John; well done! Have you tested it in the water? I like your alignment fixture. I think the hardest part is aligning the shaft in the Kort. I know it is for me.

TD; my purchased Korts taper from large, forward to smaller, aft. That configuration would create a Venturi effect adding to the amount of water flowing through the the Kort. I have no idea how that affects the prop but on real boats I’ve read it increases the thrust. As you could appropriately point out, it ain’t going to show up on our models.
For my part this is becoming a good discussion. I’m learning a lot.👍 Keep the input coming.

building the Kort nozzle and it presented me with a bit of a problem – because of its awkward size in diameter. Normally, I would have used a bit of 3” plastic drainpipe machined to the correct shape and size. This one has an outside diameter/dimension of 68 mm. So, I ended up laminating the Kort from plywood and the procedure I used for this was:
I used a cardboard tube which came from the centre of some knitting yarn which the Mrs is using at the moment. This tube had correct internal dimension required which corresponded with the outside diameter of the Kort nozzle so – this tube became the former for me to build up laminates of 1 mm thick plywood by 30 mm wide by roughly 200 mm long to begin with. The first laminate I applied some cellotape to one side of it to prevent it bonding to the inside of the cardboard tube.
This laminate was placed inside the tube first with no glue on ensuring that the side which had the cellotape on was against the inside of the cardboard tube. The second piece of plywood of similar dimensions, only slightly shorter had the glued to the inner piece of plywood already in the cardboard tube ensuring that the butt joints were opposite each other. Then an assortment of clamps clamping the laminates together – when this had dried, I applied 4 more laminates building up the required wall thickness of the Korts nozzle. This was then allowed to dry out completely and then the cardboard tubing was removed.
I have the facility of a lathe to which I machined the outside/inside angle and polished. This was then given 2 coats of epoxy resin.
For those not having the facility of a lathe – you could make a mandrel up to hold the nozzle and then machine it in an electric drill. From plasticard I also made a double jig up for aligning the nozzle with the prop shaft. Then I located and fixed the nozzle in place using 2 brass pins.
aye
john

My V4-MA-1 tug has a Kort nozzle. The inside diameter is even bow to stern. The outside has an airfoil shape I had it turned from a piece of bronze according to the original Maritime Commission plans. It is a fixed Kort.
In my head, I would think that if the ID tapered, presumably front to back, it would negate some of the advantage of the Kort. Unlike air, H2O is not compressible( I think you knew that 😂) so thrust should not increase.
But, then what do I know???🤣

For those that use Kort nozzles:
- Do you buy them or fabricate your own?
- If you make your own, do you taper the ID ?

- Do you prefer steerable Korts or fixed Korts
with single or dual rudders?

You’re absolutely right right TD; all the technical discussions that apply to the 1:1’s in reality amount to a big “so what” when we’re talking models. But my background makes me want to know “why”. I view these things through the lens of functionality. Ex, I never thought about material, component shortages, etc played a role in the direction of prop rotation for PT boats. That in itself is fascinating & enlightening for me.

No question I get into the weeds about functionality. My wife would say that’s a good thing as it slows down my building which results in lower fleet inventory numbers!

just a quick note, whilst doing a lot of research on the Vosper RTTL Launch; it was discovered that, originally the boats were triple screw all driving in the same direction. Vosper did experiment on these and it was found that there was a slight increase in speed over the prop formation of 2 outboard props turning inboard and the centre prop was turning inboard or outboard and that it made no difference to the speed. It was also found, later on, that with the props all turning in the same direction the boat did have a tendency to corkscrew when going on the plane and the boat would dive into the next wave in front. Consequently breaking planking on the bottom of the hull at the bow. So, Vosper strengthened the hull by adding a 3rd layer of planking to the bottom of these boats. The other thing which has been mentioned is that during the War the shortage of materials for making reverse drive gear boxes was one of the main reasons for all of the MTBs and PT boats shafts driving in the same direction .

