| Subject: |
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Re: More on shaft brake |
| Name: |
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Guillermo Gefaell |
| Date Posted: |
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Feb 27, 05 - 9:26 PM |
| Email: |
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bmccommodore@banjer37.net |
| Website: |
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http://www.banjer37.net |
| Message: |
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Here some additional explanations on this issue:
"To freewheel or not to freewheel has been debated in the sailing community since Ericsson invented the screw propeller. As you say, a two bladed prop that can be locked behind the deadwood offers less drag than one allowed to freewheel but not as low drag as a folding or feathering blade prop. The British journal Yachting World published a series of tests about 20 years ago that compared the drag of freewheeling and fixed three blade props. The experiment was quite ingenious. A boat with a three bladed prop was anchored in a tideway. A spring scale was inserted in the rode. As the tidal flow reached a speed of six knots, the prop was allowed to freewheel and the scale force measured. Then the prop was locked and the force measured again. The process was repeated with several props and several degrees of resistance to rotation as might be provided by a shaft in a stuffing box and turning a transmission. The conclusion was that a freewheeling prop with NO RESISTANCE offered the least drag, followed by a FIXED PROP. The most drag was offered by a SEMI-FREEWHEELING prop rotating about 1/2 to 2/3 the speed of a free spinning prop."
(Extracted from a Larry Zeitling's post in a sailing Forum)
As you can see, it would be necessary to perform a dedicated test for Banjers, because we do not know the resistance to rotation of shaft, bearings and gear, being probably quite significative, as tested by myself when trying to rotate by hand the propeller. So, most probably, a good idea for long passages should be braking the shaft, as I said before or, to solve definitively the problem, and get some extra knot, the best thing is to mount an "Autoprop" or "Maxprop" propeller type and forget about the brake. |
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