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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The internal diameter is never less than the tip diameter of the prop, so as to allow the prop to be withdrawn out the aft end (after the rudder has been removed).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.


Great looking job!!  Very  neat and crisp.


Hmm!  Good question  Toby.  In my kit built Smit Nederland they were strait in line with the keel.  On my stormforce tug I made them the same. (in line with the keel)   These are my home made ones.
 Norm thank you for the photo; it was not one I had seen.
So if I comprehend aright then the kortz at the propshaft end is of greater diameter then at the very stern.

In addition are the nozzles themselves set true to the  line of the theoretical keel spar or are they set slightly turned out each side

Hi Toby,  If I understand you correctly, any Kortz that I have seen are flared out at the leading edge tapering narrow at the trailing edge.  Not sure if I sent you the pics of the one I built but will include it here.

NEWBIES - START HERE ! / Re: Does anyone know the name of this girl?
« Last post by Toby on August 24, 2018, 07:43:46 »
Well I bought a second- hand hull and am using the plan sheet above.
I note that the plan allows either for a single screw out of the skeg or for some of the skeg to be trimmed away and twin kort nozzles to be fitted.
I have bought a pair of 11" shafts and 40mm props from MBB and decided to try the kort nozzle set up.
I have trimmed the skeg and positioned the nozzle shaft tubes and inserted the prop shafts.
There seems to be no provision for anchors and so I have decided upon cutting the hull to insert an anchor housing port and starboard something akin to that of the tug pictures above.

Re the kort nozzles would someone kindly tell me whether or not these are set straight back to the stern or are they 'flared out' (or in) for the boat to steam ahead.
Steam Tugs / Re: Gallery - Image Resource - Pictures only !
« Last post by shiploversa on August 20, 2018, 05:30:05 »
this model of the sir david hunter was up for auction awhile ago - in florida united states
Steam Tugs / Re: South African Steam Tugs
« Last post by shiploversa on August 20, 2018, 05:27:13 »
Hi George

I had never considered that, she was one of five vessels built by Cantieri Navali e Officiene Meccaniche de Venezia for the SAR&H which had most "unusual" funnel colours...

The others were: "William Weller", " Cecil G White", SJ Harrison", and "JE Eaglesham" all virtually identical and designed along the lines of much earler period British built tugs.

Dimensions 83ft x 20ft x 11ft.  110 GRT

All machinery also made by the builder. (300 IHP Triple Expansion , Coal Fired Scotch Type Boiler, 200psi working pressure service speed 10.5 Kts....)

Have also attached a pic of the Cecil G White circa 1980. Slightly different colour scheme..

I also must admit that I had little knoweledge of Italian built steamers, however since this one am quite impressed.

Unusual as was built to both act as a tug as well as carry passengers... kind of like a cross between a launch and tugboat, hence the uncluttered aft deck, teak decks etc.

Agree with what you say re the Crank Pit... oh well someone has to do it, just thank goodness it isn't me.  ;D

 the wheel house of the cecil g white in the east london museum

Steam Tugs / Re: South African Steam Tugs
« Last post by shiploversa on August 20, 2018, 05:16:28 »
Many thanks guys,

I had a sneaky little reason for asking... and am very familiar with the JR More.

Durban has two of the three remaining steam tugs left in SA. (Ulundi being the other one there)

However there is one left presently at Cape Town but not for much longer.

The "Alwyn Vintcent" is the last surviving steam pilot tug of her class, and, unlike the others was in fact Italian built in 1958.

To cut a long story short, she got herself into a bit of bother however, now she has just become privately owned and saved from the cutters torch, will shortly be coming "down under" and returning to commercial work as both tug and passenger vessel as a steamer.

I have attached a small pic of her as she was in about 2001... but a little neglected at the moment.

Some other shots taken a couple of days ago... dont like them however the "look" will be shortlived.... work is now underway.

Still have drawings of her (once converted to electronic) and her sister ships. And numerous other material.

More pics will be available next week of other internal areas, and a new website posted.



Will update more in a day or two.


the last i knew of her was she was in villiersdorp - and thats it
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