Technical, Techniques, Hints, and Tips > Other Technical Questions...
Propeller Rotation & Configuration
NFMike:
Yes, for open props I have always thought outward turning was preferable, though not for the reasons detailed in the OP.
However, many tugs and AHTS have Kort nozzles, which presumably remove these effects. Is that correct? (I have a picture on file of an AHTS with inward turning props in nozzles.)
Marty:
Inward turning props on AHTS are for one reason only that's because there fitted with controllable pitch propellers. Kort nozzles do not remove the above mentioned effects, there simply there to increase thrust. Most CPP propellers are left handed on twin screw CPP vessels (inward turning) this is a term known as a "handed" vessel. This allows them to have the same characteristics of outward turning props and achieve the same results.
Kort nozzles were initially introduced in canals and inland waterways to reduce erosion on the river banks by focussing the wash from the propeller into the centre of the canal/river, however it was noted that this had no effect other than considerably increasing the thrust produced by propeller. Korts are most useful when used at very low speeds under high load, use them above 8 knots and the vessels hull efficiency through the water is greatly reduced.
Using an azimuth thruster/pod system are CPP vessels, basically there just big outboard engines! Awesome pieces of kit if you ask me. I was working in Limassol earlier this year and we had two Damen tugs standing by our vessel. When alongside us i could see both azipods through the crystal clear water spinning away with zero pitch on the props. When the skipper put on the power the reaction was so quick, its incredible just how fast the direction of the pod can be changed all at the same time.
I've seen a lot of scale models using steerable korts and there great for towing by increasing the thrust, however for steering there not bad but could be better.(in my opinion) A large rudder fitted centre line on a kort directing the water flow provides superb means of control. The only other means better than this is the azipod system where you get maximum thrust in any direction you want. My tug has twin rudders on each kort and the minimum amount of helm gets her swinging.
Martin
Martin (Admin):
--- Quote from: Capt Podge on June 01, 2015, 11:45:28 pm ---That's an excellent piece of information Martin - thank you very much for the clarification, this will help with "discussions" at the lakeside in future - Nice one :-))
Regards,
Ray.
--- End quote ---
Totally agree with Ray, et al. . Nice to hear it direct from the "horses mouth!" Thanks for posting Marty.
NB. "inward turning propellers used on tugs and anchor handlers because in all essence there an absolute pig to handle."
1. Why would ship builders ever do that then?
2. Is there any case where inward turning propellers could be an advantage?
Martin :-)
Fastfaz:
Hi Marty,
That's a really useful post and has answered a question I was going to ask later in the year when I start the build on my Portgarth. Re the rudder on the back of a Kort nozzle I have a Lowgarth which is fitted with a 75mm 4 blade prop in a Kort nozzle, when I built the boat I did not fit the rudder blade to the back of the nozzle (had major grief with the parts in the kit and decided to use only the hull and running gear the rest scratch built). The boat sails very well and is pretty good going astern i.e. responsive and will sail straight, what I would like to know is will it make any difference, apart from making it hard to remove the prop, that is beneficial to handling especially when towing?
Thanks again for you really helpful post.
Cheers,
Pete/Faz. :-)) :-)) :-)) :-)) :-)) :-)) :-))
NFMike:
--- Quote from: Marty on June 02, 2015, 02:45:15 am ---Inward turning props on AHTS are for one reason only that's because there fitted with controllable pitch propellers. Kort nozzles do not remove the above mentioned effects, there simply there to increase thrust. Most CPP propellers are left handed on twin screw CPP vessels (inward turning) this is a term known as a "handed" vessel. This allows them to have the same characteristics of outward turning props and achieve the same results.
--- End quote ---
Ah. Ooh. Yes. Penny has dropped ... and the the machine gave me an extra bar of chocolate too O0
Yes, in that AHTS picture I have it does look like CP props.
This effect was explained to me about 50 years ago as 'paddle-wheel' effect which is why I thought that a nozzle would pretty much negate it, but the 'helical discharge' explanation makes it all fit properly (in my head anyway).
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