....
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Old 30 November 2005, 09:45   #58
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Doesn't have to thrust against anything - ask Newton!!!

I thought a waterjet was so simple to understand - but then again everything is simple when you look at the basics and don't let science get in the way.
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Old 30 November 2005, 09:46   #59
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Richard B,

Yeah your right mate soz. in my defence it was 20+ years ago. :-)

thanks
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Old 30 November 2005, 09:48   #60
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Quote:
Originally Posted by pressman
One for CODPRAWN.

Tugs also use Voith Schrieder drives, but then I suppose you know about those aswell.

Compass units are, another one, kitchener gear, there are many, best you read up on these!!

Ah...(sigh) The Kitchen Rudder! Forerunner to the jet Bucket and the best of both worlds (no fkin gearbox). Happy Days.
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