Technical Question-Jetboats

[Mark Halliday]

Just to prove I have no idea what I am talking about...........but what the hell.

Would the thrust of a jet will reduce as it gets lower in the water because the "water pressure" (hydrostatic head?) increases with depth? This will subtract from the nozzle pressure. I think it will would have to be quite deep for noticeable reduction.
Cant try a hose at the mo', because it has gone all stiff and the top of the bucket has some sort of hard film on it.

Props sort of cut a thread through the water (hence the term "screw") with the aid of the pressure differential across the blade.
Bigger diameter = more thrust (bigger surface area for Newton's theory to play with), and bigger pitch = more speed (distance per rev).
I am thinking about wood screws here . A No. 12 will pull something up good and tight but makes the wrist ache. A No. 4 will zip in nice and quick, but if you tighten it too much it will loose grip.

If you skip this reply, I don't blame you.

[Mark Halliday]

JW

I think the right hand arrow should still be there in the bottom pic, the removed end has now become the air or water outside the unit.

gArf. You are right. I can see a bit of blue sky outside.

[Scary Des]

Originally Posted by Mark Halliday

.....Would the thrust of a jet will reduce as it gets lower in the water because the "water pressure" .....increases with depth? This will subtract from the nozzle pressure.....


If you skip this reply, I don't blame you.

Yeh but won’t the pressure at the intake end increase so equalling it out
I couldn’t skip your post so I printed it out and shredded it Des

[The Garfish]

Originally Posted by Mollulnan

The amazing thing is how the hell you've managed to make a water jet sound so bloody complicated

ded rite molers yew scrowte.

Originally Posted by pressman

a couple of wines

Originally Posted by Hugh Jardon

after 3/4 a bottle of wine!!

itts cuz orl de bastuds arr onn de pis an torkin bollix. wyne drinnkin wussis. reel menn drinnk beere.

fuk mee wots dis. anuvver wus

Originally Posted by Mollulnan

I'm also into a bottle of wine

gArf

[jwalker]

Originally Posted by Mark Halliday

JW..I think the right hand arrow should still be there in the bottom pic, the removed end has now become the air or water outside the unit.

Yes, but I simplified the diagram. There seems to be a notion that the forward movement is caused by the jet stream rushing out of the open end and pushing on the air outside of the container. Whereas, I'm suggesting that it is the difference in pressure between the open end and the inside of the container. In reality it would be difficult to maintain a sufficiently high pressure in an open ended container so some form of restriction of the open end will be needed, hence, the nozzle.

[Mark Halliday]

Originally Posted by Scary Des

Yeh but won’t the pressure at the intake end increase so equalling it out

Er........... yes.
Well..........no.
Ummmm........
Head x pump boost = thrust - head.
So ........... Only by head/boost...............
I think...................

Originally Posted by Scary Des

I couldn’t skip your post so I printed it out and shredded it Des

I hope its a heavy duty shredder, else you might wear it out.

Pass me a BEER someone, my fully adjustable yard arm says it time.

[Mark Halliday]

Originally Posted by jwalker

Yes, but I simplified the diagram. There seems to be a notion that the forward movement is caused by the jet stream rushing out of the open end and pushing on the air outside of the container. Whereas, I'm suggesting that it is the difference in pressure between the open end and the inside of the container. In reality it would be difficult to maintain a sufficiently high pressure in an open ended container so some form of restriction of the open end will be needed, hence, the nozzle.

Yeah. I get your point. Sorry, I thought you had "simplified" the nozzle out.
Detail omited for clarity and all that.

[The Garfish]

Originally Posted by Mark Halliday

Pass me a BEER someone

ahh a reel mann att fkin lasst

dis mite saiv yew nobburs warin owt dem worlnutts yew corl a brayn

worterjett

gArf

[Hugh Jardon]

wow diagrams as well.....

surely a water jet is a propellor shrouded with a collar, and that collar gets narrower at the rear
water is drawn in by the propellor which speeds it up and forces it rearwards, the converging collar or narrowing collar speeds up the water still further causing a fast stream of water to exit the hole giving thrust

isnt that all its about or did i miss something? i am sober at the mo so might not be on the right wavelength.

[kitten]

spot on

[yoyo]

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 don’t 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. I’m 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

[Alex Brown]

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

[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.

[codprawn]

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!!!

[codprawn]

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.

???????????????

[codprawn]

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???????

[jwalker]

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....

[codprawn]

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.

[yoyo]

Richard B,

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

thanks

[hard1]

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.