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Old 29 November 2005, 17:57   #11
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Quote:
Originally Posted by Hugh Jardon
.. now the lift created on a wing is all about sucking and a prop is a series of wings..
I agree that a prop is a series of wings but I think you misunderstand when you talk about the suction.
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Old 29 November 2005, 17:57   #12
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Interesting points JW and HJ.

Really like the metaphor of a series of wings.


Steve/Chris does that mean that the action might be a little like a home made jet wash?

i.e. finger over the end of a hose pipe?

missus gettin the idea
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Old 29 November 2005, 17:59   #13
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Quote:
Originally Posted by pressman
The infitely adjustable directional thrust gives excellent maneouvarability(think thats spelt right ad a couple of wines) and towing power over a propellor.

You will not get the same top end speed as a prop, but not far off.
I would say towing power is greater with a prop - hence most of the world's biggest tugs have some sort of propellor.

As to top speed I would say there is quite a difference - especially when you start looking at surface piercing etc.
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Old 29 November 2005, 18:01   #14
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So does that mean there is a difference in performance in varied sea conditions also?

(Between prop / jet?)

Is it harder to maintain fwd progress in rough waters in a jet-driven boat?

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Old 29 November 2005, 18:05   #15
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tugs use large props and move large volumes of water to get the required thrust for the work they do

a friend had a small jet boat thing, was around 3.5 m long but they found the slow speed gentle manouvering quite difficult as it needs a bit of throttle to force the water thru to give the directional steering, at speed it was great but for slow speed work they found it tricky.Engine was very powerful and a little bit of throttle on such a light craft made it tricky as it would lurch. never tried it myself so is someone elses opinion.

i suspect the jets are more suited to larger boats on the slow manouvering stuff
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Old 29 November 2005, 18:05   #16
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Yes, most tugs have propellors, but these are then sat in a Kort nozzle{you may wish to look that one up on GOOGLE), this will increase the bollard pull of a prop driven vessel by 25 %, that is, it is surrounded by a tube if you like, the same as a jet impellor which, stops a lot of the thrust from a prop slipping out sideways.

Surface piercing props are a totally different aspect to what we are talking about!!!!
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Old 29 November 2005, 18:14   #17
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Swine

I wanted to say Kortz nozzle

Missus the slow speed manoverability of a jet is stunning, the downside of them is that they are not as fuel efficient as a prop and they usually run from an inboard.

Hugh. the mall jet boat things tend to have a different control system from proper rib type Jets. The jet has two controls a forward and reverse and steering. The motor has one control accelerator. You may have an optional gearbox but it isn't needed

You set the revs on a motor and then don't really need to adjust it after that as you control the momentum via the forward reverse control. when you get used to this it's very assuring

Now don't all go posting links to the outboard jet drive
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Old 29 November 2005, 18:16   #18
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Quote:
Originally Posted by The Jackeens
i.e. finger over the end of a hose pipe?
A jet is exactly like that. When the end of the pipe is completely blocked, the water pressure is acting equally in all directions and the pipe will have no tendency to move. If you partially open the end, there is then nothing for the water to lean against at the open bit* so the forces inside the pipe are no longer balanced and there is a force tending to move the pipe in the opposite direction to the open end. That's why I think, in a water jet, the impeller must compress the water in the rear chamber at a rate capable of sustaining sufficient pressure to produce the jet effect.
*There is air but it's hardly significant.

Is the outlet of the jet submerged when the craft is planing? I'm presuming not.
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Old 29 November 2005, 18:20   #19
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Quote:
Originally Posted by jwalker
Is the outlet of the jet submerged when the craft is planing? I'm presuming not.
That might explain why 'Pressman' presents such an awesome sight as it storms up the Solent?

(Very interesting points Jeff & Stu.)

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Old 29 November 2005, 18:21   #20
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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!!
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