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dannyc1234
dannyc1234

Posted: Tue Apr 03, 2012 5:01am

Post Subject: Shaft + Propeller at slight incline (7 deg)

Hello I've been informed that having the propeller shaft (and also then the propeller) at a slight incline of 7 deg (down) would normally slightly increase the thrust/effect of the engine/propulsion. Though I would still want the engine mounted straight. My understanding then is that this would require the use of a flexible drive unit, like a Python Drive or Aquadrive. Does this 7 deg angle realistically give much benefit at canal boat speeds? I don't mind buying and Aquadrive or similar if that will actually assist in improving the thrust. But I'm also considering a Centaflex coupling. This should normally also be effective at reducing vibrations. Though is this generally felt to be as effective at this as the Python/Aquadrive units. Also, it only supports 'minor' misalignment, so would need to be installed with straight shaft, so I wouldn't be implementing the 7 deg incline. Part of my reservations about the Python/Aquadrive units are that the seem to be around 300 mm long, this making my engine compartment longer (and the cabin area slightly smaller) Regards

Tony-B
Tony-B

Posted: Tue Apr 03, 2012 7:45am

Post Subject: Shaft + Propeller at slight incline (7 deg)

There is only one Cenntaflex model that is as effective as the Aquadrive or Pythondrive and that one is of similar length. Unless you are going to solidly mount your engine (on wood, not rubber mounts) I can not agree with your statement. Flexibly mounted engines tend to jump about on their mounts, especially as the mounts age. All the shorter flexible couplings only deal with angular misalignment not the radial that such movement produces. The longer flexibles also transfer the prop-thrust to the hull rather than stressing the engine mounts. Gearboxes generally allow zero radial misalignment and 0.05 mm of angular misalignment. The longer flexibles like the Aquadrive allow 2 x 3 degrees angular misalignment and radial contained within the 2 x 3 degree angles. They are not designed for allowing a horizontal engine and down angled shaft. However on GRP boats it has been standard practice to have a downward angled shaft but it has been accomplished by angeling the engine beds. Check the engine marinisers'data to see how much downward angle they allow for mounting the engine. I am away on holiday at the moment so have no access to my books but I think 7 degrees is often the maximum. I have already advised that you consult a naval architect for definitive advice and as an engineer I have no idea if a downward angled shaft would increase the thrust. My feeling is the thrust that would be trying to lift the back of the boat would be lost. I also fear that on canals the "suction" in reverse would tend to exert more "lift" on debris on the canal bed so making prop fouling more likely. Tony Brooks PS A hydraulic drive would allow a flat engine and heavily angled shaft!

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