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Go Back   SailNet Community > General Interest Forums > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #11  
Old 05-12-2006
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he Borg Warner boxes needs oil pressure from the input shaft, engine on. to engage the clutches. I guess your engine is not of the newest model, so has it been spinning this long, let it spin but keep the box in neutral position. It was not recomended by the older Borg Warner to let the propeller spin but a shaft lock to be innstaled, but as I said has it been spinning for the last 20+ years (?) I would keep on unless You plan a circum navigation.
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Old 05-12-2006
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Folding props can keep freewheeling, especially of you start sailing at speed before taking the engine out of forward gear. The resistance of the gearbox (generally in reverse) forces the prop to fold due to drag. Once folded, you can go to neutral again, but in most boats there's no need.
We have a Hurth gearbox, the recommendation is to put into reverse while sailing, and further not to let the prop drive the gearbox if the gear is in forward without the engine running. (I believe the input shaft must be powered to provide lubrication for the gearbox)
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Old 05-12-2006
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The locking of the clutch systems are different on the Borg Warner and Hurth. The hurt is purely mechanical, but the cluch neds the resistanse given by the propeller to lock. That gives that is has to be put in reverse to lock the clutc pack. This goes for all boxes with multiple disk clutch pack, Hurth, most Yanmar, ZF.
The lubrication is performed by the lower shaft splashing the oil around, no pressure, and the upper only rotates with the engine. The output shaft and bearings are not in oil and letting the prop shaft rotate means the bearing runs dry!
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Old 05-12-2006
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Quote:
Originally Posted by WanderingC's
A free spinning prop is not good. If for no other reason, it will slow the boat. Either put the transmission in gear or if hydraulic install a shaft break.
This has been argued many times over the years, always without facts as far as I have seen. If anyone knows of any experiments or measurements I would love to know of them.

My belief is that a spinning prop offers the least resistance. If it offered more resistance, it would stop spinning. But that is just a belief, not even a theory.

But I can definitely recommend you not install a "shaft break". A shaft brake would be more appropriate.
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Old 05-14-2006
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A free spinning propeller will have the same drag as a solid disc of the same diameter. DO NOT LET THE PROP SPIN FREE. Shut down the engine in gear and this should be enough to stop the prop from spinning. If you are in gear and the prop still spins, youu need a shaft brake.

Last edited by bradleyjdonaldson; 05-14-2006 at 01:23 AM.
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Old 05-14-2006
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This is base on Aerodynamics, but it is essentially the same in Hydrodynamics. The point is "The propeller produces much more drag as it windmills than it would if it simply stopped turning."

http://selair.selkirk.bc.ca/aerodyna...lti/Page2.html

http://avstop.com/AC/FlightTraingHan...eathering.html
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Old 05-14-2006
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i still think the example there doesn't fit the problem here.

for starters, the propellers on aircraft don't spin free. because they are usually atached directly to the crankshaft of the engine, a windmilling propeller has to overcome the engine's internal friction and compression. the power needed to do this produces drag.
as solution the blades are turned so they don't produce spinning force at all, reducing drag to the minimum.
it doesn't address the problem of a nonfeathered propeller spinning with no friction, or almost no friction, as only the friction of the bearings has to get overcome. and neither does it address the drag of a nonfeathered propeller stopped by a brake.

Last edited by Tea-Rex; 05-14-2006 at 08:26 AM.
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Old 05-19-2006
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Actually there is NO FRICTION in a free spinning propeller on a turbine powered aircraft. There is no mechanical linkage between the propeller and the power turbine. If you watch a pre-flight on a turbo prop aircraft, you can free spin the prop like a windmill with no resistance what so ever. It is still detrimental to fully feather the prop as soon as you loose power to keep the dead windmilling engine from dragging the aircraft down. In fact if it is an air transport category aircraft it must have a AUTOFEATHER system to make the climb requirement after an engine failure on takeoff. ANYWAY..... Try this...Get a small prop from the toy store. Hold it out the window of your car while somebody else drives. Look at the difference in force aginst you with a spinning prop and a stopped one. You will be amazed how much drag a spinning prop makes!

Last edited by bradleyjdonaldson; 05-19-2006 at 02:53 PM.
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Old 05-20-2006
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still the only conclusion those articles offer is that a feathered propeller has less drag than a nonfeathered propeller, either stopped or windmilling.
don't get me wrong here, i'm not saying that you're wrong, just that those articles don't support your statement.

however i googled around a bit and found this:

http://www.goshen.edu/physics/PropellerDrag/thesis.htm

the test involved severall propellers with different pitch. some of them had less drag windmilling than stopped, some had more.

i guess it depends too much on the individual propeller to make a general statement.
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Old 05-20-2006
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You seem to have lost contact with the 'sea' in the above discussion! The question is not what creates the less drag, the question is what the gear box may handle and tolerate. Most marine gearboxes can not tolerate free spinning due to lubrication problem!!!!!!!!!!!!!!!!!!
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