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Old 30 November 2005, 03:51   #31
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Quote:
Originally Posted by kitten
My twopenneth (as I understand it)
Taking water as a constant volume noncompressable liquid (which it's not, but it is close)
A propeller mostly drags itself through the water by creating low pressure on the front of the blade which is why cavitation (lowering water pressure below vapor pressure) destroys drive.
A jet takes in low velocity water and raises it's pressure and speeds it up by restricting it's area, ejecting it out the back producing drive via the inertia of the column of water behind it, put the column under water and most of the drive is lost, which is partly the reason jets can lose in the rough. (try it with a hose pipe in a bucket of water. End of the hose in the air it will push, end of the hose in the water it don’t so much)
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Cheers for that Kitten.

Nice bit of explaining. In the context of the forces used then the water is constant volume got that one.

Find the hosepipe analogy very helpful and appreciate that something about the density? extra opposing force? when submerged means the jet might be less powerful? So maybe slower? in rough seas.

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 )

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Old 30 November 2005, 04:38   #32
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Not sure that I really understand it but I believe that the reason you use a large prop on a tug is that it is less likely to slip (spin) when compared to a small prop. The analogue is large diameter wheel v a small diameter wheel. With a go-kart, for example, you can spin the 10” wheels quite easily, replace these with 30” wheels and providing the clutch would take it, you would never spin them
Quote:
Originally Posted by kitten
......... the inertia of the column of water behind it, put the column under water and most of the drive is lost, which is partly the reason jets can lose in the rough.
Not sure that this is right because with a jet engine on a plane it is always ‘underwater’ and yet doesn’t suffer in the way you describe
I thing that the lack of performance in rough weather attributed to jets is an urban myth that started when people used to using outdrives were shocked when the jet didn’t perform in the same way. The RN now run Pac22 MK2 jet ribs on a lot of there ships and I doubt that they would have selected this boat if it was no good in the rough. Des
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Old 30 November 2005, 04:58   #33
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From my experience the only down side of a jet in the rough is the loss of momentum if you come out of the water. If the intake looses contact with water it sucks air until it is immersed again. This can cause a bit of a dead spot. However in a big deep v boat the stern tends to stay in the water and this is not a big concern.

They can be a little skittish at high speed if not set up perfectly - i.e. slide a little and not turn as tight as a prop.

However if set up well you just cannot beat a big waterjet rib... imho of course
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Old 30 November 2005, 05:12   #34
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Quote:
Originally Posted by Rodan
From my experience the only down side of a jet in the rough is the loss of momentum if you come out of the water. If the intake looses contact with water it sucks air until it is immersed again.....
Isn’t this also true for a prop
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Old 30 November 2005, 05:29   #35
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Yes but a prop seems to pick up again quicker - this is from experience but is not gospel!
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Old 30 November 2005, 05:49   #36
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There is no easy one sentence answer to how or why props or jets work.
I didn’t say that submerging the jet losses all drive, but it does lose a lot as it makes it a small prop running in a tube and not a jet.
RN use jets for many reasons and efficiency and speed are not the main concerns floating obstacles is one, ease of maintenance another.
Mr Scary. Bigger wheels raises the gearing, you do that on a prop with pitch. Bigger blades spread a given load over a bigger area so you get more drive, like with a tug. Wider tyres would be a better analogy.
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Old 30 November 2005, 06:16   #37
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Quote:
Originally Posted by kitten
.....I didn’t say that submerging the jet losses all drive, but it does lose a lot as it makes it a small prop running in a tube and not a jet......
Sorry still don't see it By increasing the velocity you are getting the ‘equal and opposite’ so does it matter where the jet discharges unless there is something else going on such as drag on the side of the water jet
Quote:
Originally Posted by kitten
.....RN use jets for many reasons and efficiency and speed are not the main concerns floating obstacles is one, ease of maintenance another........
Safety would be a concern for the RN so poor rough weather performance would be an issue.

Quote:
Originally Posted by kitten
....Mr Scary. .......
THAT'S Des to you

Quote:
Originally Posted by kitten
Bigger wheels raises the gearing, you do that on a prop with pitch. Bigger blades spread a given load over a bigger area so you get more drive, like with a tug. Wider tyres would be a better analogy.....
OK that makes sense but what is the relationship between big dia. Low pitch small dia high pitch at some point they much be equal and is that the point when shear becomes an issue. Des
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Old 30 November 2005, 06:52   #38
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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?!!
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Old 30 November 2005, 07:11   #39
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I feel thare are a number of misconceptions creeping in to this discussion.

I've had similar conversations with others and no one has given me a satisfactory explanation of the jet. In the absence of this, my deduction is that it works as shown in the diagram.

The top picture is a container at high pressure, the lower is a container at high pressure with one end removed. Look at the force arrows. There has to be forward movement of the container. I've used a very simplified shape in order to show the principle.

Comments please?
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Old 30 November 2005, 07:19   #40
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