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[Kmot]

I suppose a "conventional tube" would be a straight tube. The tapered nozzle would increase pressure, so that is where the alleged 30% increase in power would come from.

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[mfr02]

The wonderful world of advertising. The magic words are "as much as", but they don't say what the comparison is with. Does look a nice bit of kit, though.

[der kapitan]

The wonderful world of advertising. The magic words are "as much as", but they don't say what the comparison is with. Does look a nice bit of kit, though.

"as much as---" ;)

Yes, the wonderful world of advertising, indeed! :rolleyes:

Uh huh, and taking an enema can give YOU an increase in thrust too---. :D

[Shaun Hendricks]

I know about this from aerodynamics, aquadynamics are probably similar. The shaping does indeed increase thrust, but at the cost of speed. To a tug, this is a piffle, but to an aircraft, it would be a considerable loss. Thus aircraft ducts (like ducted fans, etc) are designed to be a bit more neutral to both.

I was reading about the Korts having 2 types, one inside taper, one outside and they had different effects. Directing more water into the nozzle with an inside taper resulted in more thrust. Directing the water around the outside with a taper resulted in more speed. Both at the expense of steering but it was worse with the outside taper. This nozzle appears to have both so I'm wondering what they are accomplishing...

Dunno, any nautical engineers out there that can solve this dilemma. I'm perplexed.

[pimp_squeak]

Would an internal taper not simply increase drag? If I recall correctly (and I probably don't) water is not compressible (air is) so if there was any decrease in size for the nozzle it would simply block the water (not a sealed system like a water-jet)

[keith S]

I know about this from aerodynamics, aquadynamics are probably similar. The shaping does indeed increase thrust, but at the cost of speed. To a tug, this is a piffle, but to an aircraft, it would be a considerable loss. Thus aircraft ducts (like ducted fans, etc) are designed to be a bit more neutral to both.

I was reading about the Korts having 2 types, one inside taper, one outside and they had different effects. Directing more water into the nozzle with an inside taper resulted in more thrust. Directing the water around the outside with a taper resulted in more speed. Both at the expense of steering but it was worse with the outside taper. This nozzle appears to have both so I'm wondering what they are accomplishing...

Dunno, any nautical engineers out there that can solve this dilemma. I'm perplexed.

Not an engineer but talked to several that have explained the designof the knozzel. The inside tappers (as there are several when veiwing the real ones) is what creates the increased thrust in combination with the type of prop. The large opening allows for lots of water to be sucked in to the knozzel. As it is reduced in size very rapidly, the force creates an increase in thrust through compression of the water. As the trust is exited by the props, it is allow to decompress at a gradual rate ( compared to the intake ). In it's simpilest form (visual) imagine a funnel with an impeller. The water will flow, but the impeller will increase the amount of water trying to be forced out the end, thus creating more thrust.
The out side of the knozzel does the same in revearse--creating less drag like an airfoil. The thing to remember is that on a full size knozzel they run very close tollerances between the prop and the knozzel ( usally less than a 1.5" closer to 1/8th" ( depends on type of sevice it is being used on). That is as simple as I can explain. The real ones, by the way are built hollow and in peices welded ( around 64 sections welded together to get the shap to gether--not solid.) Some of the smaller ones are solid metal.

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[Kmot]

Smoke and mirrors. Snake oil. Magic potion.

Been the same for centuries.

[Tugboat Andy]

I emailed Mobile Marine Models and questioned their claims asking for details of boat models tested and test data relating to their kort's...they wont divulge their test results..

Well, well..... ;)

Thanks for following up on this Toes. I am a bit puzzled by their response.

My R & D lab will continue to pursue the next logical steps in model kort nozzle development....whatever that is :p

[mfr02]

As water in its liquid form is incompressible, but in this case does have somewhere to go, when the prop spins and moves a given quantity of water through the hole at the front, the same quantity will exit at the back, but as the hole is smaller, it will exit a whole lot faster. The design probably seeks to minimise losses due to turbulence both internal and external. The nozzel must present a surface area to the oncoming water which will generate resistance which will increase as the square of the speed, so any gains will be effective at low speeds, but overcome at higher speeds.
The unfair trading in the UK - you tend to get sued for what is known as "knocking copy". This is especially true if you prove the point with just your research, but in some areas government labs do the research and that information is published and can be used.
Like I said before, it still looks a nice piece of kit, much nicer than a length of drainpipe.

[Boatfox]

Ohhh...the science behind the design..Kort design (http://www.solarnavigator.net/kort_nozzle.htm)

The light reading saysKort nozzles or ducted propellers can be significantly more efficient than un-ducted propellers at low speeds, producing greater thrust in a smaller package. For the Bollard Pull it may produce as much as 50% greater thrust per unit power than a propeller without a duct. Tugboats are the most common application for Kort nozzles as highly loaded propellers on slow moving vessels benefit the most.

The additional shrouding adds drag, however, and Kort nozzles lose their advantage over propellers at about ten knots (18,52 km/h), although with the latest advances in design the maximum working speed may be increased to 15 knots.

