Author Topic: Does anyone know the name of this girl? KEENOMA apparently.  (Read 1259 times)

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Toby

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Re: Does anyone know the name of this girl? KEENOMA apparently.
« Reply #15 on: August 24, 2018, 14:33:50 »
  Norm,

Great looking job!!  Very  neat and crisp.

Toby


des

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Re: Does anyone know the name of this girl? KEENOMA apparently.
« Reply #16 on: August 24, 2018, 15:34:28 »
Hi Toby

The Kort nozzles have an airfoil cross-section, usually thicker at the forward end and thinner at the outlet end (although there are some exceptions); flat in the area of the prop ring - the smallest internal diameter of the nozzle is in the region of the prop ring itself;  the nozzle then opens up slightly towards the outlet, thereby forming a venturi shape.  The outer surface is always tapered down, in a straight line, from the greater diameter of the leading edge, to the thinner diameter of the trailing edge;  this shape promotes laminar flow of the water over the outer surfaces of the nozzles.  (The nozzle is never partially flat and parallel, and partially tapered, as seen on some freebie model plans.)

There are two different airfoil shapes in common use for fixed or steerable nozzles (not sure about Schottel drives though) -

- one airfoil shape is used when thrust is most usually required in one direction only (usually forward) than the other - eg for towing over the stern, or pushing with the bow.  (I would think that this is the nozzle shape most likely to be used for Schottel and other ASD drives.)

- a different airfoil is used if the tug is of a type which can equally tow over the bow while travelling backwards, as well as towing over the stern.  This nozzle is not as efficient as the first when towing in the principal direction, but is more efficient in the "other" direction.

The internal diameter is never less than the tip diameter of the prop, so as to allow the prop to be withdrawn out the aft end (after the rudder has been removed).  In practice, some clearance is required, so in most model setups the internal nozzle diameter is usually 2-3 mm greater than the tip diameter of the prop - this makes for some interesting considerations when trying to source compatible props and nozzles from different suppliers, as some manufacturers of both state true diameters, and others state "nominal" diameters.

The nozzles are always set parallel to the shaftline - this is a matter of physics and fluid dynamics, as well as the need to maintain a constant propeller tip clearance within a narrow space.  Canting of the nozzles relative to the shaftline will decrease propeller tip clearance in one region, while opening it up in another, thereby resulting in vortices forming in the water flow through the nozzle.  If the shaftline itself is set running in or outboard, then the nozzles will be set accordingly.  Similarly, if the shaftline is set running upwards or downwards, then the nozzles will be set accordingly.  (I am currently working up working plans for a model where the prototype tug's prop shaftlines run both slightly downward, and slightly outboard.)

Hope this is helpful.

Des.

Toby

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Re: Does anyone know the name of this girl? KEENOMA apparently.
« Reply #17 on: August 24, 2018, 16:59:41 »
Hello Des!

Wow, what a lot of useful and interesting info.
Thank you.
I shall re- read it for kort nozzles are not something I have dealt with before and this all quite new.

Toby