Technical Question-Jetboats

[The Jackeens]

This is a technical question but I'm only a woman ribster so I'll post it nice and quietly down here outta the way

Following a discussion tonight with some fellow ribsters I was trying to understand the physics of a jet boat.

A discussion about aeroplane wings followed () but no-one explained how jet boats operate. Is it about suction or expulsion?

I know Rogue Wave has some experience ( Bugle Billy : what a great name for a boat) and we are about to try out Pressman's monster craft next weekend.

Does such a system suit RIBs? Or is the jet system better for hardboats?

Thanks in advance for any replies/ photos of jetboats!

missus hated physics

[pressman]

The jet unit is basically a water pump.

Water is drawn in through a large intake then forced out through a smaller opening under speed and pressure. you are forcing out the same volume of water from the large intake through a smaller outlet.

Before this leaves the outlet it has to go through a stator, this concentrates the water in a straight line, without the stator the water would just spin in the same direction as the impellor that is driving it.

The jet can be used to great effect on all kinds of vessel from heavy displacement to fast planing craft.

the majority of fast cat ferry's use water jets, the vomit comits use four water jets with 38,000 HP driving them.

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.

If a jet leaves the water a lot of people say it takes a long time to pick up again, not true, takes no longer than a prop.

The other advantages are, no exsposed propellors to worry about, less mechanical problems as most jets have only one moving part.

Can explain more on Sunday Kathleen and would welcome any sceptics to run out should they wish

[Poly]

(i'm a chemist so forgive the mickey mouse explanation)

Is it about suction or expulsion?

The boat sucks water in an opening somewhere at the front. A large impellor (basically a propellor inside a tube) sucks the water in, and then forces it out the back. Newtonian physics says that every action has an equal and opposite reaction so the force of the water coming out the back results i an equal but opposite reaction against the boat - pushing it forwards. This is exactly the same principal that makes Jet planes move forward (probably how the conversation digressed in the pub!) or rockets fly. How the "jet" is generated is of course different.

I can't comment on the suitability for a rib, but there are some advantages where there is any risk of people being in the water nearby. On the otherhand if there is a risk you can suck in stones/gravel from beaching it is very bad.

HTH

Neil

[The Jackeens]

Now that's a top answer Steve, many thanks!

(OOPS Neil, just saw your post..will read also. OK that explains why they were going on about planes If they'd only mentioned Newton I'd have got it straightaway Thanks for the clear explanation also. I particularly like the safety aspect of not having a limb chopper (prop) at the back!)

And it all made sense

Certainly makes a good argument for jet propulsion. Will be well prepared for next Sunday's Pressman Jetboat tryout. Correct terminology on board too.

Found on Jetboat John's (Robin's mate's) boat that it would stop on a sixpence as it were !

Felt totally different from prop. action.

gArf used to say that when Rogue Wave took Bugle Billy out in the Solent, the water level used to drop by a foot

So how's the slow handling? Do you ram it towards a pontoon and shut things down quickly while closing your eyes and praying all at the same time?



missus

[pressman]

We will reveal all on Sunday

You will see how quick it stops and turns at 10, 20 30 knots and maybe faster if you are feeling brave.

[The Jackeens]

Excellent!


I also observed that it may not be best to be up in the bow when you do those big stops!


missus

[pressman]

[jwalker]

I don't fully understand this either. I do understand how a jet works and I understand how a turbine works but in both of these fuel is being burnt to produce the pressure within the engine. The fact that they have a high internal pressure and one end is open to the atmosphere causes the engine to move. However, a water jet is more like a hairdryer in that the same quantity of air (water) goes in as comes out.

The only way I can see the waterjet working is by taking in the water with little restriction and compressing it in the rear chamber at such a pressure that it becomes a true jet as the water is emitted.

Over to those who know.

[pressman]

The only way I can see the waterjet working is by taking in the water with little restriction and compressing it in the rear chamber at such a pressure that it becomes a true jet as the water is emitted.

Over to those who know. [/QUOTE]

You are right with that, if you look at a firemans hose a large pump pushing a large volume of water from a small outlet, you must have seen the hose when no one's holding it!!

[Hugh Jardon]

i would say it is less of a sucking along and more of a blowing along. aircraft do not suck themselves along in the air they push themselves along by the force of the exhausted gas stream, which has been heated and a continious controlled explosion has taken place

with a fast jet you can stand within x feet of the intake, x being different depending on the engine or jet but you can not stand the same distance behind it as you will be blown away.

on a hercules you can stand in front of the props at around 15 ft no problem but the wind at 600 feet behind them is 60 mph at full chat, now the lift created on a wing is all about sucking and a prop is a series of wings

now with a water craft as water does not compress so easily as air i am unclear if the above principal is truly the same, intersting to find out, i suspect they tend to suck a little bit and blow a lot, they draw water in at low pressure thru a larger hole and force the water thru a smaller hole speeding it up considerably giving forward propulsion
oooooowsat,,

[jwalker]

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.

[The Jackeens]

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

[codprawn]

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.

[The Jackeens]

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?

missus

[Hugh Jardon]

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

[pressman]

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!!!!

[Rogue Wave]

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

[jwalker]

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.

[The Jackeens]

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.)

missus

[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!!