Technical Question-Jetboats

[Simon B]

Sounds plausible there seems to be very little restriction and little in the way of head so pressure would not be high. With the previously posted monster food pumps if we get a blockage there's not that much pressure or flow it doesnt ramp up and bust the pump - the impellor simply spins and the motor goes out on over speed (if invertor controlled) When the whole system is running tickety boo the motor is working really hard so high current is drawn. This equates to speed as it's volume shifted per minute.

Our veg oil is pumped in a similar way but its a tiny pump only does 400 -600 cubic metres and hour.

[Jonny Fuller]

Originally Posted by Simon B

only does 400 -600 cubic metres and hour.

[jwalker]

Originally Posted by Jonny Fuller

And, I don't think velocity is the key word, I think it's the shere weight of water that you grab hold of, and throw back quickly (accelerate) that gives the thrust.

If that was the case, you could have a big tube open at the rear and a fekkin great impeller shoving the water back. You're back to a tube with a propeller inside it. Tricky init?

[pressman]

Were back to KORT KNOZZLES now!!

[Jonny Fuller]

Originally Posted by jwalker

If that was the case, you could have a big tube open at the rear and a fekkin great impeller shoving the water back. You're back to a tube with a propeller inside it. Tricky init?

Well I think that if I were trying to pull a ship into harbour, thats the setup i'd have, this is what I mean about the ballance of mass, and velocity being correct for a given purpose, if your jetsprinter needs to do 80mph, you'd engineer the pump to move water considerably quicker than 80mph, but that pump wouldn't be right for a 30 knt ferry.

[Jonny Fuller]

Question:

when does a prop in a tube, become a waterjet drive?

ie, if your bowthruster had a huge motor, nice efficient impeller, and stators, but no constriction, and you mounted it at the stern, with a through-hull intake, would it be a low speed water jet?

I think it would, better at slow hard pushing than tearing along at 80 maybe, but still doing the same basic job.

[jwalker]

Originally Posted by Jonny Fuller

... if your bowthruster had a huge motor, nice efficient impeller, and stators, but no constriction, and you mounted it at the stern, with a through-hull intake, would it be a low speed water jet?

Nah, I don't think it would. But I do think that it would be good for low speed pulling because it would constrain the water which would otherwise be thrown out of the blades by centrifugal force and the water which is trying to escape over the blade tips because of the pressure difference front to rear. I guess the propellers which have a ring around them work in a similar way.

[Jonny Fuller]

Oh well, that was fun, but I still think it rather unlikely I'll ever own one.

[DGR]

Originally Posted by Jonny Fuller

Oh well, that was fun...

[kitten]

Can I just say inertia (again)
Also as I understand potetial energy is stored, and kinetic energy is in motion and not stored.

[The Jackeens]

Fair bit of catching up to do here for some of us


missus

[Rogue Wave]

Originally Posted by The Jackeens

Fair bit of catching up to do here for some of us


missus

I think you'll find that you can catch the jet boat kwite kwickly with your 200 hp outboard.

[Hugh Jardon]

Well, today went out on Steve and Carolines (Pressman) Jet Rib and what an awsome beast it is but handles lovely. the slow speed manouverability and control is superb, it can spin on a sixpense, you can cruise at 40 knots and then stop almost instantly. At high speed it can do some incredible tight turns. On the mooring it can move sideways and at slow speed it will go any direction you want it to.

I can see why folks have Jet Drives now, fantastic bits of Kit

[Hightower]

And a photo of the Power units.

[The Jackeens]

Most recent helpful ideas (thanks) include :

• water acting as 'bullets in machine gun fire'
• the importance of the nozzle in the whole performance (without it an impellor acting as a prop in a tube i.e. bow-thruster) It also can change boat's direction :nozzle co-efficient ?
• more of a focus on pressure rather than mass (although both play their part: the former more so in speed)
• accelerating the water gives the action & subsequently there is a reaction (moving forward)
• anyone got that video clip of the crazy jet-sprint boat spin mentioned where the acceleration is seemingly space age?
• stator 'straightens out' water (flow?)
• potential energy stored : kinetic energy released?
• impellor applies force on water (mass)
• there are one, two & three-stage jets
• Bucket 'brakes'
• Potatoes !!! Now you're talking my language

Please correct any misconceptions above.

Now stuck on the terms axial and centrifugal (and any other varieties!)

Please explain.

missus

[Mark Halliday]

Originally Posted by The Jackeens

Now stuck on the terms axial and centrifugal (and any other varieties!)

Please explain.

missus

You want to keep that spoon don't you? (It has to be a new thread to qualify)

Axial - the shaft of the fan/pump is in line with the flow. (Co-Axial) The action is to push/pull along the pipe. Think bow thruster or modern hair drier.

Centrifugal - the shaft is at right angles to the exhaust flow. The stuff you are pumping goes in at the centre of the pump and is spun off the end of the vanes and usually exits ar 90º to the inlet. Think old style hair drier or submersible bilge pump.

Do you want to know about Vane, Roots, Screw and Piston (positive displacement) pumps?

[The Jackeens]

Originally Posted by Mark Halliday

You want to keep that spoon don't you? (It has to be a new thread to qualify)

Very thoughtful & I was chuffed

Axial - the shaft of the fan/pump is in line with the flow. (Co-Axial) The action is to push/pull along the pipe. Think bow thruster or modern hair drier.

Centrifugal - the shaft is at right angles to the exhaust flow. The stuff you are pumping goes in at the centre of the pump and is spun off the end of the vanes and usually exits ar 90º to the inlet. Think old style hair drier or submersible bilge pump.

Need to digest that a bit more

Do you want to know about Vane, Roots, Screw and Piston (positive displacement) pumps?

In for a penny, in for a pound. (Thing is I actually thought vanes were those yokes on top of chimneys with farmyard animals on them).

If they're central to jetboat function then please elaborate Sir Mark et al.
Thanks then!

missus

[Rogue Wave]

Originally Posted by Mark Halliday

Do you want to know about Vane, Roots, Screw and Piston (positive displacement) pumps?

DE missis knoze orl abowt de rootz! er pAwl woz telin mee e ad tooo giv er fiftee kwid tooo gett herr rootz touchd upp. eye fink dat's wot shee ad touchd upp enywey

[The Jackeens]

How does one report a bad post again. Must consult with the forum whingers



missus

(And you can behave and all Pierre Sept )

[The Jackeens]

This is the business end of Pressman's 2 jets



and

This is what happens when you get too close to these jets.
me feckin camera took a bashing not to mention me





paul