[der kapitan]

Smoke and mirrors. Snake oil. Magic potion.
Been the same for centuries.
Sounds like the promises for Dr. Pifflesnort's Root & Herb Potion---. :rolleyes:

Cures virtually everything under the sun---. ;)

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[Captbill]

Since i do have experience with the real thing...I'll offer this.

As some as said water cannot be compressed. Period. Maybe some are missing the whole point of the nozzle. Aircraft depend on the THRUST of the air put out by the prop (they operate in air)

Jet boats are (sort of) the same in that their power is based on the ability of the impeller to blast water out the back (see pump) and are measured on thrust out.

Propeller driven boats are measured by Bollard pull (Toes has the idea).

Anyhow, in a propeller driven boat we measure how much power is required to "screw" (advance) the propeller through the water. That is pitch. (e.g., an 18" pitch prop advances 18 inches in one turn). We don't care what the thrust is.

With a Kort Nozzle you are increasing the ability of the prop (screw) to advance through the water by directly channeling it to the prop, side slip (waste) is greatly reduced. The nozzle size/taper helps for control (allot).

It's like driving a screw through soft wood and it strips, with the nozzle it's more like driving the screw (prop) through Oak.


My .o2, not a engineer either but been in it a while. Hope this helps.

Best to ya'll

Bill

[Aerominded]

It's like driving a screw through soft wood and it strips, with the nozzle it's more like driving the screw (prop) through Oak.




That is a great analogy!!! :)

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[Captbill]

Anytime you can reduce side loss...as the shaped nozzle will (both at the side of the prop and aft), the better you'll be as opposed to a straight nozzle (to a point).

Engine power, gearing for the given prop size would be first consideration and quite technical). After that is perfectly tuned then the addition of a properly fitted nozzle helps efficiency. I've never heard the 30% increase stated earlier...I recall like 16-24ish %

But the props that were perfectly matched to the nozzles were always best (blade tip clearances).

Very technical stuff to be sure...fun to play with I bet!!!!

I think the Tami D Springer will stay like she is (stock Springer)...having a blast with her...

Keep us posted... :)

Bill

[Tugboat Andy]

...props that were perfectly matched to the nozzles were always best (blade tip clearances).

Bill

Good point Cap. There are some pretty wild and radical propeller designs with swept blades...mostly unavailable to us backyard boat builders of limited means ;)

[Aerominded]

I think the Tami D Springer will stay like she is (stock Springer)...having a blast with her...



The mods will be for the second one then? :p ;)

Very interesting points you make about the nozzle, Bill! :)

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[Tugboat Andy]

How about something like this Andy?..

Yeeoow! That's what I'm talking about!

Um.....is that Kmot's propeller...? :o

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[Aerominded]

PMSL Andy :D:D:D



Et tu, Toes? txt? :p

[Captbill]

The mods will be for the second one then? :p ;)



I like the way you think! :D ;)

(of course!)

[CaptLee]

Toes Been studying these Kort Nozzles for a bit, found out there is three things to consider. Main influence is the propeller and its tip clearance, closer is better but swiveling nozzles take more clearance and lose efficiencey. Nozzle props need more area in the tip area, standard types are not as efficent in a nozzle. Nozzle profile is next factor to work on, model seems to choke more and are less efficent. Profile drag is the least factor to consider as it is drag as and as speed rises so does the drag(Seems to be exponitally).

[Captbill]

Toes Been studying these Kort Nozzles for a bit, found out there is three things to consider. Main influence is the propeller and its tip clearance, closer is better but swiveling nozzles take more clearance and lose efficiencey. Nozzle props need more area in the tip area, standard types are not as efficent in a nozzle. Nozzle profile is next factor to work on, model seems to choke more and are less efficent. Profile drag is the least factor to consider as it is drag as and as speed rises so does the drag(Seems to be exponitally).

Yep and yep.

For DECADES consumer groups (read: Attorneys) have been trying to sue (over injury cases) that MerCruiser, OMC, etc., etc., should have "nozzles" over the props to help prevent injury. In each case they have lost. Why? In a planing boat the drag created by the nozzle (or protective shroud as they called it) dropped efficiency immensely (negative gain).

The loss of efficiency with a nozzle ("guard" in lawyer speak) is so bad the USCG said NO WAY (planing boats).

They, to this day still try to sue (I know of 2 active cases just here).

The recreational lawsuit "prop injury" cases have been going on since the 50's.

Each new graduating class of lawyers thinks that injury because of the unprotected (nozzled) prop is going to set them in retirement.

(Just look at a new ladder--all of those waring labels are from lawsuits!)

Off topic....but

I was once in a case with a Bayliner (big one) that ran up the beach 62 feet, the operator was .12 OUI. The VP of Bayliner made a statement to the judge in federal court..."I guess what we need your honor is yet another warning label at the helm:" "CAUTION FOR USE ON WATER ONLY"



Best toYa'll

Bill

On Edit-- I don't mean to offend any lawyers here...just stating what I'm experienced with.

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[Kmot]

I don't mean to offend any lawyers here...just stating what I'm experienced with.

. :D .