Use pressurized water as manuevering thruster?

I am not sure I have understood what you mean but the following might help your understanding.

If you have a view that placing the nozzle below the waterline helps then this is incorrect. The best place for the jet to exhaust is just above the water level. I can appreciate this may create a problem for neighbouring vessels. However this is the reason jet boats discharge above the water level. It is more efficient.

The force you can generate with a jet is given in a post above. The power required to produce a given force increases as you increase jet velocity.

As a first approximation the least power as a function of thrust can be determined as:
Power = (4/pi/9/rho)^0.5 x Thrust^(3/2)/D in Watts

rho = 1025 for salt water
D in metres.

This relationship is the fundamental jet relationship. In practice there are pipe losses, rotor losses, lift losses and back pressure losses to contend with. In real life these will almost double the required power.

Point is the bigger the nozzle the less power required to produce a particular force.

Working out what you will get from a particular system is not very difficult. It is far better than just fiddling with hoses and pumps. Determine what is needed to do the job and then build it. It will still take a bit of development but at least you know where you are heading.

Rick W

 

"However this is the reason jet boats discharge above the water level. It is more efficient."

This is not strictly correct. Waterjet boats ideally discharge at the waterline, there is zero head. I have also never had KMW,LIPS,MJP, Hamilton et al ever suggest to me doing anything otherwise on any vessels I've designed too. Ive designed vessels ranging from KMW FF450 jets right up to KMWs 120 S series

 

I have facts and science to back up the information I provide unlike others who provide unfounded claims to knowledge that is just nonsense.

The most significant development in establishing water jet technology was to set the discharge above the waterline. This results in very little head increase and is far better than dealing with the backpressure from an entrained stream.

Rick W

 

If you were to read what i wrote, you will see that is not what I'm saying. However if you feel so strongly then please tell KMW, LIPS, MJP et al, that they should change their recommendations and warranties of their equipment.

Ive been down this road with all of them before....their kit, their warranty.

I don't know how many water jet powered boats you have designed but whenever you have the centreline of the duct/shaft above the waterline, it becomes increasingly more difficult to prime!..hence why all the manufacturers recommend in line with DWL..regardless of what theories are out there.

 

I would like to see any picture you have of a jet powered boat where the discharge nozzle is underwater when the boat is operating at design condition.

Rick W

 

Rick Ad Hoc has been espousing his design virtues since he came onboard, he even claimed to have "talked' the British M.O.D, into changing from steel to al al, yet on the 3 occasions I asked him for his credentials, whom he had worked for, his name etc, he chose to ignore, as he is by myself: so do not fret my man, we are listening to you, as per

 

Rick

There you go again, you utterly fail to comprehend what has been written, and meander down pointless meaningless paths of your own choosing.

Where have i written "... where the discharge nozzle is underwater ..."..? please explain to me where this is in my text?..can't find it....oh dear!

Also if you are so 100% convinced that you have this all singing all dancing new theory, then you better tell KMW, LIPS MJP et all very quickly so you can make a killing on a patent. They have only been doing this for 30 odd years...clearly you have better resources than all of them combined.

Next time you're on a jet boat..ask the Capt to place loads of ballast on the bow, to make her bow down trim so the jets are above the waterline. Then ask him to engage...and listen to the racing as the pumps cannot prime!

Ah, since you're such an expert on jets, you know all about the KMW AWJ-21, but I'm sure you've told KMW that this is not possible as your are asserting incorrectly again.

For those with the intellegnce to read and deisgn vessels with waterjets, you see that it is an underwater discharge waterjet design with remarkable stealth characteristics. But hey who am I compared to someone who knows far more than all the waterjet manufactures in the world combined.!

 

whoosh
This fellow Ad Hoc is bad news for the forum. He has killed some very useful input from contributors unwilling to challenge him on his pumped up claims.

He provides silly advice and has turned off a number of good forum contributors by his pompous stance. Taking any opportunity to put people down. I have not yet identified a single positive contribution or any useful advice.

If you go back through his posts it is like a train wreck.

Some examples:
He advised a fellow wanting to build a shallow mudflat hull that he needs to start with the hydrostatic head on the hull to determine how thick the sheeting should be. How useful is that?

He told Leo Lazauskas that he did not know what he was talking about when Leo advised depthofit that his model needed to be at least 1.5m long to get results valid for scale up design.

He has no idea of what Stitch and Glue construction is.

He has no idea who Dave Gerr is.

He believes the cross beams of a catamaran, point supported at port stern and starboard bow, will have zero moment to resolve.

He has no idea that the units for endurance limit of materials is a stress value.

He suggested to an experienced aluminium fabricator that the fabricator had no idea about controlling distortion in aluminium boat construction.

Have a look at this thread to see how this AH fellow can kill useful contributions:
http://www.boatdesign.net/forums/multihulls/designing-structure-38-catamaran-25944-2.html
Who would bother to continue to contribute when you get this sort of response. I hope Ernie returns. Look at the put-down for TYD in post #6.

Rick W

 

YES, I am quite distressed abt it, I simply could not let it go, the fellow is quite plainly a fabricator of the first magnitude
I would implore you to contact Jeff, I have had no end of people contact me over this
By the times he is online, I suspect he is unemployed, a sad individual living some dream in his head
i think the best thing is ignore as I do, pretty soon ostrasim works
Trouble is, some people may listen to his nonsense and take it as Gospel, particulary boat builders who are coming into al al
And he tried lecture myself abt distortion after 29 years building al al

 

Rick

see you have done it again..you fail to address the issue you raised. You just ignore them when countered, and don't answer the questions posed as you consistently say must be answered by others when reciprocating.

Perhaps you should practice what you preach!

 

You wrote:
"Waterjet boats ideally discharge at the waterline, there is zero head."

For it to have zero head it must be below the water level or are you going to deny this basic physics. Of course it could be minutely thin and infinity wide I guess but I believe most would agree this is not how they are built.

Rick W

 

In your mind you may believe you are the most important person in the world requiring an immediate response but I was responding to someone who makes useful contributions - something I cannot find from anything you have posted.

Rick W

 

Zero head means on the waterline when designing jet powered boats.

second, "...discharge above the waterline.." your words. This means the cone and hence the shaft line is above the static waterline. So, as stated above, you better tell KMW et al that they have been designing waterjet incorrectly, not me. Secondly tell them how easy it is to prime their jets when the shaft-line is above the waterline as that is where you wish to place their unit in your design.

Also you said " ..I would like to see any picture you have of a jet powered boat where the discharge nozzle is underwater..."

I gave you a reference which you obviously havent not bothered to look at or know about. Since you're a fan of cutting an pasting. Just cut and paste it into google, under MKW...you'll see the boat which you claim cannot be done. (Not so good at cutting and pasting as you cannot even copy my text correctly, you have used someone else's in your haste to reply above).

So, unless I am mistaken you clearly have the edge on KMW, LIPS et al, and you must have designed so many waterjet units that the whole world now uses them and that "normal" waterline discharge jets are grossly out numbered by your discharging nozzle above water.

 

It is a pity that this thread has gone down such a petty trail. Since, most of what you wrote was fine...but when you sprouted on about the usage of waterjets, it needed rectifying.

However you clearly feel that you have cornered the market on waterjet units and every other manufacturer is wrong and you are the worlds authority on waterjet units with your successful above dwl pumping units, given what you are saying above. Since that is your corollary.

 

as we know ad hoc means
improvised or impromptu. so that abt sums it up