Chumm,
Unless you are operating at the David Taylor Center( USN hydrodynamic research center for proving of ship/hull designs and characteristics in a scaled environment) I think that us modelers worry too much.
There are so many variables that effect a ship's response and we, as modelers, have almost zero control over the majority of them. How true is your prop? What is the actual pitch? How about drag coefficients? Are our power plants( motors) scale to the HP of the 1:1 ship? The list goes on.
In the end, it is a hobby, and we try to make our models work prototypically which really all we want.

Thank you both Alessandro & TD. In the future I’ll ask if various parameters from 1:1 vessels really make a difference in our models. In most cases I suspect that they don’t; to wit airfoil profile for internal diameter of a Kort since our model props don’t move a large enough volume of water.

I certainly am no expert in marine hydro dynamics. What knowledge I do have is based on experimentation & experience; not empirical analysis. Any naval archs in the forum?

I looked at a custom manufactured Kort I have. If I REALLY squint I might see an airfoil profile for the nozzle ID. Very difficult to tell. Again, experimentally based, I have found no difference in mass produced model Korts vs ones I have fabricated from PVC pipe. Throw is what creates the most noticeable differences in maneuverability.

Allessandro,
Thank you for the correction!
Chumm,
Right handed and left handed props are equally efficient. So why did US Pt's have all like handed wheels? Expediency and cost. The choice of R vs L handed was based upon the engine rotation direction.
Why all three? It saved on the cost and weight of additional reverse gearing. The only down side to these decisions, that I know of, was a different turning diameter, Port vs Stbd.

In the long run, on our models it hardly makes any difference, except to scale appearance.

I believe the issue is complex enough to discuss briefly, and I certainly don't have the expertise to do so.

However, from what I know, I hold this opinion.
The adoption of the third propeller was an evolution of twin-screw hulls.
Compared to these, hulls with three propellers are (or were considered) more maneuverable.
The adoption of four propellers on four separate shafts was a further evolution and also a gain in terms of power. Three propellers were adopted for a certain period on some ships, but then this solution was abandoned.
Toradog is right, but he makes a mistake in citing the Italian battleships. The latest ones built (the Littorio class) were all designed with four propellers, not three.
The older ones, however (the Cavour class), switched from four to two propellers after the modernization that completely changed them.
It is true, however, that the Bismarck remained with the old design.
The later ones, however, had four propellers.
Ditto Yamato and Yowa class.

Thanks for the explanation TD; makes sense. Now I would appreciate your explanation of prop rotation on PT boats. I can understand 3 shafts for needed hp, but all three rotating in same direction? Which direction & why?

Onto Korts. SS; Upon further research I found they do have an airfoil profile on the I.D. I’ll have to look at the commercially produced ones I have. So this has already been a good learning experience. There is an excellent discussion by the skipper of a real tug on another UK Forum. Is it permissible in this forum to provide a link to it? He prefers a fixed Kort with twin rudders on each. Yet another variation to consider! I haven’t tried that arrangement.

If there are skippers of real vessels on this site who captain a boat equipped with Korts I would REALLY appreciate your thoughts.

Chum,
First, steam powered vessels were reversible, at least reciprocating drive one's were. Steam turbines are not, hence the addition of a reverse turbine. Early steam turbine plants generally lacked a reverse turbine. They would come later when turbine drive became more reliable, economic and sophisticated. Early turbines also lacked the HP relative to reciprocating engines.
Early in their development, steam turbines proved their worth as an easy way to add increased HP without bulk.
Looking at the design histories of later ships, the three engine plant was a compromise. The turbines had by this time become matured and the HP available from them was such that three shaft would provide the speed needed, without the internal space demands for
an additional turbine, boilers and fuel supply. This, in turn allowed the re-allocation of tonnage to other" more important" items, such as armor and armament.
There is an interesting correlation between anticipated range of a ship and the shaft arrangements. The USA, Britain, and Japan, quickly adopted the four shaft arrangement, while Germany, and Italy chose three shafts.
There is quite a bit more to this than what I present, but hopefully the idea is conveyed.
In the end, the shaft arrangement often reflects the HP available per shaft in relation to the speed target of the vessel.😊

Well I don't know about smaller craft but the Titanic's center screw was driven by a steam turbine utilizing the last energy from the triple expansion engines driving the outboard screws. fwiw😁