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shroud around props:

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rathikrishna:
Good after noon friends.i am going to make a new TUG, which is intented for long time operation.i was looking for a good motor for that. and now i got it, a wiper motor from an old car. it drains only around 800 ma even while trying to hold its shaft at speed.its much tourquey also.so my doubt is this. i could find a ring covering around the props, in every tugs, just we see in a duct fan used in aircrafts.i have furnished a full unit of propultion, the props goes in to 3 inch dia.an apropriate sized pvc pipe is added as a duct.but i am in a confused state that, when i checks the motor prop unit under water it drops around 600 ma.at 12 volts. but when i adds the duct to the prop the current drops around 500 ma.means motor is less strained after fixing the duct..means less peformance...?..will any one guide me once...so simply my doubt is this. will a prop shroud increase performance ?. please..

Pirate:
A brief history of shrouds around props:
during the industrial revolution in England, much of the goods traveled by horse drawn barges, but when the internal combustion engine came into being it was quickly adapted to barge propulsion. Great fear was aroused to the new propeller driven craft eroding the canal bank sides with its wash, So it was found that a shroud around the prop reduced the side thrust and thus reduced the erosion. As a side issue it was also discovered that the engine was using less fuel for the same speed. (This is what you have discovered) the wasted side thrust is sent as drive thrust, thus the engine revs can be reduced to obtain the same level of propulsive work as before. This shroud has undergone vast research and development and is now the modern Kort nozzle. These Kort's have an airfoil section and act like a continuous circular wing and jet drive at the same time. For tugs this adds significant propulsive drive as compared to an open propeller. I feel that you with your aeronautical background will do great service to your Kort/shroud propeller driven craft.

Kind regards
Pirate

woody_294:
Great reply Pirate! I'm glad I now know that, I love the fact that they came across it completely by accident!

Tug-Kenny RIP:

Thank you for that explanation Pirate.  I have often wondered about their design.

You mention aeronautical shape. Does the 'tube shape' of the 'Kort nozzle'  taper in it's internal and external diameter from front to back ? and does the thickness of the skin vary as well as being curved on the outside ?

The information would help Rathi and many others (including me!)

Cheers

Ken

Colin Bishop:
Kort Nozzles were described in Terry Gosling's article on Propulsion published in Model Boats in 1999 and recently reprinted in this year's Commemorative Special issue. Well worth reading for anyone thinking of installing Korts in their model boat. An extract from Terry's article below would seem to answer your query Ken.

Colin

Mr Kort, who was an aerodynamicist, rather than a naval architect, proposed that a shroud be fitted around the propeller. With his aerodynamics experience he naturally made this shroud of a streamline shape similar to an aeroplane wing cross section to reduce the resistance as much as possible. It proved to be very successful as a preventer of erosion and still is so, but somewhat to Kort's surprise, a marked increase in efficiency and performance was noted whenever the shroud was fitted.

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