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CONTRA AND COUNTER rotating Props . Actually both are correct. Probably counter rotating is more correct for ships. We could argue for days!! Anyway they both go in different directions. One clock wise when the other is anticlockwise, but make sure they have left and right hand props, not the same type or you will be in trouble.
Actually for ships with props on separate prop shafts it's counter rotating. Contra refers to aircraft equipped with contra-rotating propellers with rotation about the same axis in opposite directions. Contra-rotating propellers should not be confused with counter-rotating propellers on separate shafts turning in opposite directions. I was intrigued by this discussion, possibly because it reminded me of my model flying days. What would we do without Wiki! Dave
with my 4 foot fireboat, I use contra rotating props, from propshop. I was advised by them to have them turning outwards, as viewed from the rear, there was an explanation, I've forgotten it, however, on the water, at speed, turning was difficult, the boat just wanted to roll. I swopped them over, so turning inwards, and the handling was cured in an instance. Its easy enough to try one way, then the other, see what is best for you, but remember to use some sort of none permanent thread lock or you might lose one of your props
Hi from Munich, basically if you have two props and only one rudder you want to get maximum force (i.e. water pressure) on the rudder. Therefore the props should turn inward to concentrate the flow over the rudder. The Admiral's experience confirms this!! My own experience with a 1:72 scale H Class destroyer (136cm) was the same. 😉 Cheers Doug 😎
Inwards or outwards - should the props be turning to meet together above the shaft or below it?
Also from Wikipedia "Contra-rotating is where parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of torque. Contra-rotating propellers should not be confused with counter-rotating propellers, a term which describes non-coaxial propellers on separate shafts; one turning clockwise and the other counter-clockwise." Torpedoes are a marine example of contra-rotating props. Roy
If they're turning inwards they (or better the water flow) 'meet' above the shafts, conversely: outwards = below the shafts where it disperses more easily, and probably also mostly below the rudder 🤔 Of course the hull form at the stern plays a major role. Which is why most modern fast naval vessels have a relatively flat hull at the stern if the only have one rudder. (Spent the last 31 years designing integrated COMMS systems for naval ships so have seen several such plans and GAs and many ships during the build.) With two rudders (1 per shaft) it's different. Classic example RN vs KM 1935/36 destroyers in WW2: RN 2 shafts 1 rudder; KM 2 shafts and 2 rudders. Giving them much better rudder response & manoeuvrability. Your comment re Contra is spot on 😉 Cheers from Munich.
An excellent discussion with very valid arguments. I have always had my twin shaft/prop vessels with outward turning propellers, but I will now reverse one of them to inwards turning to see how much difference it makes. Always learning...
Now here's a question for the experts. I have two identical waterjets for fitting into a model; same size and same rotation for forward motion. Will I see the same effect as if I had two open water props which were the same? Roy
Simple answer is no as they use a totally different way to provide power. Water jets suck water into a tube and eject a high pressure jet of water. My water jets have a steerable nozzle to control the direction of the water jet and a bucket to reverse the flow for reverse, exactly the same as on the full size Shannon lifeboat. You will see a much greater speed using water jets as they are optimised for high performance at the design stage. Dave